Add --advertise_host option to serve command

Allows overwriting the --host for the json/version payload. When --host is set
to 0.0.0.0, we want to provide a mechanism to specify the specific address to
connect to in /json/version (or anywhere else that we "advertise" the address).

Inspired by https://github.com/lightpanda-io/browser/pull/1923 but rather than
defaulting to 127.0.0.1 (which seems just as unsafe), adds the explicit config
option.

src/Config.zig

@@ -163,6 +163,20 @@ pub fn cdpTimeout(self: *const Config) usize {
     };
 }
 
+pub fn port(self: *const Config) u16 {
+    return switch (self.mode) {
+        .serve => |opts| opts.port,
+        else => unreachable,
+    };
+}
+
+pub fn advertiseHost(self: *const Config) []const u8 {
+    return switch (self.mode) {
+        .serve => |opts| opts.advertise_host orelse opts.host,
+        else => unreachable,
+    };
+}
+
 pub fn webBotAuth(self: *const Config) ?WebBotAuthConfig {
     return switch (self.mode) {
         inline .serve, .fetch, .mcp => |opts| WebBotAuthConfig{
@@ -199,6 +213,7 @@ pub const Mode = union(RunMode) {
 pub const Serve = struct {
     host: []const u8 = "127.0.0.1",
     port: u16 = 9222,
+    advertise_host: ?[]const u8 = null,
     timeout: u31 = 10,
     cdp_max_connections: u16 = 16,
     cdp_max_pending_connections: u16 = 128,
@@ -416,6 +431,11 @@ pub fn printUsageAndExit(self: *const Config, success: bool) void {
         \\--port          Port of the CDP server
         \\                Defaults to 9222
         \\
+        \\--advertise_host
+        \\                The host to advertise, e.g. in the /json/version response.
+        \\                Useful, for example, when --host is 0.0.0.0.
+        \\                Defaults to --host value
+        \\
         \\--timeout       Inactivity timeout in seconds before disconnecting clients
         \\                Defaults to 10 (seconds). Limited to 604800 (1 week).
         \\
@@ -557,6 +577,15 @@ fn parseServeArgs(
             continue;
         }
 
+        if (std.mem.eql(u8, "--advertise_host", opt)) {
+            const str = args.next() orelse {
+                log.fatal(.app, "missing argument value", .{ .arg = "--advertise_host" });
+                return error.InvalidArgument;
+            };
+            serve.advertise_host = try allocator.dupe(u8, str);
+            continue;
+        }
+
         if (std.mem.eql(u8, "--timeout", opt)) {
             const str = args.next() orelse {
                 log.fatal(.app, "missing argument value", .{ .arg = "--timeout" });

src/Server.zig

@@ -45,7 +45,7 @@ clients_pool: std.heap.MemoryPool(Client),
 
 pub fn init(app: *App, address: net.Address) !*Server {
     const allocator = app.allocator;
-    const json_version_response = try buildJSONVersionResponse(allocator, address);
+    const json_version_response = try buildJSONVersionResponse(app);
     errdefer allocator.free(json_version_response);
 
     const self = try allocator.create(Server);
@@ -484,11 +484,17 @@ pub const Client = struct {
 // --------
 
 fn buildJSONVersionResponse(
-    allocator: Allocator,
+    app: *const App,
-    address: net.Address,
 ) ![]const u8 {
-    const body_format = "{{\"webSocketDebuggerUrl\": \"ws://{f}/\"}}";
+    const port = app.config.port();
-    const body_len = std.fmt.count(body_format, .{address});
+    const host = app.config.advertiseHost();
+    if (std.mem.eql(u8, host, "0.0.0.0")) {
+        log.info(.cdp, "unreachable advertised host", .{
+            .message = "when --host is set to 0.0.0.0 consider setting --advertise_host to a reachable address",
+        });
+    }
+    const body_format = "{{\"webSocketDebuggerUrl\": \"ws://{s}:{d}/\"}}";
+    const body_len = std.fmt.count(body_format, .{host, port});
 
     // We send a Connection: Close (and actually close the connection)
     // because chromedp (Go driver) sends a request to /json/version and then
@@ -502,7 +508,7 @@ fn buildJSONVersionResponse(
         "Connection: Close\r\n" ++
         "Content-Type: application/json; charset=UTF-8\r\n\r\n" ++
         body_format;
-    return try std.fmt.allocPrint(allocator, response_format, .{ body_len, address });
+    return try std.fmt.allocPrint(app.allocator, response_format, .{ body_len, host, port });
 }
 
 pub const timestamp = @import("datetime.zig").timestamp;