cdp: refacto message JSON read

src/cdp/browser.zig

@@ -22,7 +22,7 @@ const server = @import("../server.zig");
 const Ctx = server.Ctx;
 const cdp = @import("cdp.zig");
 const result = cdp.result;
-const getMsg = cdp.getMsg;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 
 const log = std.log.scoped(.cdp);
 
@@ -35,18 +35,17 @@ const Methods = enum {
 
 pub fn browser(
     alloc: std.mem.Allocator,
-    id: ?u16,
+    msg: *IncomingMessage,
     action: []const u8,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
     const method = std.meta.stringToEnum(Methods, action) orelse
         return error.UnknownMethod;
     return switch (method) {
-        .getVersion => getVersion(alloc, id, scanner, ctx),
+        .getVersion => getVersion(alloc, msg, ctx),
-        .setDownloadBehavior => setDownloadBehavior(alloc, id, scanner, ctx),
+        .setDownloadBehavior => setDownloadBehavior(alloc, msg, ctx),
-        .getWindowForTarget => getWindowForTarget(alloc, id, scanner, ctx),
+        .getWindowForTarget => getWindowForTarget(alloc, msg, ctx),
-        .setWindowBounds => setWindowBounds(alloc, id, scanner, ctx),
+        .setWindowBounds => setWindowBounds(alloc, msg, ctx),
     };
 }
 
@@ -59,14 +58,12 @@ const JsVersion = "12.4.254.8";
 
 fn getVersion(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
-    const msg = try getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "browser.getVersion" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "browser.getVersion" });
 
     // ouput
     const Res = struct {
@@ -76,17 +73,15 @@ fn getVersion(
         userAgent: []const u8 = UserAgent,
         jsVersion: []const u8 = JsVersion,
     };
-    return result(alloc, msg.id, Res, .{}, null);
+    return result(alloc, input.id, Res, .{}, null);
 }
 
 // TODO: noop method
 fn setDownloadBehavior(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         behavior: []const u8,
@@ -94,11 +89,11 @@ fn setDownloadBehavior(
         downloadPath: ?[]const u8 = null,
         eventsEnabled: ?bool = null,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("REQ > id {d}, method {s}", .{ msg.id, "browser.setDownloadBehavior" });
+    log.debug("REQ > id {d}, method {s}", .{ input.id, "browser.setDownloadBehavior" });
 
     // output
-    return result(alloc, msg.id, null, null, null);
+    return result(alloc, input.id, null, null, null);
 }
 
 // TODO: hard coded ID
@@ -106,8 +101,7 @@ const DevToolsWindowID = 1923710101;
 
 fn getWindowForTarget(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
 
@@ -115,9 +109,9 @@ fn getWindowForTarget(
     const Params = struct {
         targetId: ?[]const u8 = null,
     };
-    const msg = try cdp.getMsg(alloc, _id, ?Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    std.debug.assert(msg.sessionID != null);
+    std.debug.assert(input.sessionId != null);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "browser.getWindowForTarget" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "browser.getWindowForTarget" });
 
     // output
     const Resp = struct {
@@ -130,21 +124,20 @@ fn getWindowForTarget(
             windowState: []const u8 = "normal",
         } = .{},
     };
-    return result(alloc, msg.id, Resp, Resp{}, msg.sessionID.?);
+    return result(alloc, input.id, Resp, Resp{}, input.sessionId);
 }
 
 // TODO: noop method
 fn setWindowBounds(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
+    const input = try msg.getInput(alloc, void);
 
     // input
-    const msg = try cdp.getMsg(alloc, _id, void, scanner);
+    log.debug("Req > id {d}, method {s}", .{ input.id, "browser.setWindowBounds" });
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "browser.setWindowBounds" });
 
     // output
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }

src/cdp/cdp.zig

@@ -30,6 +30,7 @@ const network = @import("network.zig").network;
 const emulation = @import("emulation.zig").emulation;
 const fetch = @import("fetch.zig").fetch;
 const performance = @import("performance.zig").performance;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 
 const log_cdp = std.log.scoped(.cdp);
 
@@ -70,51 +71,29 @@ pub fn do(
     ctx: *Ctx,
 ) ![]const u8 {
 
-    // JSON scanner
+    // incoming message parser
-    var scanner = std.json.Scanner.initCompleteInput(alloc, s);
+    var msg = IncomingMessage.init(alloc, s);
-    defer scanner.deinit();
+    defer msg.deinit();
 
-    std.debug.assert(try scanner.next() == .object_begin);
+    const method = try msg.getMethod();
-
-    // handle 2 possible orders:
-    // - id, method <...>
-    // - method, id <...>
-    var method_key = try nextString(&scanner);
-    var method_token: std.json.Token = undefined;
-    var id: ?u16 = null;
-    // check swap order
-    if (std.mem.eql(u8, method_key, "id")) {
-        id = try getId(&scanner, method_key);
-        method_key = try nextString(&scanner);
-        method_token = try scanner.next();
-    } else {
-        method_token = try scanner.next();
-    }
-    try checkKey(method_key, "method");
-
-    // retrieve method
-    if (method_token != .string) {
-        return error.WrongTokenType;
-    }
-    const method_name = method_token.string;
 
     // retrieve domain from method
-    var iter = std.mem.splitScalar(u8, method_name, '.');
+    var iter = std.mem.splitScalar(u8, method, '.');
     const domain = std.meta.stringToEnum(Domains, iter.first()) orelse
         return error.UnknonwDomain;
 
     // select corresponding domain
     const action = iter.next() orelse return error.BadMethod;
     return switch (domain) {
-        .Browser => browser(alloc, id, action, &scanner, ctx),
+        .Browser => browser(alloc, &msg, action, ctx),
-        .Target => target(alloc, id, action, &scanner, ctx),
+        .Target => target(alloc, &msg, action, ctx),
-        .Page => page(alloc, id, action, &scanner, ctx),
+        .Page => page(alloc, &msg, action, ctx),
-        .Log => log(alloc, id, action, &scanner, ctx),
+        .Log => log(alloc, &msg, action, ctx),
-        .Runtime => runtime(alloc, id, action, &scanner, s, ctx),
+        .Runtime => runtime(alloc, &msg, action, ctx),
-        .Network => network(alloc, id, action, &scanner, ctx),
+        .Network => network(alloc, &msg, action, ctx),
-        .Emulation => emulation(alloc, id, action, &scanner, ctx),
+        .Emulation => emulation(alloc, &msg, action, ctx),
-        .Fetch => fetch(alloc, id, action, &scanner, ctx),
+        .Fetch => fetch(alloc, &msg, action, ctx),
-        .Performance => performance(alloc, id, action, &scanner, ctx),
+        .Performance => performance(alloc, &msg, action, ctx),
     };
 }
 
@@ -133,14 +112,6 @@ pub const State = struct {
 // Utils
 // -----
 
-fn nextString(scanner: *std.json.Scanner) ![]const u8 {
-    const token = try scanner.next();
-    if (token != .string) {
-        return error.WrongTokenType;
-    }
-    return token.string;
-}
-
 pub fn dumpFile(
     alloc: std.mem.Allocator,
     id: u16,
@@ -158,10 +129,6 @@ pub fn dumpFile(
     defer alloc.free(p);
 }
 
-fn checkKey(key: []const u8, token: []const u8) !void {
-    if (!std.mem.eql(u8, key, token)) return error.WrongToken;
-}
-
 // caller owns the slice returned
 pub fn stringify(alloc: std.mem.Allocator, res: anytype) ![]const u8 {
     var out = std.ArrayList(u8).init(alloc);
@@ -229,97 +196,6 @@ pub fn sendEvent(
     try server.sendAsync(ctx, event_msg);
 }
 
-fn getParams(
-    alloc: std.mem.Allocator,
-    comptime T: type,
-    scanner: *std.json.Scanner,
-    key: []const u8,
-) !?T {
-
-    // check key is "params"
-    if (!std.mem.eql(u8, "params", key)) return null;
-
-    // skip "params" if not requested
-    if (T == void) {
-        var finished: usize = 0;
-        while (true) {
-            switch (try scanner.next()) {
-                .object_begin => finished += 1,
-                .object_end => finished -= 1,
-                else => continue,
-            }
-            if (finished == 0) break;
-        }
-        return void{};
-    }
-
-    // parse "params"
-    const options = std.json.ParseOptions{
-        .max_value_len = scanner.input.len,
-        .allocate = .alloc_if_needed,
-    };
-    return try std.json.innerParse(T, alloc, scanner, options);
-}
-
-fn getId(scanner: *std.json.Scanner, key: []const u8) !?u16 {
-
-    // check key is "id"
-    if (!std.mem.eql(u8, "id", key)) return null;
-
-    // parse "id"
-    return try std.fmt.parseUnsigned(u16, (try scanner.next()).number, 10);
-}
-
-fn getSessionId(scanner: *std.json.Scanner, key: []const u8) !?[]const u8 {
-
-    // check key is "sessionId"
-    if (!std.mem.eql(u8, "sessionId", key)) return null;
-
-    // parse "sessionId"
-    return try nextString(scanner);
-}
-
-pub fn getMsg(
-    alloc: std.mem.Allocator,
-    _id: ?u16,
-    comptime params_T: type,
-    scanner: *std.json.Scanner,
-) !struct { id: u16, params: ?params_T, sessionID: ?[]const u8 } {
-    var id_msg: ?u16 = null;
-    var params: ?params_T = null;
-    var sessionID: ?[]const u8 = null;
-
-    var t: std.json.Token = undefined;
-
-    while (true) {
-        t = try scanner.next();
-        if (t == .object_end) break;
-        if (t != .string) {
-            return error.WrongTokenType;
-        }
-        if (_id == null and id_msg == null) {
-            id_msg = try getId(scanner, t.string);
-            if (id_msg != null) continue;
-        }
-        if (params == null) {
-            params = try getParams(alloc, params_T, scanner, t.string);
-            if (params != null) continue;
-        }
-        if (sessionID == null) {
-            sessionID = try getSessionId(scanner, t.string);
-        }
-    }
-
-    // end
-    t = try scanner.next();
-    if (t != .end_of_document) return error.CDPMsgEnd;
-
-    // check id
-    if (_id == null and id_msg == null) return error.RequestWithoutID;
-
-    return .{ .id = _id orelse id_msg.?, .params = params, .sessionID = sessionID };
-}
-
 // Common
 // ------
 

src/cdp/emulation.zig

@@ -22,8 +22,8 @@ const server = @import("../server.zig");
 const Ctx = server.Ctx;
 const cdp = @import("cdp.zig");
 const result = cdp.result;
-const getMsg = cdp.getMsg;
 const stringify = cdp.stringify;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 
 const log = std.log.scoped(.cdp);
 
@@ -36,18 +36,17 @@ const Methods = enum {
 
 pub fn emulation(
     alloc: std.mem.Allocator,
-    id: ?u16,
+    msg: *IncomingMessage,
     action: []const u8,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
     const method = std.meta.stringToEnum(Methods, action) orelse
         return error.UnknownMethod;
     return switch (method) {
-        .setEmulatedMedia => setEmulatedMedia(alloc, id, scanner, ctx),
+        .setEmulatedMedia => setEmulatedMedia(alloc, msg, ctx),
-        .setFocusEmulationEnabled => setFocusEmulationEnabled(alloc, id, scanner, ctx),
+        .setFocusEmulationEnabled => setFocusEmulationEnabled(alloc, msg, ctx),
-        .setDeviceMetricsOverride => setDeviceMetricsOverride(alloc, id, scanner, ctx),
+        .setDeviceMetricsOverride => setDeviceMetricsOverride(alloc, msg, ctx),
-        .setTouchEmulationEnabled => setTouchEmulationEnabled(alloc, id, scanner, ctx),
+        .setTouchEmulationEnabled => setTouchEmulationEnabled(alloc, msg, ctx),
     };
 }
 
@@ -59,8 +58,7 @@ const MediaFeature = struct {
 // TODO: noop method
 fn setEmulatedMedia(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
 
@@ -69,57 +67,52 @@ fn setEmulatedMedia(
         media: ?[]const u8 = null,
         features: ?[]MediaFeature = null,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "emulation.setEmulatedMedia" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "emulation.setEmulatedMedia" });
 
     // output
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }
 
 // TODO: noop method
 fn setFocusEmulationEnabled(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         enabled: bool,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "emulation.setFocusEmulationEnabled" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "emulation.setFocusEmulationEnabled" });
 
     // output
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }
 
 // TODO: noop method
 fn setDeviceMetricsOverride(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
-    const msg = try cdp.getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "emulation.setDeviceMetricsOverride" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "emulation.setDeviceMetricsOverride" });
 
     // output
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }
 
 // TODO: noop method
 fn setTouchEmulationEnabled(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-    const msg = try cdp.getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "emulation.setTouchEmulationEnabled" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "emulation.setTouchEmulationEnabled" });
 
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }

src/cdp/fetch.zig

@@ -22,7 +22,7 @@ const server = @import("../server.zig");
 const Ctx = server.Ctx;
 const cdp = @import("cdp.zig");
 const result = cdp.result;
-const getMsg = cdp.getMsg;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 
 const log = std.log.scoped(.cdp);
 
@@ -32,28 +32,26 @@ const Methods = enum {
 
 pub fn fetch(
     alloc: std.mem.Allocator,
-    id: ?u16,
+    msg: *IncomingMessage,
     action: []const u8,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
     const method = std.meta.stringToEnum(Methods, action) orelse
         return error.UnknownMethod;
 
     return switch (method) {
-        .disable => disable(alloc, id, scanner, ctx),
+        .disable => disable(alloc, msg, ctx),
     };
 }
 
 // TODO: noop method
 fn disable(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-    const msg = try getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "fetch.disable" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "fetch.disable" });
 
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }

src/cdp/log.zig

@@ -22,7 +22,7 @@ const server = @import("../server.zig");
 const Ctx = server.Ctx;
 const cdp = @import("cdp.zig");
 const result = cdp.result;
-const getMsg = cdp.getMsg;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 const stringify = cdp.stringify;
 
 const log_cdp = std.log.scoped(.cdp);
@@ -33,27 +33,25 @@ const Methods = enum {
 
 pub fn log(
     alloc: std.mem.Allocator,
-    id: ?u16,
+    msg: *IncomingMessage,
     action: []const u8,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
     const method = std.meta.stringToEnum(Methods, action) orelse
         return error.UnknownMethod;
 
     return switch (method) {
-        .enable => enable(alloc, id, scanner, ctx),
+        .enable => enable(alloc, msg, ctx),
     };
 }
 
 fn enable(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-    const msg = try getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log_cdp.debug("Req > id {d}, method {s}", .{ msg.id, "log.enable" });
+    log_cdp.debug("Req > id {d}, method {s}", .{ input.id, "log.enable" });
 
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }

src/cdp/msg.zig

@@ -0,0 +1,214 @@
+// Copyright (C) 2023-2024  Lightpanda (Selecy SAS)
+//
+// Francis Bouvier <francis@lightpanda.io>
+// Pierre Tachoire <pierre@lightpanda.io>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as
+// published by the Free Software Foundation, either version 3 of the
+// License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+const std = @import("std");
+
+// Parse incoming protocol message in json format.
+pub const IncomingMessage = struct {
+    scanner: std.json.Scanner,
+    json: []const u8,
+
+    obj_begin: bool = false,
+    obj_end: bool = false,
+
+    id: ?u16 = null,
+    scan_sessionId: bool = false,
+    sessionId: ?[]const u8 = null,
+    method: ?[]const u8 = null,
+    params_skip: bool = false,
+
+    pub fn init(alloc: std.mem.Allocator, json: []const u8) IncomingMessage {
+        return .{
+            .json = json,
+            .scanner = std.json.Scanner.initCompleteInput(alloc, json),
+        };
+    }
+
+    pub fn deinit(self: *IncomingMessage) void {
+        self.scanner.deinit();
+    }
+
+    fn scanUntil(self: *IncomingMessage, key: []const u8) !void {
+        while (true) {
+            switch (try self.scanner.next()) {
+                .end_of_document => return error.EndOfDocument,
+                .object_begin => {
+                    if (self.obj_begin) return error.InvalidObjectBegin;
+                    self.obj_begin = true;
+                },
+                .object_end => {
+                    if (!self.obj_begin) return error.InvalidObjectEnd;
+                    if (self.obj_end) return error.InvalidObjectEnd;
+                    self.obj_end = true;
+                },
+                .string => |s| {
+                    // is the key what we expects?
+                    if (std.mem.eql(u8, s, key)) return;
+
+                    // save other known keys
+                    if (std.mem.eql(u8, s, "id")) try self.scanId();
+                    if (std.mem.eql(u8, s, "sessionId")) try self.scanSessionId();
+                    if (std.mem.eql(u8, s, "method")) try self.scanMethod();
+                    if (std.mem.eql(u8, s, "params")) try self.scanParams();
+
+                    // TODO should we skip unknown key?
+                },
+                else => return error.InvalidToken,
+            }
+        }
+    }
+
+    fn scanId(self: *IncomingMessage) !void {
+        const t = try self.scanner.next();
+        if (t != .number) return error.InvalidId;
+        self.id = try std.fmt.parseUnsigned(u16, t.number, 10);
+    }
+
+    fn getId(self: *IncomingMessage) !u16 {
+        if (self.id != null) return self.id.?;
+
+        try self.scanUntil("id");
+        try self.scanId();
+        return self.id.?;
+    }
+
+    fn scanSessionId(self: *IncomingMessage) !void {
+        switch (try self.scanner.next()) {
+            // session id can be null.
+            .null => return,
+            .string => |s| self.sessionId = s,
+            else => return error.InvalidSessionId,
+        }
+
+        self.scan_sessionId = true;
+    }
+
+    fn getSessionId(self: *IncomingMessage) !?[]const u8 {
+        if (self.scan_sessionId) return self.sessionId;
+
+        self.scanUntil("sessionId") catch |err| {
+            if (err != error.EndOfDocument) return err;
+            // if the document doesn't contains any session id key, we must
+            // return null value.
+            self.scan_sessionId = true;
+            return null;
+        };
+        try self.scanSessionId();
+        return self.sessionId;
+    }
+
+    fn scanMethod(self: *IncomingMessage) !void {
+        const t = try self.scanner.next();
+        if (t != .string) return error.InvalidMethod;
+        self.method = t.string;
+    }
+
+    pub fn getMethod(self: *IncomingMessage) ![]const u8 {
+        if (self.method != null) return self.method.?;
+
+        try self.scanUntil("method");
+        try self.scanMethod();
+        return self.method.?;
+    }
+
+    // scanParams skip found parameters b/c if we encounter params *before*
+    // asking for getParams, we don't know how to parse them.
+    fn scanParams(self: *IncomingMessage) !void {
+        const tt = try self.scanner.peekNextTokenType();
+        if (tt != .object_begin) return error.InvalidParams;
+        try self.scanner.skipValue();
+        self.params_skip = true;
+    }
+
+    // getParams restart the JSON parsing
+    fn getParams(self: *IncomingMessage, alloc: std.mem.Allocator, T: type) !T {
+        if (T == void) return void{};
+
+        if (self.params_skip) {
+            // TODO if the params have been skipped, we have to retart the
+            // parsing from start.
+            return error.SkippedParams;
+        }
+
+        try self.scanUntil("params");
+
+        // parse "params"
+        const options = std.json.ParseOptions{
+            .max_value_len = self.scanner.input.len,
+            .allocate = .alloc_if_needed,
+        };
+        return try std.json.innerParse(T, alloc, &self.scanner, options);
+    }
+
+    pub fn getInput(self: *IncomingMessage, alloc: std.mem.Allocator, T: type) !struct { id: u16, sessionId: ?[]const u8, params: T } {
+        return .{
+            .params = try self.getParams(alloc, T),
+            .id = try self.getId(),
+            .sessionId = try self.getSessionId(),
+        };
+    }
+};
+
+test "read incoming message" {
+    const inputs = [_][]const u8{
+        \\{"id":1,"method":"foo","sessionId":"bar","params":{"bar":"baz"}}
+        ,
+        \\{"params":{"bar":"baz"},"id":1,"method":"foo","sessionId":"bar"}
+        ,
+        \\{"sessionId":"bar","params":{"bar":"baz"},"id":1,"method":"foo"}
+        ,
+        \\{"method":"foo","sessionId":"bar","params":{"bar":"baz"},"id":1}
+        ,
+    };
+
+    for (inputs) |input| {
+        var msg = IncomingMessage.init(std.testing.allocator, input);
+        defer msg.deinit();
+
+        try std.testing.expectEqual(1, try msg.getId());
+        try std.testing.expectEqualSlices(u8, "foo", try msg.getMethod());
+        try std.testing.expectEqualSlices(u8, "bar", (try msg.getSessionId()).?);
+
+        const T = struct { bar: []const u8 };
+        const in = msg.getInput(std.testing.allocator, T) catch |err| {
+            if (err != error.SkippedParams) return err;
+            // TODO remove this check when params in the beginning is handled.
+            continue;
+        };
+        try std.testing.expectEqualSlices(u8, "baz", in.params.bar);
+    }
+}
+
+test "read incoming message with null session id" {
+    const inputs = [_][]const u8{
+        \\{"id":1}
+        ,
+        \\{"params":{"bar":"baz"},"id":1,"method":"foo"}
+        ,
+        \\{"sessionId":null,"params":{"bar":"baz"},"id":1,"method":"foo"}
+        ,
+    };
+
+    for (inputs) |input| {
+        var msg = IncomingMessage.init(std.testing.allocator, input);
+        defer msg.deinit();
+
+        try std.testing.expect(try msg.getSessionId() == null);
+        try std.testing.expectEqual(1, try msg.getId());
+    }
+}

src/cdp/network.zig

@@ -22,7 +22,7 @@ const server = @import("../server.zig");
 const Ctx = server.Ctx;
 const cdp = @import("cdp.zig");
 const result = cdp.result;
-const getMsg = cdp.getMsg;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 
 const log = std.log.scoped(.cdp);
 
@@ -33,45 +33,40 @@ const Methods = enum {
 
 pub fn network(
     alloc: std.mem.Allocator,
-    id: ?u16,
+    msg: *IncomingMessage,
     action: []const u8,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
     const method = std.meta.stringToEnum(Methods, action) orelse
         return error.UnknownMethod;
 
     return switch (method) {
-        .enable => enable(alloc, id, scanner, ctx),
+        .enable => enable(alloc, msg, ctx),
-        .setCacheDisabled => setCacheDisabled(alloc, id, scanner, ctx),
+        .setCacheDisabled => setCacheDisabled(alloc, msg, ctx),
     };
 }
 
 fn enable(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
-    const msg = try getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "network.enable" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "network.enable" });
 
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }
 
 // TODO: noop method
 fn setCacheDisabled(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
-    const msg = try getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "network.setCacheDisabled" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "network.setCacheDisabled" });
 
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }

src/cdp/page.zig

@@ -22,9 +22,9 @@ const server = @import("../server.zig");
 const Ctx = server.Ctx;
 const cdp = @import("cdp.zig");
 const result = cdp.result;
-const getMsg = cdp.getMsg;
 const stringify = cdp.stringify;
 const sendEvent = cdp.sendEvent;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 
 const log = std.log.scoped(.cdp);
 
@@ -41,35 +41,32 @@ const Methods = enum {
 
 pub fn page(
     alloc: std.mem.Allocator,
-    id: ?u16,
+    msg: *IncomingMessage,
     action: []const u8,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
     const method = std.meta.stringToEnum(Methods, action) orelse
         return error.UnknownMethod;
     return switch (method) {
-        .enable => enable(alloc, id, scanner, ctx),
+        .enable => enable(alloc, msg, ctx),
-        .getFrameTree => getFrameTree(alloc, id, scanner, ctx),
+        .getFrameTree => getFrameTree(alloc, msg, ctx),
-        .setLifecycleEventsEnabled => setLifecycleEventsEnabled(alloc, id, scanner, ctx),
+        .setLifecycleEventsEnabled => setLifecycleEventsEnabled(alloc, msg, ctx),
-        .addScriptToEvaluateOnNewDocument => addScriptToEvaluateOnNewDocument(alloc, id, scanner, ctx),
+        .addScriptToEvaluateOnNewDocument => addScriptToEvaluateOnNewDocument(alloc, msg, ctx),
-        .createIsolatedWorld => createIsolatedWorld(alloc, id, scanner, ctx),
+        .createIsolatedWorld => createIsolatedWorld(alloc, msg, ctx),
-        .navigate => navigate(alloc, id, scanner, ctx),
+        .navigate => navigate(alloc, msg, ctx),
     };
 }
 
 fn enable(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
-    const msg = try getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "page.enable" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "page.enable" });
 
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }
 
 const Frame = struct {
@@ -89,14 +86,12 @@ const Frame = struct {
 
 fn getFrameTree(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
-    const msg = try cdp.getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "page.getFrameTree" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "page.getFrameTree" });
 
     // output
     const FrameTree = struct {
@@ -135,27 +130,25 @@ fn getFrameTree(
             },
         },
     };
-    return result(alloc, msg.id, FrameTree, frameTree, msg.sessionID);
+    return result(alloc, input.id, FrameTree, frameTree, input.sessionId);
 }
 
 fn setLifecycleEventsEnabled(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         enabled: bool,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "page.setLifecycleEventsEnabled" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "page.setLifecycleEventsEnabled" });
 
     ctx.state.page_life_cycle_events = true;
 
     // output
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }
 
 const LifecycleEvent = struct {
@@ -168,11 +161,9 @@ const LifecycleEvent = struct {
 // TODO: hard coded method
 fn addScriptToEvaluateOnNewDocument(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         source: []const u8,
@@ -180,8 +171,8 @@ fn addScriptToEvaluateOnNewDocument(
         includeCommandLineAPI: bool = false,
         runImmediately: bool = false,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "page.addScriptToEvaluateOnNewDocument" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "page.addScriptToEvaluateOnNewDocument" });
 
     // output
     const Res = struct {
@@ -199,27 +190,24 @@ fn addScriptToEvaluateOnNewDocument(
             try writer.writeAll(" }");
         }
     };
-    return result(alloc, msg.id, Res, Res{}, msg.sessionID);
+    return result(alloc, input.id, Res, Res{}, input.sessionId);
 }
 
 // TODO: hard coded method
 fn createIsolatedWorld(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         frameId: []const u8,
         worldName: []const u8,
         grantUniveralAccess: bool,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    std.debug.assert(msg.sessionID != null);
+    std.debug.assert(input.sessionId != null);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "page.createIsolatedWorld" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "page.createIsolatedWorld" });
-    const params = msg.params.?;
 
     // noop executionContextCreated event
     try Runtime.executionContextCreated(
@@ -227,15 +215,15 @@ fn createIsolatedWorld(
         ctx,
         0,
         "",
-        params.worldName,
+        input.params.worldName,
         // TODO: hard coded ID
         "7102379147004877974.3265385113993241162",
         .{
             .isDefault = false,
             .type = "isolated",
-            .frameId = params.frameId,
+            .frameId = input.params.frameId,
         },
-        msg.sessionID,
+        input.sessionId,
     );
 
     // output
@@ -243,16 +231,14 @@ fn createIsolatedWorld(
         executionContextId: u8 = 0,
     };
 
-    return result(alloc, msg.id, Resp, .{}, msg.sessionID);
+    return result(alloc, input.id, Resp, .{}, input.sessionId);
 }
 
 fn navigate(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         url: []const u8,
@@ -261,13 +247,12 @@ fn navigate(
         frameId: ?[]const u8 = null,
         referrerPolicy: ?[]const u8 = null, // TODO: enum
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    std.debug.assert(msg.sessionID != null);
+    std.debug.assert(input.sessionId != null);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "page.navigate" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "page.navigate" });
-    const params = msg.params.?;
 
     // change state
-    ctx.state.url = params.url;
+    ctx.state.url = input.params.url;
     // TODO: hard coded ID
     ctx.state.loaderID = "AF8667A203C5392DBE9AC290044AA4C2";
 
@@ -289,7 +274,7 @@ fn navigate(
         "Page.frameStartedLoading",
         FrameStartedLoading,
         frame_started_loading,
-        msg.sessionID,
+        input.sessionId,
     );
     if (ctx.state.page_life_cycle_events) {
         life_event.name = "init";
@@ -300,7 +285,7 @@ fn navigate(
             "Page.lifecycleEvent",
             LifecycleEvent,
             life_event,
-            msg.sessionID,
+            input.sessionId,
         );
     }
 
@@ -330,14 +315,14 @@ fn navigate(
         .frameId = ctx.state.frameID,
         .loaderId = ctx.state.loaderID,
     };
-    const res = try result(alloc, msg.id, Resp, resp, msg.sessionID);
+    const res = try result(alloc, input.id, Resp, resp, input.sessionId);
     try server.sendAsync(ctx, res);
 
     // TODO: at this point do we need async the following actions to be async?
 
     // Send Runtime.executionContextsCleared event
     // TODO: noop event, we have no env context at this point, is it necesarry?
-    try sendEvent(alloc, ctx, "Runtime.executionContextsCleared", void, {}, msg.sessionID);
+    try sendEvent(alloc, ctx, "Runtime.executionContextsCleared", void, {}, input.sessionId);
 
     // Launch navigate
     const p = try ctx.browser.session.createPage();
@@ -349,7 +334,7 @@ fn navigate(
         .{ctx.state.frameID},
     );
     defer alloc.free(auxData);
-    try p.navigate(params.url, auxData);
+    try p.navigate(input.params.url, auxData);
 
     // Events
 
@@ -364,7 +349,7 @@ fn navigate(
             "Page.lifecycleEvent",
             LifecycleEvent,
             life_event,
-            msg.sessionID,
+            input.sessionId,
         );
     }
 
@@ -388,7 +373,7 @@ fn navigate(
         "Page.frameNavigated",
         FrameNavigated,
         frame_navigated,
-        msg.sessionID,
+        input.sessionId,
     );
 
     // domContentEventFired event
@@ -400,7 +385,7 @@ fn navigate(
         "Page.domContentEventFired",
         cdp.TimestampEvent,
         ts_event,
-        msg.sessionID,
+        input.sessionId,
     );
 
     // lifecycle DOMContentLoaded event
@@ -414,7 +399,7 @@ fn navigate(
             "Page.lifecycleEvent",
             LifecycleEvent,
             life_event,
-            msg.sessionID,
+            input.sessionId,
         );
     }
 
@@ -427,7 +412,7 @@ fn navigate(
         "Page.loadEventFired",
         cdp.TimestampEvent,
         ts_event,
-        msg.sessionID,
+        input.sessionId,
     );
 
     // lifecycle DOMContentLoaded event
@@ -441,7 +426,7 @@ fn navigate(
             "Page.lifecycleEvent",
             LifecycleEvent,
             life_event,
-            msg.sessionID,
+            input.sessionId,
         );
     }
 
@@ -453,7 +438,7 @@ fn navigate(
         "Page.frameStoppedLoading",
         FrameStoppedLoading,
         .{ .frameId = ctx.state.frameID },
-        msg.sessionID,
+        input.sessionId,
     );
 
     return "";

src/cdp/performance.zig

@@ -22,7 +22,7 @@ const server = @import("../server.zig");
 const Ctx = server.Ctx;
 const cdp = @import("cdp.zig");
 const result = cdp.result;
-const getMsg = cdp.getMsg;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 
 const log = std.log.scoped(.cdp);
 
@@ -32,29 +32,26 @@ const Methods = enum {
 
 pub fn performance(
     alloc: std.mem.Allocator,
-    id: ?u16,
+    msg: *IncomingMessage,
     action: []const u8,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
     const method = std.meta.stringToEnum(Methods, action) orelse
         return error.UnknownMethod;
 
     return switch (method) {
-        .enable => enable(alloc, id, scanner, ctx),
+        .enable => enable(alloc, msg, ctx),
     };
 }
 
 fn enable(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
-    const msg = try getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "performance.enable" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "performance.enable" });
 
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }

src/cdp/runtime.zig

@@ -25,7 +25,7 @@ const server = @import("../server.zig");
 const Ctx = server.Ctx;
 const cdp = @import("cdp.zig");
 const result = cdp.result;
-const getMsg = cdp.getMsg;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 const stringify = cdp.stringify;
 
 const log = std.log.scoped(.cdp);
@@ -41,27 +41,23 @@ const Methods = enum {
 
 pub fn runtime(
     alloc: std.mem.Allocator,
-    id: ?u16,
+    msg: *IncomingMessage,
     action: []const u8,
-    scanner: *std.json.Scanner,
-    s: []const u8,
     ctx: *Ctx,
 ) ![]const u8 {
     const method = std.meta.stringToEnum(Methods, action) orelse
         // NOTE: we could send it anyway to the JS runtime but it's good to check it
         return error.UnknownMethod;
     return switch (method) {
-        .runIfWaitingForDebugger => runIfWaitingForDebugger(alloc, id, scanner, ctx),
+        .runIfWaitingForDebugger => runIfWaitingForDebugger(alloc, msg, ctx),
-        else => sendInspector(alloc, method, id, s, scanner, ctx),
+        else => sendInspector(alloc, method, msg, ctx),
     };
 }
 
 fn sendInspector(
     alloc: std.mem.Allocator,
     method: Methods,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    s: []const u8,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
 
@@ -69,8 +65,8 @@ fn sendInspector(
     if (std.log.defaultLogEnabled(.debug)) {
 
         // input
-        var script: ?[]const u8 = null;
         var id: u16 = undefined;
+        var script: ?[]const u8 = null;
 
         if (method == .evaluate) {
             const Params = struct {
@@ -81,11 +77,10 @@ fn sendInspector(
                 userGesture: ?bool = null,
             };
 
-            const msg = try getMsg(alloc, _id, Params, scanner);
+            const input = try msg.getInput(alloc, Params);
-            log.debug("Req > id {d}, method {s} (script saved on cache)", .{ msg.id, "runtime.evaluate" });
+            log.debug("Req > id {d}, method {s} (script saved on cache)", .{ input.id, "runtime.evaluate" });
-            const params = msg.params.?;
+            script = input.params.expression;
-            script = params.expression;
+            id = input.id;
-            id = msg.id;
         } else if (method == .callFunctionOn) {
             const Params = struct {
                 functionDeclaration: []const u8,
@@ -100,11 +95,10 @@ fn sendInspector(
                 userGesture: ?bool = null,
             };
 
-            const msg = try getMsg(alloc, _id, Params, scanner);
+            const input = try msg.getInput(alloc, Params);
-            log.debug("Req > id {d}, method {s} (script saved on cache)", .{ msg.id, "runtime.callFunctionOn" });
+            log.debug("Req > id {d}, method {s} (script saved on cache)", .{ input.id, "runtime.callFunctionOn" });
-            const params = msg.params.?;
+            script = input.params.functionDeclaration;
-            script = params.functionDeclaration;
+            id = input.id;
-            id = msg.id;
         }
 
         if (script) |src| {
@@ -115,12 +109,12 @@ fn sendInspector(
     // remove awaitPromise true params
     // TODO: delete when Promise are correctly handled by zig-js-runtime
     if (method == .callFunctionOn or method == .evaluate) {
-        const buf = try alloc.alloc(u8, s.len + 1);
+        const buf = try alloc.alloc(u8, msg.json.len + 1);
         defer alloc.free(buf);
-        _ = std.mem.replace(u8, s, "\"awaitPromise\":true", "\"awaitPromise\":false", buf);
+        _ = std.mem.replace(u8, msg.json, "\"awaitPromise\":true", "\"awaitPromise\":false", buf);
         ctx.sendInspector(buf);
     } else {
-        ctx.sendInspector(s);
+        ctx.sendInspector(msg.json);
     }
     return "";
 }
@@ -166,14 +160,11 @@ pub fn executionContextCreated(
 // should we be passing this also to the JS Inspector?
 fn runIfWaitingForDebugger(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
+    const input = try msg.getInput(alloc, void);
+    log.debug("Req > id {d}, method {s}", .{ input.id, "runtime.runIfWaitingForDebugger" });
 
-    // input
+    return result(alloc, input.id, null, null, input.sessionId);
-    const msg = try getMsg(alloc, _id, void, scanner);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "runtime.runIfWaitingForDebugger" });
-
-    return result(alloc, msg.id, null, null, msg.sessionID);
 }

src/cdp/target.zig

@@ -22,8 +22,8 @@ const server = @import("../server.zig");
 const Ctx = server.Ctx;
 const cdp = @import("cdp.zig");
 const result = cdp.result;
-const getMsg = cdp.getMsg;
 const stringify = cdp.stringify;
+const IncomingMessage = @import("msg.zig").IncomingMessage;
 
 const log = std.log.scoped(.cdp);
 
@@ -41,23 +41,22 @@ const Methods = enum {
 
 pub fn target(
     alloc: std.mem.Allocator,
-    id: ?u16,
+    msg: *IncomingMessage,
     action: []const u8,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
     const method = std.meta.stringToEnum(Methods, action) orelse
         return error.UnknownMethod;
     return switch (method) {
-        .setDiscoverTargets => setDiscoverTargets(alloc, id, scanner, ctx),
+        .setDiscoverTargets => setDiscoverTargets(alloc, msg, ctx),
-        .setAutoAttach => setAutoAttach(alloc, id, scanner, ctx),
+        .setAutoAttach => setAutoAttach(alloc, msg, ctx),
-        .attachToTarget => attachToTarget(alloc, id, scanner, ctx),
+        .attachToTarget => attachToTarget(alloc, msg, ctx),
-        .getTargetInfo => getTargetInfo(alloc, id, scanner, ctx),
+        .getTargetInfo => getTargetInfo(alloc, msg, ctx),
-        .getBrowserContexts => getBrowserContexts(alloc, id, scanner, ctx),
+        .getBrowserContexts => getBrowserContexts(alloc, msg, ctx),
-        .createBrowserContext => createBrowserContext(alloc, id, scanner, ctx),
+        .createBrowserContext => createBrowserContext(alloc, msg, ctx),
-        .disposeBrowserContext => disposeBrowserContext(alloc, id, scanner, ctx),
+        .disposeBrowserContext => disposeBrowserContext(alloc, msg, ctx),
-        .createTarget => createTarget(alloc, id, scanner, ctx),
+        .createTarget => createTarget(alloc, msg, ctx),
-        .closeTarget => closeTarget(alloc, id, scanner, ctx),
+        .closeTarget => closeTarget(alloc, msg, ctx),
     };
 }
 
@@ -69,17 +68,15 @@ const BrowserContextID = "65618675CB7D3585A95049E9DFE95EA9";
 // TODO: noop method
 fn setDiscoverTargets(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
-    const msg = try getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "target.setDiscoverTargets" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "target.setDiscoverTargets" });
 
     // output
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }
 
 const AttachToTarget = struct {
@@ -104,11 +101,9 @@ const TargetFilter = struct {
 // TODO: noop method
 fn setAutoAttach(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         autoAttach: bool,
@@ -116,11 +111,11 @@ fn setAutoAttach(
         flatten: bool = true,
         filter: ?[]TargetFilter = null,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "target.setAutoAttach" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "target.setAutoAttach" });
 
     // attachedToTarget event
-    if (msg.sessionID == null) {
+    if (input.sessionId == null) {
         const attached = AttachToTarget{
             .sessionId = cdp.BrowserSessionID,
             .targetInfo = .{
@@ -134,14 +129,13 @@ fn setAutoAttach(
     }
 
     // output
-    return result(alloc, msg.id, null, null, msg.sessionID);
+    return result(alloc, input.id, null, null, input.sessionId);
 }
 
 // TODO: noop method
 fn attachToTarget(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
 
@@ -150,11 +144,11 @@ fn attachToTarget(
         targetId: []const u8,
         flatten: bool = true,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "target.setAutoAttach" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "target.attachToTarget" });
 
     // attachedToTarget event
-    if (msg.sessionID == null) {
+    if (input.sessionId == null) {
         const attached = AttachToTarget{
             .sessionId = cdp.BrowserSessionID,
             .targetInfo = .{
@@ -172,24 +166,22 @@ fn attachToTarget(
         sessionId: []const u8,
     };
     const output = SessionId{
-        .sessionId = msg.sessionID orelse BrowserContextID,
+        .sessionId = input.sessionId orelse BrowserContextID,
     };
-    return result(alloc, msg.id, SessionId, output, null);
+    return result(alloc, input.id, SessionId, output, null);
 }
 
 fn getTargetInfo(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     _: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         targetId: ?[]const u8 = null,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "target.getTargetInfo" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "target.getTargetInfo" });
 
     // output
     const TargetInfo = struct {
@@ -208,7 +200,7 @@ fn getTargetInfo(
         .targetId = BrowserTargetID,
         .type = "browser",
     };
-    return result(alloc, msg.id, TargetInfo, targetInfo, null);
+    return result(alloc, input.id, TargetInfo, targetInfo, null);
 }
 
 // Browser context are not handled and not in the roadmap for now
@@ -217,14 +209,12 @@ fn getTargetInfo(
 // TODO: noop method
 fn getBrowserContexts(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
-    const msg = try getMsg(alloc, _id, void, scanner);
+    const input = try msg.getInput(alloc, void);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "target.getBrowserContexts" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "target.getBrowserContexts" });
 
     // ouptut
     const Resp = struct {
@@ -238,7 +228,7 @@ fn getBrowserContexts(
         const contextIDs = [0][]const u8{};
         resp = .{ .browserContextIds = &contextIDs };
     }
-    return result(alloc, msg.id, Resp, resp, null);
+    return result(alloc, input.id, Resp, resp, null);
 }
 
 const ContextID = "22648B09EDCCDD11109E2D4FEFBE4F89";
@@ -246,11 +236,9 @@ const ContextID = "22648B09EDCCDD11109E2D4FEFBE4F89";
 // TODO: noop method
 fn createBrowserContext(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         disposeOnDetach: bool = false,
@@ -258,8 +246,8 @@ fn createBrowserContext(
         proxyBypassList: ?[]const u8 = null,
         originsWithUniversalNetworkAccess: ?[][]const u8 = null,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "target.createBrowserContext" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "target.createBrowserContext" });
 
     ctx.state.contextID = ContextID;
 
@@ -279,25 +267,23 @@ fn createBrowserContext(
             try writer.writeAll(" }");
         }
     };
-    return result(alloc, msg.id, Resp, Resp{}, msg.sessionID);
+    return result(alloc, input.id, Resp, Resp{}, input.sessionId);
 }
 
 fn disposeBrowserContext(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         browserContextId: []const u8,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "target.disposeBrowserContext" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "target.disposeBrowserContext" });
 
     // output
-    const res = try result(alloc, msg.id, null, .{}, null);
+    const res = try result(alloc, input.id, null, .{}, null);
     try server.sendAsync(ctx, res);
 
     return error.DisposeBrowserContext;
@@ -309,11 +295,9 @@ const LoaderID = "DD4A76F842AA389647D702B4D805F49A";
 
 fn createTarget(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         url: []const u8,
@@ -325,8 +309,8 @@ fn createTarget(
         background: bool = false,
         forTab: ?bool = null,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "target.createTarget" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "target.createTarget" });
 
     // change CDP state
     ctx.state.frameID = TargetID;
@@ -346,7 +330,7 @@ fn createTarget(
         },
         .waitingForDebugger = true,
     };
-    try cdp.sendEvent(alloc, ctx, "Target.attachedToTarget", AttachToTarget, attached, msg.sessionID);
+    try cdp.sendEvent(alloc, ctx, "Target.attachedToTarget", AttachToTarget, attached, input.sessionId);
 
     // output
     const Resp = struct {
@@ -364,28 +348,26 @@ fn createTarget(
             try writer.writeAll(" }");
         }
     };
-    return result(alloc, msg.id, Resp, Resp{}, msg.sessionID);
+    return result(alloc, input.id, Resp, Resp{}, input.sessionId);
 }
 
 fn closeTarget(
     alloc: std.mem.Allocator,
-    _id: ?u16,
+    msg: *IncomingMessage,
-    scanner: *std.json.Scanner,
     ctx: *Ctx,
 ) ![]const u8 {
-
     // input
     const Params = struct {
         targetId: []const u8,
     };
-    const msg = try getMsg(alloc, _id, Params, scanner);
+    const input = try msg.getInput(alloc, Params);
-    log.debug("Req > id {d}, method {s}", .{ msg.id, "target.closeTarget" });
+    log.debug("Req > id {d}, method {s}", .{ input.id, "target.closeTarget" });
 
     // output
     const Resp = struct {
         success: bool = true,
     };
-    const res = try result(alloc, msg.id, Resp, Resp{}, null);
+    const res = try result(alloc, input.id, Resp, Resp{}, null);
     try server.sendAsync(ctx, res);
 
     // Inspector.detached event
@@ -398,7 +380,7 @@ fn closeTarget(
         "Inspector.detached",
         InspectorDetached,
         .{},
-        msg.sessionID orelse cdp.ContextSessionID,
+        input.sessionId orelse cdp.ContextSessionID,
     );
 
     // detachedFromTarget event
@@ -412,8 +394,8 @@ fn closeTarget(
         "Target.detachedFromTarget",
         TargetDetached,
         .{
-            .sessionId = msg.sessionID orelse cdp.ContextSessionID,
+            .sessionId = input.sessionId orelse cdp.ContextSessionID,
-            .targetId = msg.params.?.targetId,
+            .targetId = input.params.targetId,
         },
         null,
     );