internal/poll: keep copying after successful Sendfile return on BSD

The BSD implementation of poll.SendFile incorrectly halted
copying after succesfully writing one full chunk of data.
Adjust the copy loop to match the Linux and Solaris
implementations.

In testing, empirically macOS appears to sometimes return
EAGAIN from sendfile after successfully copying a full
chunk. Add a check to all implementations to return nil
after successfully copying all data if the last sendfile
call returns EAGAIN.

For #70000

Change-Id: I57ba649491fc078c7330310b23e1cfd85135c8ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/622235
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/internal/poll/sendfile_bsd.go

@@ -41,22 +41,25 @@ func SendFile(dstFD *FD, src int, pos, remain int64) (written int64, err error,
 			pos += int64(n)
 			written += int64(n)
 			remain -= int64(n)
+			continue
+		} else if err != syscall.EAGAIN && err != syscall.EINTR {
+			// This includes syscall.ENOSYS (no kernel
+			// support) and syscall.EINVAL (fd types which
+			// don't implement sendfile), and other errors.
+			// We should end the loop when there is no error
+			// returned from sendfile(2) or it is not a retryable error.
+			break
 		}
 		if err == syscall.EINTR {
 			continue
 		}
-		// This includes syscall.ENOSYS (no kernel
-		// support) and syscall.EINVAL (fd types which
-		// don't implement sendfile), and other errors.
-		// We should end the loop when there is no error
-		// returned from sendfile(2) or it is not a retryable error.
-		if err != syscall.EAGAIN {
-			break
-		}
 		if err = dstFD.pd.waitWrite(dstFD.isFile); err != nil {
 			break
 		}
 	}
+	if err == syscall.EAGAIN {
+		err = nil
+	}
 	handled = written != 0 || (err != syscall.ENOSYS && err != syscall.EINVAL)
 	return
 }

src/internal/poll/sendfile_linux.go

@@ -53,6 +53,9 @@ func SendFile(dstFD *FD, src int, remain int64) (written int64, err error, handl
 			break
 		}
 	}
+	if err == syscall.EAGAIN {
+		err = nil
+	}
 	handled = written != 0 || (err != syscall.ENOSYS && err != syscall.EINVAL)
 	return
 }

src/internal/poll/sendfile_solaris.go

@@ -71,6 +71,9 @@ func SendFile(dstFD *FD, src int, pos, remain int64) (written int64, err error,
 			break
 		}
 	}
+	if err == syscall.EAGAIN {
+		err = nil
+	}
 	handled = written != 0 || (err != syscall.ENOSYS && err != syscall.EINVAL)
 	return
 }

src/os/copy_test.go

@@ -0,0 +1,154 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os_test
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"math/rand/v2"
+	"net"
+	"os"
+	"runtime"
+	"sync"
+	"testing"
+
+	"golang.org/x/net/nettest"
+)
+
+// Exercise sendfile/splice fast paths with a moderately large file.
+//
+// https://go.dev/issue/70000
+
+func TestLargeCopyViaNetwork(t *testing.T) {
+	const size = 10 * 1024 * 1024
+	dir := t.TempDir()
+
+	src, err := os.Create(dir + "/src")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer src.Close()
+	if _, err := io.CopyN(src, newRandReader(), size); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := src.Seek(0, 0); err != nil {
+		t.Fatal(err)
+	}
+
+	dst, err := os.Create(dir + "/dst")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer dst.Close()
+
+	client, server := createSocketPair(t, "tcp")
+	var wg sync.WaitGroup
+	wg.Add(2)
+	go func() {
+		defer wg.Done()
+		if n, err := io.Copy(dst, server); n != size || err != nil {
+			t.Errorf("copy to destination = %v, %v; want %v, nil", n, err, size)
+		}
+	}()
+	go func() {
+		defer wg.Done()
+		defer client.Close()
+		if n, err := io.Copy(client, src); n != size || err != nil {
+			t.Errorf("copy from source = %v, %v; want %v, nil", n, err, size)
+		}
+	}()
+	wg.Wait()
+
+	if _, err := dst.Seek(0, 0); err != nil {
+		t.Fatal(err)
+	}
+	if err := compareReaders(dst, io.LimitReader(newRandReader(), size)); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func compareReaders(a, b io.Reader) error {
+	bufa := make([]byte, 4096)
+	bufb := make([]byte, 4096)
+	for {
+		na, erra := io.ReadFull(a, bufa)
+		if erra != nil && erra != io.EOF {
+			return erra
+		}
+		nb, errb := io.ReadFull(b, bufb)
+		if errb != nil && errb != io.EOF {
+			return errb
+		}
+		if !bytes.Equal(bufa[:na], bufb[:nb]) {
+			return errors.New("contents mismatch")
+		}
+		if erra == io.EOF && errb == io.EOF {
+			break
+		}
+	}
+	return nil
+}
+
+type randReader struct {
+	rand *rand.Rand
+}
+
+func newRandReader() *randReader {
+	return &randReader{rand.New(rand.NewPCG(0, 0))}
+}
+
+func (r *randReader) Read(p []byte) (int, error) {
+	var v uint64
+	var n int
+	for i := range p {
+		if n == 0 {
+			v = r.rand.Uint64()
+			n = 8
+		}
+		p[i] = byte(v & 0xff)
+		v >>= 8
+		n--
+	}
+	return len(p), nil
+}
+
+func createSocketPair(t *testing.T, proto string) (client, server net.Conn) {
+	t.Helper()
+	if !nettest.TestableNetwork(proto) {
+		t.Skipf("%s does not support %q", runtime.GOOS, proto)
+	}
+
+	ln, err := nettest.NewLocalListener(proto)
+	if err != nil {
+		t.Fatalf("NewLocalListener error: %v", err)
+	}
+	t.Cleanup(func() {
+		if ln != nil {
+			ln.Close()
+		}
+		if client != nil {
+			client.Close()
+		}
+		if server != nil {
+			server.Close()
+		}
+	})
+	ch := make(chan struct{})
+	go func() {
+		var err error
+		server, err = ln.Accept()
+		if err != nil {
+			t.Errorf("Accept new connection error: %v", err)
+		}
+		ch <- struct{}{}
+	}()
+	client, err = net.Dial(proto, ln.Addr().String())
+	<-ch
+	if err != nil {
+		t.Fatalf("Dial new connection error: %v", err)
+	}
+	return client, server
+}

src/os/readfrom_linux_test.go

@@ -14,14 +14,11 @@ import (
 	"net"
 	. "os"
 	"path/filepath"
-	"runtime"
 	"strconv"
 	"sync"
 	"syscall"
 	"testing"
 	"time"
-
-	"golang.org/x/net/nettest"
 )
 
 func TestSpliceFile(t *testing.T) {
@@ -479,41 +476,3 @@ func testGetPollFDAndNetwork(t *testing.T, proto string) {
 		t.Fatalf("server Control error: %v", err)
 	}
 }
-
-func createSocketPair(t *testing.T, proto string) (client, server net.Conn) {
-	t.Helper()
-	if !nettest.TestableNetwork(proto) {
-		t.Skipf("%s does not support %q", runtime.GOOS, proto)
-	}
-
-	ln, err := nettest.NewLocalListener(proto)
-	if err != nil {
-		t.Fatalf("NewLocalListener error: %v", err)
-	}
-	t.Cleanup(func() {
-		if ln != nil {
-			ln.Close()
-		}
-		if client != nil {
-			client.Close()
-		}
-		if server != nil {
-			server.Close()
-		}
-	})
-	ch := make(chan struct{})
-	go func() {
-		var err error
-		server, err = ln.Accept()
-		if err != nil {
-			t.Errorf("Accept new connection error: %v", err)
-		}
-		ch <- struct{}{}
-	}()
-	client, err = net.Dial(proto, ln.Addr().String())
-	<-ch
-	if err != nil {
-		t.Fatalf("Dial new connection error: %v", err)
-	}
-	return client, server
-}