refactor: add ErrMessageTooBig sentinel error for limited reads (#535)

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

Author: DanielleMaywood

conn_test.go

@@ -421,6 +421,25 @@ func TestConn(t *testing.T) {
 		err = c1.Close(websocket.StatusNormalClosure, "")
 		assert.Success(t, err)
 	})
+
+	t.Run("ReadLimitExceededReturnsErrMessageTooBig", func(t *testing.T) {
+		tt, c1, c2 := newConnTest(t, nil, nil)
+
+		c1.SetReadLimit(1024)
+		_ = c2.CloseRead(tt.ctx)
+
+		writeDone := xsync.Go(func() error {
+			payload := strings.Repeat("x", 4096)
+			return c2.Write(tt.ctx, websocket.MessageText, []byte(payload))
+		})
+
+		_, _, err := c1.Read(tt.ctx)
+		assert.ErrorIs(t, websocket.ErrMessageTooBig, err)
+		assert.Contains(t, err, "read limited at 1025 bytes")
+
+		_ = c2.CloseNow()
+		<-writeDone
+	})
 }
 
 func TestWasm(t *testing.T) {

errors.go

@@ -0,0 +1,8 @@
+package websocket
+
+import (
+	"errors"
+)
+
+// ErrMessageTooBig is returned when a message exceeds the read limit.
+var ErrMessageTooBig = errors.New("websocket: message too big")

read.go

@@ -90,7 +90,8 @@ func (c *Conn) CloseRead(ctx context.Context) context.Context {
 //
 // By default, the connection has a message read limit of 32768 bytes.
 //
-// When the limit is hit, the connection will be closed with StatusMessageTooBig.
+// When the limit is hit, reads return an error wrapping ErrMessageTooBig and
+// the connection is closed with StatusMessageTooBig.
 //
 // Set to -1 to disable.
 func (c *Conn) SetReadLimit(n int64) {
@@ -522,9 +523,9 @@ func (lr *limitReader) Read(p []byte) (int, error) {
 	}
 
 	if lr.n == 0 {
-		err := fmt.Errorf("read limited at %v bytes", lr.limit.Load())
-		lr.c.writeError(StatusMessageTooBig, err)
-		return 0, err
+		reason := fmt.Errorf("read limited at %d bytes", lr.limit.Load())
+		lr.c.writeError(StatusMessageTooBig, reason)
+		return 0, fmt.Errorf("%w: %v", ErrMessageTooBig, reason)
 	}
 
 	if int64(len(p)) > lr.n {

ws_js.go

@@ -144,9 +144,9 @@ func (c *Conn) Read(ctx context.Context) (MessageType, []byte, error) {
 	}
 	readLimit := c.msgReadLimit.Load()
 	if readLimit >= 0 && int64(len(p)) > readLimit {
-		err := fmt.Errorf("read limited at %v bytes", c.msgReadLimit.Load())
-		c.Close(StatusMessageTooBig, err.Error())
-		return 0, nil, err
+		reason := fmt.Errorf("read limited at %d bytes", c.msgReadLimit.Load())
+		c.Close(StatusMessageTooBig, reason.Error())
+		return 0, nil, fmt.Errorf("%w: %v", ErrMessageTooBig, reason)
 	}
 	return typ, p, nil
 }