internal/poll: don't wait for unpollable files

If we get an EAGAIN error on an unpollable file, don't try to wait for
it to be ready; just return EAGAIN.

It's possible that we should instead ensure that when Stdin is a pipe
in non-blocking mode, we wait for data to appear. For now take the
conservative approach of doing what we did in previous releases.

Based on https://golang.org/cl/47555 by Totoro W.

Fixes #20915

Change-Id: Icc9e97a5a877b0a3583ec056c35412d1afab62d1
Reviewed-on: https://go-review.googlesource.com/48490
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

4 files changed, 51 insertions, 9 deletions

diff --git a/src/internal/poll/fd_poll_nacl.go b/src/internal/poll/fd_poll_nacl.go
index 8fa75c5a26..2df30030b1 100644
--- a/src/internal/poll/fd_poll_nacl.go
+++ b/src/internal/poll/fd_poll_nacl.go
@@ -49,6 +49,8 @@ func (pd *pollDesc) waitWrite(isFile bool) error { return pd.wait('w', isFile) }
 
 func (pd *pollDesc) waitCanceled(mode int) {}
 
+func (pd *pollDesc) pollable() bool { return true }
+
 // SetDeadline sets the read and write deadlines associated with fd.
 func (fd *FD) SetDeadline(t time.Time) error {
 	return setDeadlineImpl(fd, t, 'r'+'w')
diff --git a/src/internal/poll/fd_poll_runtime.go b/src/internal/poll/fd_poll_runtime.go
index 08b40c2720..bfbe3c7de4 100644
--- a/src/internal/poll/fd_poll_runtime.go
+++ b/src/internal/poll/fd_poll_runtime.go
@@ -101,6 +101,10 @@ func (pd *pollDesc) waitCanceled(mode int) {
 	runtime_pollWaitCanceled(pd.runtimeCtx, mode)
 }
 
+func (pd *pollDesc) pollable() bool {
+	return pd.runtimeCtx != 0
+}
+
 func convertErr(res int, isFile bool) error {
 	switch res {
 	case 0:
diff --git a/src/internal/poll/fd_unix.go b/src/internal/poll/fd_unix.go
index 3ca6e157c5..c40c701f59 100644
--- a/src/internal/poll/fd_unix.go
+++ b/src/internal/poll/fd_unix.go
@@ -121,7 +121,7 @@ func (fd *FD) Read(p []byte) (int, error) {
 		n, err := syscall.Read(fd.Sysfd, p)
 		if err != nil {
 			n = 0
-			if err == syscall.EAGAIN {
+			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
@@ -165,7 +165,7 @@ func (fd *FD) ReadFrom(p []byte) (int, syscall.Sockaddr, error) {
 		n, sa, err := syscall.Recvfrom(fd.Sysfd, p, 0)
 		if err != nil {
 			n = 0
-			if err == syscall.EAGAIN {
+			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
@@ -189,7 +189,7 @@ func (fd *FD) ReadMsg(p []byte, oob []byte) (int, int, int, syscall.Sockaddr, er
 		n, oobn, flags, sa, err := syscall.Recvmsg(fd.Sysfd, p, oob, 0)
 		if err != nil {
 			// TODO(dfc) should n and oobn be set to 0
-			if err == syscall.EAGAIN {
+			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
@@ -222,7 +222,7 @@ func (fd *FD) Write(p []byte) (int, error) {
 		if nn == len(p) {
 			return nn, err
 		}
-		if err == syscall.EAGAIN {
+		if err == syscall.EAGAIN && fd.pd.pollable() {
 			if err = fd.pd.waitWrite(fd.isFile); err == nil {
 				continue
 			}
@@ -278,7 +278,7 @@ func (fd *FD) WriteTo(p []byte, sa syscall.Sockaddr) (int, error) {
 	}
 	for {
 		err := syscall.Sendto(fd.Sysfd, p, 0, sa)
-		if err == syscall.EAGAIN {
+		if err == syscall.EAGAIN && fd.pd.pollable() {
 			if err = fd.pd.waitWrite(fd.isFile); err == nil {
 				continue
 			}
@@ -301,7 +301,7 @@ func (fd *FD) WriteMsg(p []byte, oob []byte, sa syscall.Sockaddr) (int, int, err
 	}
 	for {
 		n, err := syscall.SendmsgN(fd.Sysfd, p, oob, sa, 0)
-		if err == syscall.EAGAIN {
+		if err == syscall.EAGAIN && fd.pd.pollable() {
 			if err = fd.pd.waitWrite(fd.isFile); err == nil {
 				continue
 			}
@@ -330,8 +330,10 @@ func (fd *FD) Accept() (int, syscall.Sockaddr, string, error) {
 		}
 		switch err {
 		case syscall.EAGAIN:
-			if err = fd.pd.waitRead(fd.isFile); err == nil {
-				continue
+			if fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
 			}
 		case syscall.ECONNABORTED:
 			// This means that a socket on the listen
@@ -364,7 +366,7 @@ func (fd *FD) ReadDirent(buf []byte) (int, error) {
 		n, err := syscall.ReadDirent(fd.Sysfd, buf)
 		if err != nil {
 			n = 0
-			if err == syscall.EAGAIN {
+			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
diff --git a/src/os/pipe_test.go b/src/os/pipe_test.go
index 7c4cbb17ae..9d79d84575 100644
--- a/src/os/pipe_test.go
+++ b/src/os/pipe_test.go
@@ -186,3 +186,37 @@ func TestClosedPipeRaceRead(t *testing.T) {
 func TestClosedPipeRaceWrite(t *testing.T) {
 	testClosedPipeRace(t, false)
 }
+
+// Issue 20915: Reading on nonblocking fd should not return "waiting
+// for unsupported file type." Currently it returns EAGAIN; it is
+// possible that in the future it will simply wait for data.
+func TestReadNonblockingFd(t *testing.T) {
+	if os.Getenv("GO_WANT_READ_NONBLOCKING_FD") == "1" {
+		fd := int(os.Stdin.Fd())
+		syscall.SetNonblock(fd, true)
+		defer syscall.SetNonblock(fd, false)
+		_, err := os.Stdin.Read(make([]byte, 1))
+		if err != nil {
+			if perr, ok := err.(*os.PathError); !ok || perr.Err != syscall.EAGAIN {
+				t.Fatalf("read on nonblocking stdin got %q, should have gotten EAGAIN", err)
+			}
+		}
+		os.Exit(0)
+	}
+
+	testenv.MustHaveExec(t)
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer r.Close()
+	defer w.Close()
+	cmd := osexec.Command(os.Args[0], "-test.run="+t.Name())
+	cmd.Env = append(os.Environ(), "GO_WANT_READ_NONBLOCKING_FD=1")
+	cmd.Stdin = r
+	output, err := cmd.CombinedOutput()
+	t.Logf("%s", output)
+	if err != nil {
+		t.Errorf("child process failed: %v", err)
+	}
+}