MsgBuffer to handle both combined and multipart read

Signed-off-by: Francis Bouvier <francis@lightpanda.io>

src/server.zig

@@ -18,6 +18,8 @@ const Error = IOError || std.fmt.ParseIntError || cdp.Error || NoError;
 // I/O Recv
 // --------
 
+const BufReadSize = 1024;
+
 pub const Cmd = struct {
 
     // internal fields
@@ -25,6 +27,8 @@ pub const Cmd = struct {
     buf: []u8, // only for read operations
     err: ?Error = null,
 
+    msg_buf: *MsgBuffer,
+
     // CDP
     state: cdp.State = .{},
 
@@ -57,7 +61,7 @@ pub const Cmd = struct {
         }
 
         // read and execute input
-        readInput(input, Cmd.do, self) catch unreachable;
+        self.msg_buf.read(self.alloc(), input, self, Cmd.do) catch unreachable;
 
         // continue receving incomming messages asynchronously
         self.loop().io.recv(*Cmd, self, cbk, completion, self.socket, self.buf);
@@ -74,7 +78,7 @@ pub const Cmd = struct {
         return self.browser.currentSession().loop;
     }
 
-    fn do(self: *Cmd, cmd: []const u8) !void {
+    fn do(self: *Cmd, cmd: []const u8) anyerror!void {
         const res = try cdp.do(self.alloc(), cmd, self);
 
         // send result
@@ -85,60 +89,153 @@ pub const Cmd = struct {
     }
 };
 
-fn readInput(buf: []const u8, func: anytype, data: anytype) !void {
-    var input = buf;
+/// MsgBuffer return messages from a raw text read stream,
+/// according to the following format `<msg_size>:<msg>`.
+/// It handles both:
+/// - combined messages in one read
+/// - single message in several read (multipart)
+/// It is safe (and good practice) to reuse the same MsgBuffer
+/// on several reads of the same stream.
+const MsgBuffer = struct {
+    size: usize = 0,
+    buf: []u8,
+    pos: usize = 0,
+
+    fn init(alloc: std.mem.Allocator, size: usize) std.mem.Allocator.Error!MsgBuffer {
+        const buf = try alloc.alloc(u8, size);
+        return .{ .buf = buf };
+    }
+
+    fn deinit(self: MsgBuffer, alloc: std.mem.Allocator) void {
+        alloc.free(self.buf);
+    }
+
+    fn isFinished(self: *MsgBuffer) bool {
+        return self.pos >= self.size;
+    }
+
+    fn isEmpty(self: MsgBuffer) bool {
+        return self.size == 0 and self.pos == 0;
+    }
+
+    fn reset(self: *MsgBuffer) void {
+        self.size = 0;
+        self.pos = 0;
+    }
+
+    // read input
+    // - `do_func` is a callback to execute on each message of the input
+    // - `data` is a arbitrary payload that will be passed to the callback along with
+    // the message itself
+    fn read(
+        self: *MsgBuffer,
+        alloc: std.mem.Allocator,
+        input: []const u8,
+        data: anytype,
+        comptime do_func: fn (data: @TypeOf(data), msg: []const u8) anyerror!void,
+    ) !void {
+        var _input = input; // make input writable
 
         while (true) {
-        var cmd: []const u8 = undefined;
+            var msg: []const u8 = undefined;
 
-        // size msg
-        var size_msg: usize = undefined;
+            // msg size
+            var msg_size: usize = undefined;
+            if (self.isEmpty()) {
+                // parse msg size metadata
+                const size_pos = std.mem.indexOfScalar(u8, _input, ':').?;
+                const size_str = _input[0..size_pos];
+                msg_size = try std.fmt.parseInt(u32, size_str, 10);
+                _input = _input[size_pos + 1 ..];
+            } else {
+                msg_size = self.size;
+            }
 
-        // parse json msg size
-        const size_pos = std.mem.indexOfScalar(u8, input, ':').?;
-        std.log.debug("msg size pos: {d}", .{size_pos});
-        const size_str = input[0..size_pos];
-        input = input[size_pos + 1 ..];
-        size_msg = try std.fmt.parseInt(u32, size_str, 10);
-        // }
-        std.log.debug("msg size: {d}", .{size_msg});
+            // multipart
+            const is_multipart = !self.isEmpty() or _input.len < msg_size;
+            if (is_multipart) {
+
+                // set msg size on empty MsgBuffer
+                if (self.isEmpty()) {
+                    self.size = msg_size;
+                }
+
+                // get the new position of the cursor
+                const new_pos = self.pos + _input.len;
+
+                // check if the current input can fit in MsgBuffer
+                if (new_pos > self.buf.len) {
+                    // max_size is the max between msg size and current new cursor position
+                    const max_size = @max(self.size, new_pos);
+                    // resize the MsgBuffer to fit
+                    self.buf = try alloc.realloc(self.buf, max_size);
+                }
+
+                // copy the current input into MsgBuffer
+                @memcpy(self.buf[self.pos..new_pos], _input[0..]);
+
+                // set the new cursor position
+                self.pos = new_pos;
+
+                // if multipart is not finished, go fetch the next input
+                if (!self.isFinished()) return;
+
+                // otherwhise multipart is finished, use its buffer as input
+                _input = self.buf[0..self.pos];
+                self.reset();
+            }
 
             // handle several JSON msg in 1 read
-        const is_multi = input.len > size_msg;
-        std.log.debug("is_multi: {any}", .{is_multi});
-        cmd = input[0..size_msg];
-        std.log.debug("cmd: {s}", .{cmd[0..@min(BufReadSize, size_msg)]});
-        if (is_multi) {
-            input = input[size_msg..];
-            std.log.debug("rest: {s}", .{input});
+            const is_combined = _input.len > msg_size;
+            msg = _input[0..msg_size];
+            std.log.debug("msg: {s}", .{msg[0..@min(BufReadSize, msg_size)]});
+            if (is_combined) {
+                _input = _input[msg_size..];
             }
 
-        try @call(.auto, func, .{ data, cmd });
+            try @call(.auto, do_func, .{ data, msg });
 
-        if (!is_multi) break;
-
-        // TODO: handle 1 read smaller than a complete JSON msg
+            if (!is_combined) break;
         }
     }
+};
 
 fn doTest(nb: *u8, _: []const u8) anyerror!void {
     nb.* += 1;
 }
 
-test {
+test "MsgBuffer" {
     const Case = struct {
         input: []const u8,
         nb: u8,
     };
+    const alloc = std.testing.allocator;
     const cases = [_]Case{
         // simple
         .{ .input = "2:ok", .nb = 1 },
-        // multi
+        // combined
         .{ .input = "2:ok3:foo7:bar2:ok", .nb = 3 }, // "bar2:ok" is a message, no need to escape "2:" here
+        // multipart
+        .{ .input = "9:multi", .nb = 0 },
+        .{ .input = "part", .nb = 1 },
+        // multipart & combined
+        .{ .input = "9:multi", .nb = 0 },
+        .{ .input = "part2:ok", .nb = 2 },
+        // several multipart
+        .{ .input = "23:multi", .nb = 0 },
+        .{ .input = "several", .nb = 0 },
+        .{ .input = "complex", .nb = 0 },
+        .{ .input = "part", .nb = 1 },
+        // combined & multipart
+        .{ .input = "2:ok9:multi", .nb = 1 },
+        .{ .input = "part", .nb = 1 },
     };
+    var nb: u8 = undefined;
+    var msg_buf = try MsgBuffer.init(alloc, 10);
+    defer msg_buf.deinit(alloc);
     for (cases) |case| {
-        var nb: u8 = 0;
-        try readInput(case.input, doTest, &nb);
+        nb = 0;
+        try msg_buf.read(alloc, case.input, &nb, doTest);
         try std.testing.expect(nb == case.nb);
     }
 }
@@ -232,14 +329,20 @@ const Accept = struct {
 // ------
 
 pub fn listen(browser: *Browser, socket: std.os.socket_t) anyerror!void {
+    const loop = browser.currentSession().loop;
+
+    // MsgBuffer
+    var msg_buf = try MsgBuffer.init(loop.alloc, BufReadSize);
+    defer msg_buf.deinit(loop.alloc);
 
     // create I/O contexts and callbacks
     // for accepting connections and receving messages
-    var input: [1024]u8 = undefined;
+    var ctxInput: [BufReadSize]u8 = undefined;
     var cmd = Cmd{
         .browser = browser,
         .socket = undefined,
-        .buf = &input,
+        .buf = &ctxInput,
+        .msg_buf = &msg_buf,
     };
     var accept = Accept{
         .cmd = &cmd,
@@ -247,7 +350,6 @@ pub fn listen(browser: *Browser, socket: std.os.socket_t) anyerror!void {
     };
 
     // accepting connection asynchronously on internal server
-    const loop = browser.currentSession().loop;
     var completion: Completion = undefined;
     loop.io.accept(*Accept, &accept, Accept.cbk, &completion, socket);