syscall: avoid convT2I allocs for ERROR_IO_PENDING instead of WSAEINPROGRESS

CL 28484 mistakenly assumed that WSARecv returns WSAEINPROGRESS
when there is nothing to read. But the error is ERROR_IO_PENDING.
Fix that mistake.

I was about to write a test for it. But I have found
TestTCPReadWriteAllocs in net package that does nearly what I need,
but was conveniently disabled. So enable and extend the test.

Fixes #16988

Change-Id: I55e5cf8998a9cf29e92b398d702280bdf7d6fc85
Reviewed-on: https://go-review.googlesource.com/28990
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/tcpsock_test.go

@@ -460,11 +460,9 @@ func TestTCPConcurrentAccept(t *testing.T) {
 
 func TestTCPReadWriteAllocs(t *testing.T) {
 	switch runtime.GOOS {
-	case "nacl", "windows":
+	case "nacl":
 		// NaCl needs to allocate pseudo file descriptor
 		// stuff. See syscall/fd_nacl.go.
-		// Windows uses closures and channels for IO
-		// completion port-based netpoll. See fd_windows.go.
 		t.Skipf("not supported on %s", runtime.GOOS)
 	}
 
@@ -474,7 +472,7 @@ func TestTCPReadWriteAllocs(t *testing.T) {
 	}
 	defer ln.Close()
 	var server Conn
-	errc := make(chan error)
+	errc := make(chan error, 1)
 	go func() {
 		var err error
 		server, err = ln.Accept()
@@ -489,6 +487,7 @@ func TestTCPReadWriteAllocs(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer server.Close()
+
 	var buf [128]byte
 	allocs := testing.AllocsPerRun(1000, func() {
 		_, err := server.Write(buf[:])
@@ -503,6 +502,28 @@ func TestTCPReadWriteAllocs(t *testing.T) {
 	if allocs > 0 {
 		t.Fatalf("got %v; want 0", allocs)
 	}
+
+	var bufwrt [128]byte
+	ch := make(chan bool)
+	defer close(ch)
+	go func() {
+		for <-ch {
+			_, err := server.Write(bufwrt[:])
+			errc <- err
+		}
+	}()
+	allocs = testing.AllocsPerRun(1000, func() {
+		ch <- true
+		if _, err = io.ReadFull(client, buf[:]); err != nil {
+			t.Fatal(err)
+		}
+		if err := <-errc; err != nil {
+			t.Fatal(err)
+		}
+	})
+	if allocs > 0 {
+		t.Fatalf("got %v; want 0", allocs)
+	}
 }
 
 func TestTCPStress(t *testing.T) {

src/syscall/mksyscall_windows.go

@@ -831,12 +831,8 @@ var _ unsafe.Pointer
 
 // Do the interface allocations only once for common
 // Errno values.
-const (
-	errnoWSAEINPROGRESS = 10036
-)
-
 var (
-	errWSAEINPROGRESS error = {{syscalldot}}Errno(errnoWSAEINPROGRESS)
+	errERROR_IO_PENDING error = {{syscalldot}}Errno(ERROR_IO_PENDING)
 )
 
 // errnoErr returns common boxed Errno values, to prevent
@@ -845,8 +841,8 @@ func errnoErr(e {{syscalldot}}Errno) error {
 	switch e {
 	case 0:
 		return nil
-	case errnoWSAEINPROGRESS:
-		return errWSAEINPROGRESS
+	case ERROR_IO_PENDING:
+		return errERROR_IO_PENDING
 	}
 	// TODO: add more here, after collecting data on the common
 	// error values see on Windows. (perhaps when running

src/syscall/zsyscall_windows.go

@@ -11,12 +11,8 @@ var _ unsafe.Pointer
 
 // Do the interface allocations only once for common
 // Errno values.
-const (
-	errnoWSAEINPROGRESS = 10036
-)
-
 var (
-	errWSAEINPROGRESS error = Errno(errnoWSAEINPROGRESS)
+	errERROR_IO_PENDING error = Errno(ERROR_IO_PENDING)
 )
 
 // errnoErr returns common boxed Errno values, to prevent
@@ -25,8 +21,8 @@ func errnoErr(e Errno) error {
 	switch e {
 	case 0:
 		return nil
-	case errnoWSAEINPROGRESS:
-		return errWSAEINPROGRESS
+	case ERROR_IO_PENDING:
+		return errERROR_IO_PENDING
 	}
 	// TODO: add more here, after collecting data on the common
 	// error values see on Windows. (perhaps when running