cdp: handle nullable Type for params

src/cdp/browser.zig

@@ -112,7 +112,7 @@ fn getWindowForTarget(
     const Params = struct {
         targetId: ?[]const u8 = null,
     };
-    const input = try Input(Params).get(alloc, msg);
+    const input = try Input(?Params).get(alloc, msg);
     defer input.deinit();
     std.debug.assert(input.sessionId != null);
     log.debug("Req > id {d}, method {s}", .{ input.id, "browser.getWindowForTarget" });

src/cdp/msg.zig

@@ -146,7 +146,15 @@ pub const IncomingMessage = struct {
             return error.SkippedParams;
         }
 
-        try self.scanUntil("params");
+        self.scanUntil("params") catch |err| {
+            // handle nullable type
+            if (@typeInfo(T) == .Optional) {
+                if (err == error.InvalidToken or err == error.EndOfDocument) {
+                    return null;
+                }
+            }
+            return err;
+        };
 
         // parse "params"
         const options = std.json.ParseOptions{
@@ -250,3 +258,34 @@ test "read incoming message with null session id" {
         try std.testing.expectEqual(1, try msg.getId());
     }
 }
+
+test "message with nullable params" {
+    const T = struct {
+        bar: []const u8,
+    };
+
+    // nullable type, params is present => value
+    const not_null =
+        \\{"id": 1,"method":"foo","params":{"bar":"baz"}}
+    ;
+    var msg = IncomingMessage.init(std.testing.allocator, not_null);
+    defer msg.deinit();
+    const input = try Input(?T).get(std.testing.allocator, &msg);
+    defer input.deinit();
+    try std.testing.expectEqualStrings(input.params.?.bar, "baz");
+
+    // nullable type, params is not present => null
+    const is_null =
+        \\{"id": 1,"method":"foo","sessionId":"AAA"}
+    ;
+    var msg_null = IncomingMessage.init(std.testing.allocator, is_null);
+    defer msg_null.deinit();
+    const input_null = try Input(?T).get(std.testing.allocator, &msg_null);
+    defer input_null.deinit();
+    try std.testing.expectEqual(null, input_null.params);
+    try std.testing.expectEqualStrings("AAA", input_null.sessionId.?);
+
+    // not nullable type, params is not present => error
+    const params_or_error = msg_null.getParams(std.testing.allocator, T);
+    try std.testing.expectError(error.EndOfDocument, params_or_error);
+}

src/cdp/target.zig

@@ -184,7 +184,7 @@ fn getTargetInfo(
     const Params = struct {
         targetId: ?[]const u8 = null,
     };
-    const input = try Input(Params).get(alloc, msg);
+    const input = try Input(?Params).get(alloc, msg);
     defer input.deinit();
     log.debug("Req > id {d}, method {s}", .{ input.id, "target.getTargetInfo" });