fix: return a correct error when failed to access concurrent connecti…

…ons (#332)

Co-authored-by: TremblingV5 <[email protected]>

connection_errors.go

@@ -36,6 +36,8 @@ const (
 	ErrEOF = syscall.Errno(0x106)
 	// Write I/O buffer timeout, calling by Connection.Writer
 	ErrWriteTimeout = syscall.Errno(0x107)
+	// Concurrent connection access error
+	ErrConcurrentAccess = syscall.Errno(0x108)
 )
 
 const ErrnoMask = 0xFF
@@ -110,11 +112,12 @@ func (e *exception) Temporary() bool {
 
 // Errors defined in netpoll
 var errnos = [...]string{
-	ErrnoMask & ErrConnClosed:     "connection has been closed",
-	ErrnoMask & ErrReadTimeout:    "connection read timeout",
-	ErrnoMask & ErrDialTimeout:    "dial wait timeout",
-	ErrnoMask & ErrDialNoDeadline: "dial no deadline",
-	ErrnoMask & ErrUnsupported:    "netpoll dose not support",
-	ErrnoMask & ErrEOF:            "EOF",
-	ErrnoMask & ErrWriteTimeout:   "connection write timeout",
+	ErrnoMask & ErrConnClosed:       "connection has been closed",
+	ErrnoMask & ErrReadTimeout:      "connection read timeout",
+	ErrnoMask & ErrDialTimeout:      "dial wait timeout",
+	ErrnoMask & ErrDialNoDeadline:   "dial no deadline",
+	ErrnoMask & ErrUnsupported:      "netpoll does not support",
+	ErrnoMask & ErrEOF:              "EOF",
+	ErrnoMask & ErrWriteTimeout:     "connection write timeout",
+	ErrnoMask & ErrConcurrentAccess: "concurrent connection access",
 }

connection_impl.go

@@ -220,10 +220,15 @@ func (c *connection) MallocLen() (length int) {
 // If empty, it will call syscall.Write to send data directly,
 // otherwise the buffer will be sent asynchronously by the epoll trigger.
 func (c *connection) Flush() error {
-	if !c.IsActive() || !c.lock(flushing) {
+	if !c.IsActive() {
 		return Exception(ErrConnClosed, "when flush")
 	}
+
+	if !c.lock(flushing) {
+		return Exception(ErrConcurrentAccess, "when flush")
+	}
 	defer c.unlock(flushing)
+
 	c.outputBuffer.Flush()
 	return c.flush()
 }
@@ -282,9 +287,13 @@ func (c *connection) Read(p []byte) (n int, err error) {
 
 // Write will Flush soon.
 func (c *connection) Write(p []byte) (n int, err error) {
-	if !c.IsActive() || !c.lock(flushing) {
+	if !c.IsActive() {
 		return 0, Exception(ErrConnClosed, "when write")
 	}
+
+	if !c.lock(flushing) {
+		return 0, Exception(ErrConcurrentAccess, "when write")
+	}
 	defer c.unlock(flushing)
 
 	dst, _ := c.outputBuffer.Malloc(len(p))