os: don't let sendFile put a pipe into blocking mode

Use SyscallConn to avoid calling the Fd method in sendFile on Unix
systems, since Fd has the side effect of putting the descriptor into
blocking mode.

Fixes #28330

Change-Id: If093417a225fe44092bd2c0dbbc3937422e98c0b
Reviewed-on: https://go-review.googlesource.com/c/155137
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/sendfile_linux.go

@@ -32,7 +32,19 @@ func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
 		return 0, nil, false
 	}
 
-	written, err = poll.SendFile(&c.pfd, int(f.Fd()), remain)
+	sc, err := f.SyscallConn()
+	if err != nil {
+		return 0, nil, false
+	}
+
+	var werr error
+	err = sc.Read(func(fd uintptr) bool {
+		written, werr = poll.SendFile(&c.pfd, int(fd), remain)
+		return true
+	})
+	if werr == nil {
+		werr = err
+	}
 
 	if lr != nil {
 		lr.N = remain - written

src/net/sendfile_test.go

@@ -12,8 +12,12 @@ import (
 	"encoding/hex"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
+	"runtime"
+	"sync"
 	"testing"
+	"time"
 )
 
 const (
@@ -210,3 +214,103 @@ func TestSendfileSeeked(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+// Test that sendfile doesn't put a pipe into blocking mode.
+func TestSendfilePipe(t *testing.T) {
+	switch runtime.GOOS {
+	case "nacl", "plan9", "windows":
+		// These systems don't support deadlines on pipes.
+		t.Skipf("skipping on %s", runtime.GOOS)
+	}
+
+	t.Parallel()
+
+	ln, err := newLocalListener("tcp")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer w.Close()
+	defer r.Close()
+
+	copied := make(chan bool)
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		// Accept a connection and copy 1 byte from the read end of
+		// the pipe to the connection. This will call into sendfile.
+		defer wg.Done()
+		conn, err := ln.Accept()
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		defer conn.Close()
+		_, err = io.CopyN(conn, r, 1)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		// Signal the main goroutine that we've copied the byte.
+		close(copied)
+	}()
+
+	wg.Add(1)
+	go func() {
+		// Write 1 byte to the write end of the pipe.
+		defer wg.Done()
+		_, err := w.Write([]byte{'a'})
+		if err != nil {
+			t.Error(err)
+		}
+	}()
+
+	wg.Add(1)
+	go func() {
+		// Connect to the server started two goroutines up and
+		// discard any data that it writes.
+		defer wg.Done()
+		conn, err := Dial("tcp", ln.Addr().String())
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		defer conn.Close()
+		io.Copy(ioutil.Discard, conn)
+	}()
+
+	// Wait for the byte to be copied, meaning that sendfile has
+	// been called on the pipe.
+	<-copied
+
+	// Set a very short deadline on the read end of the pipe.
+	if err := r.SetDeadline(time.Now().Add(time.Microsecond)); err != nil {
+		t.Fatal(err)
+	}
+
+	wg.Add(1)
+	go func() {
+		// Wait for much longer than the deadline and write a byte
+		// to the pipe.
+		defer wg.Done()
+		time.Sleep(50 * time.Millisecond)
+		w.Write([]byte{'b'})
+	}()
+
+	// If this read does not time out, the pipe was incorrectly
+	// put into blocking mode.
+	_, err = r.Read(make([]byte, 1))
+	if err == nil {
+		t.Error("Read did not time out")
+	} else if !os.IsTimeout(err) {
+		t.Errorf("got error %v, expected a time out", err)
+	}
+
+	wg.Wait()
+}

src/net/sendfile_unix_alt.go

@@ -58,7 +58,19 @@ func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
 		return 0, err, false
 	}
 
-	written, err = poll.SendFile(&c.pfd, int(f.Fd()), pos, remain)
+	sc, err := f.SyscallConn()
+	if err != nil {
+		return 0, nil, false
+	}
+
+	var werr error
+	err = sc.Read(func(fd uintptr) bool {
+		written, werr = poll.SendFile(&c.pfd, int(fd), pos, remain)
+		return true
+	})
+	if werr == nil {
+		werr = err
+	}
 
 	if lr != nil {
 		lr.N = remain - written