internal/poll: better panic message for lock overflow

Instead of "inconsistent poll.fdMutex", panic with
"too many concurrent operations on a single file or socket (max 1048575)".

Fixes #25558

Change-Id: I5cad3633aa539fb6f48cca236c6656c86acfb663
Reviewed-on: https://go-review.googlesource.com/119956
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/internal/poll/fd_mutex.go

@@ -34,6 +34,8 @@ const (
 	mutexWMask   = (1<<20 - 1) << 43
 )
 
+const overflowMsg = "too many concurrent operations on a single file or socket (max 1048575)"
+
 // Read operations must do rwlock(true)/rwunlock(true).
 //
 // Write operations must do rwlock(false)/rwunlock(false).
@@ -56,7 +58,7 @@ func (mu *fdMutex) incref() bool {
 		}
 		new := old + mutexRef
 		if new&mutexRefMask == 0 {
-			panic("inconsistent poll.fdMutex")
+			panic(overflowMsg)
 		}
 		if atomic.CompareAndSwapUint64(&mu.state, old, new) {
 			return true
@@ -75,7 +77,7 @@ func (mu *fdMutex) increfAndClose() bool {
 		// Mark as closed and acquire a reference.
 		new := (old | mutexClosed) + mutexRef
 		if new&mutexRefMask == 0 {
-			panic("inconsistent poll.fdMutex")
+			panic(overflowMsg)
 		}
 		// Remove all read and write waiters.
 		new &^= mutexRMask | mutexWMask
@@ -136,13 +138,13 @@ func (mu *fdMutex) rwlock(read bool) bool {
 			// Lock is free, acquire it.
 			new = (old | mutexBit) + mutexRef
 			if new&mutexRefMask == 0 {
-				panic("inconsistent poll.fdMutex")
+				panic(overflowMsg)
 			}
 		} else {
 			// Wait for lock.
 			new = old + mutexWait
 			if new&mutexMask == 0 {
-				panic("inconsistent poll.fdMutex")
+				panic(overflowMsg)
 			}
 		}
 		if atomic.CompareAndSwapUint64(&mu.state, old, new) {

src/internal/poll/fd_mutex_test.go

@@ -8,6 +8,7 @@ import (
 	. "internal/poll"
 	"math/rand"
 	"runtime"
+	"strings"
 	"testing"
 	"time"
 )
@@ -121,6 +122,27 @@ func TestMutexPanic(t *testing.T) {
 	mu.RWUnlock(false)
 }
 
+func TestMutexOverflowPanic(t *testing.T) {
+	defer func() {
+		r := recover()
+		if r == nil {
+			t.Fatal("did not panic")
+		}
+		msg, ok := r.(string)
+		if !ok {
+			t.Fatalf("unexpected panic type %T", r)
+		}
+		if !strings.Contains(msg, "too many") || strings.Contains(msg, "inconsistent") {
+			t.Fatalf("wrong panic message %q", msg)
+		}
+	}()
+
+	var mu1 FDMutex
+	for i := 0; i < 1<<21; i++ {
+		mu1.Incref()
+	}
+}
+
 func TestMutexStress(t *testing.T) {
 	P := 8
 	N := int(1e6)