os: employ sendfile(2) for file-to-file copying on Linux when needed

Go utilizes copy_file_range(2) for file-to-file copying only on kernel 5.3+,
but even on 5.3+ this system call can still go wrong for some reason (check
out the comment inside poll.CopyFileRange).

Before Linux 2.6.33, out_fd must refer to a socket, but since Linux 2.6.33
it can be any file. Thus, we can employ sendfile(2) for copy between files
when copy_file_range(2) fails to handle the copy, that way we can still
benefit from the zero-copy technique on kernel <5.3 and wherever
copy_file_range(2) is available but broken.

Change-Id: I3922218c95ad34ee649ccdf3ccfbd1ce692bebcc
Reviewed-on: https://go-review.googlesource.com/c/go/+/603295
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/os/readfrom_linux_test.go

@@ -25,7 +25,7 @@ import (
 	"golang.org/x/net/nettest"
 )
 
-func TestCopyFileRange(t *testing.T) {
+func TestCopyFileRangeAndSendFile(t *testing.T) {
 	sizes := []int{
 		1,
 		42,
@@ -37,6 +37,7 @@ func TestCopyFileRange(t *testing.T) {
 		for _, size := range sizes {
 			t.Run(strconv.Itoa(size), func(t *testing.T) {
 				testCopyFileRange(t, int64(size), -1)
+				testSendfileOverCopyFileRange(t, int64(size), -1)
 			})
 		}
 	})
@@ -45,6 +46,7 @@ func TestCopyFileRange(t *testing.T) {
 			for _, size := range sizes {
 				t.Run(strconv.Itoa(size), func(t *testing.T) {
 					testCopyFileRange(t, int64(size), int64(size)-1)
+					testSendfileOverCopyFileRange(t, int64(size), int64(size)-1)
 				})
 			}
 		})
@@ -52,6 +54,7 @@ func TestCopyFileRange(t *testing.T) {
 			for _, size := range sizes {
 				t.Run(strconv.Itoa(size), func(t *testing.T) {
 					testCopyFileRange(t, int64(size), int64(size)/2)
+					testSendfileOverCopyFileRange(t, int64(size), int64(size)/2)
 				})
 			}
 		})
@@ -59,173 +62,207 @@ func TestCopyFileRange(t *testing.T) {
 			for _, size := range sizes {
 				t.Run(strconv.Itoa(size), func(t *testing.T) {
 					testCopyFileRange(t, int64(size), int64(size)+7)
+					testSendfileOverCopyFileRange(t, int64(size), int64(size)+7)
 				})
 			}
 		})
 	})
 	t.Run("DoesntTryInAppendMode", func(t *testing.T) {
-		dst, src, data, hook := newCopyFileRangeTest(t, 42)
+		for _, newTest := range []func(*testing.T, int64) (*File, *File, []byte, *copyFileHook, string){
+			newCopyFileRangeTest, newSendfileOverCopyFileRangeTest} {
+			dst, src, data, hook, testName := newTest(t, 42)
 
-		dst2, err := OpenFile(dst.Name(), O_RDWR|O_APPEND, 0755)
+			dst2, err := OpenFile(dst.Name(), O_RDWR|O_APPEND, 0755)
-		if err != nil {
+			if err != nil {
-			t.Fatal(err)
+				t.Fatalf("%s: %v", testName, err)
-		}
+			}
-		defer dst2.Close()
+			defer dst2.Close()
 
-		if _, err := io.Copy(dst2, src); err != nil {
+			if _, err := io.Copy(dst2, src); err != nil {
-			t.Fatal(err)
+				t.Fatalf("%s: %v", testName, err)
+			}
+			if hook.called {
+				t.Fatalf("%s: hook shouldn't be called with destination in O_APPEND mode", testName)
+			}
+			mustSeekStart(t, dst2)
+			mustContainData(t, dst2, data) // through traditional means
 		}
-		if hook.called {
-			t.Fatal("called poll.CopyFileRange for destination in O_APPEND mode")
-		}
-		mustSeekStart(t, dst2)
-		mustContainData(t, dst2, data) // through traditional means
 	})
 	t.Run("CopyFileItself", func(t *testing.T) {
-		hook := hookCopyFileRange(t)
+		for _, hookFunc := range []func(*testing.T) (*copyFileHook, string){hookCopyFileRange, hookSendFileOverCopyFileRange} {
+			hook, testName := hookFunc(t)
 
-		f, err := CreateTemp("", "file-readfrom-itself-test")
+			f, err := CreateTemp("", "file-readfrom-itself-test")
-		if err != nil {
+			if err != nil {
-			t.Fatalf("failed to create tmp file: %v", err)
+				t.Fatalf("%s: failed to create tmp file: %v", testName, err)
-		}
+			}
-		t.Cleanup(func() {
+			t.Cleanup(func() {
-			f.Close()
+				f.Close()
-			Remove(f.Name())
+				Remove(f.Name())
-		})
+			})
 
-		data := []byte("hello world!")
+			data := []byte("hello world!")
-		if _, err := f.Write(data); err != nil {
+			if _, err := f.Write(data); err != nil {
-			t.Fatalf("failed to create and feed the file: %v", err)
+				t.Fatalf("%s: failed to create and feed the file: %v", testName, err)
-		}
+			}
 
-		if err := f.Sync(); err != nil {
+			if err := f.Sync(); err != nil {
-			t.Fatalf("failed to save the file: %v", err)
+				t.Fatalf("%s: failed to save the file: %v", testName, err)
-		}
+			}
 
-		// Rewind it.
+			// Rewind it.
-		if _, err := f.Seek(0, io.SeekStart); err != nil {
+			if _, err := f.Seek(0, io.SeekStart); err != nil {
-			t.Fatalf("failed to rewind the file: %v", err)
+				t.Fatalf("%s: failed to rewind the file: %v", testName, err)
-		}
+			}
 
-		// Read data from the file itself.
+			// Read data from the file itself.
-		if _, err := io.Copy(f, f); err != nil {
+			if _, err := io.Copy(f, f); err != nil {
-			t.Fatalf("failed to read from the file: %v", err)
+				t.Fatalf("%s: failed to read from the file: %v", testName, err)
-		}
+			}
 
-		if !hook.called || hook.written != 0 || hook.handled || hook.err != nil {
+			if hook.written != 0 || hook.handled || hook.err != nil {
-			t.Fatalf("poll.CopyFileRange should be called and return the EINVAL error, but got hook.called=%t, hook.err=%v", hook.called, hook.err)
+				t.Fatalf("%s: File.readFrom is expected not to use any zero-copy techniques when copying itself."+
-		}
+					"got hook.written=%d, hook.handled=%t, hook.err=%v; expected hook.written=0, hook.handled=false, hook.err=nil",
+					testName, hook.written, hook.handled, hook.err)
+			}
 
-		// Rewind it.
+			switch testName {
-		if _, err := f.Seek(0, io.SeekStart); err != nil {
+			case "hookCopyFileRange":
-			t.Fatalf("failed to rewind the file: %v", err)
+				// For copy_file_range(2), it fails and returns EINVAL when the source and target
-		}
+				// refer to the same file and their ranges overlap. The hook should be called to
+				// get the returned error and fall back to generic copy.
+				if !hook.called {
+					t.Fatalf("%s: should have called the hook", testName)
+				}
+			case "hookSendFileOverCopyFileRange":
+				// For sendfile(2), it allows the source and target to refer to the same file and overlap.
+				// The hook should not be called and just fall back to generic copy directly.
+				if hook.called {
+					t.Fatalf("%s: shouldn't have called the hook", testName)
+				}
+			default:
+				t.Fatalf("%s: unexpected test", testName)
+			}
 
-		data2, err := io.ReadAll(f)
+			// Rewind it.
-		if err != nil {
+			if _, err := f.Seek(0, io.SeekStart); err != nil {
-			t.Fatalf("failed to read from the file: %v", err)
+				t.Fatalf("%s: failed to rewind the file: %v", testName, err)
-		}
+			}
 
-		// It should wind up a double of the original data.
+			data2, err := io.ReadAll(f)
-		if strings.Repeat(string(data), 2) != string(data2) {
+			if err != nil {
-			t.Fatalf("data mismatch: %s != %s", string(data), string(data2))
+				t.Fatalf("%s: failed to read from the file: %v", testName, err)
+			}
+
+			// It should wind up a double of the original data.
+			if s := strings.Repeat(string(data), 2); s != string(data2) {
+				t.Fatalf("%s: file contained %s, expected %s", testName, data2, s)
+			}
 		}
 	})
 	t.Run("NotRegular", func(t *testing.T) {
 		t.Run("BothPipes", func(t *testing.T) {
-			hook := hookCopyFileRange(t)
+			for _, hookFunc := range []func(*testing.T) (*copyFileHook, string){hookCopyFileRange, hookSendFileOverCopyFileRange} {
+				hook, testName := hookFunc(t)
 
-			pr1, pw1, err := Pipe()
+				pr1, pw1, err := Pipe()
-			if err != nil {
+				if err != nil {
-				t.Fatal(err)
+					t.Fatalf("%s: %v", testName, err)
-			}
+				}
-			defer pr1.Close()
+				defer pr1.Close()
-			defer pw1.Close()
+				defer pw1.Close()
 
-			pr2, pw2, err := Pipe()
+				pr2, pw2, err := Pipe()
-			if err != nil {
+				if err != nil {
-				t.Fatal(err)
+					t.Fatalf("%s: %v", testName, err)
-			}
+				}
-			defer pr2.Close()
+				defer pr2.Close()
-			defer pw2.Close()
+				defer pw2.Close()
 
-			// The pipe is empty, and PIPE_BUF is large enough
+				// The pipe is empty, and PIPE_BUF is large enough
-			// for this, by (POSIX) definition, so there is no
+				// for this, by (POSIX) definition, so there is no
-			// need for an additional goroutine.
+				// need for an additional goroutine.
-			data := []byte("hello")
+				data := []byte("hello")
-			if _, err := pw1.Write(data); err != nil {
+				if _, err := pw1.Write(data); err != nil {
-				t.Fatal(err)
+					t.Fatalf("%s: %v", testName, err)
-			}
+				}
-			pw1.Close()
+				pw1.Close()
 
-			n, err := io.Copy(pw2, pr1)
+				n, err := io.Copy(pw2, pr1)
-			if err != nil {
+				if err != nil {
-				t.Fatal(err)
+					t.Fatalf("%s: %v", testName, err)
+				}
+				if n != int64(len(data)) {
+					t.Fatalf("%s: transferred %d, want %d", testName, n, len(data))
+				}
+				if !hook.called {
+					t.Fatalf("%s: should have called the hook", testName)
+				}
+				pw2.Close()
+				mustContainData(t, pr2, data)
 			}
-			if n != int64(len(data)) {
-				t.Fatalf("transferred %d, want %d", n, len(data))
-			}
-			if !hook.called {
-				t.Fatalf("should have called poll.CopyFileRange")
-			}
-			pw2.Close()
-			mustContainData(t, pr2, data)
 		})
 		t.Run("DstPipe", func(t *testing.T) {
-			dst, src, data, hook := newCopyFileRangeTest(t, 255)
+			for _, newTest := range []func(*testing.T, int64) (*File, *File, []byte, *copyFileHook, string){
-			dst.Close()
+				newCopyFileRangeTest, newSendfileOverCopyFileRangeTest} {
+				dst, src, data, hook, testName := newTest(t, 255)
+				dst.Close()
 
-			pr, pw, err := Pipe()
+				pr, pw, err := Pipe()
-			if err != nil {
+				if err != nil {
-				t.Fatal(err)
+					t.Fatalf("%s: %v", testName, err)
-			}
+				}
-			defer pr.Close()
+				defer pr.Close()
-			defer pw.Close()
+				defer pw.Close()
 
-			n, err := io.Copy(pw, src)
+				n, err := io.Copy(pw, src)
-			if err != nil {
+				if err != nil {
-				t.Fatal(err)
+					t.Fatalf("%s: %v", testName, err)
+				}
+				if n != int64(len(data)) {
+					t.Fatalf("%s: transferred %d, want %d", testName, n, len(data))
+				}
+				if !hook.called {
+					t.Fatalf("%s: should have called the hook", testName)
+				}
+				pw.Close()
+				mustContainData(t, pr, data)
 			}
-			if n != int64(len(data)) {
-				t.Fatalf("transferred %d, want %d", n, len(data))
-			}
-			if !hook.called {
-				t.Fatalf("should have called poll.CopyFileRange")
-			}
-			pw.Close()
-			mustContainData(t, pr, data)
 		})
 		t.Run("SrcPipe", func(t *testing.T) {
-			dst, src, data, hook := newCopyFileRangeTest(t, 255)
+			for _, newTest := range []func(*testing.T, int64) (*File, *File, []byte, *copyFileHook, string){
-			src.Close()
+				newCopyFileRangeTest, newSendfileOverCopyFileRangeTest} {
+				dst, src, data, hook, testName := newTest(t, 255)
+				src.Close()
 
-			pr, pw, err := Pipe()
+				pr, pw, err := Pipe()
-			if err != nil {
+				if err != nil {
-				t.Fatal(err)
+					t.Fatalf("%s: %v", testName, err)
-			}
+				}
-			defer pr.Close()
+				defer pr.Close()
-			defer pw.Close()
+				defer pw.Close()
 
-			// The pipe is empty, and PIPE_BUF is large enough
+				// The pipe is empty, and PIPE_BUF is large enough
-			// for this, by (POSIX) definition, so there is no
+				// for this, by (POSIX) definition, so there is no
-			// need for an additional goroutine.
+				// need for an additional goroutine.
-			if _, err := pw.Write(data); err != nil {
+				if _, err := pw.Write(data); err != nil {
-				t.Fatal(err)
+					t.Fatalf("%s: %v", testName, err)
-			}
+				}
-			pw.Close()
+				pw.Close()
 
-			n, err := io.Copy(dst, pr)
+				n, err := io.Copy(dst, pr)
-			if err != nil {
+				if err != nil {
-				t.Fatal(err)
+					t.Fatalf("%s: %v", testName, err)
+				}
+				if n != int64(len(data)) {
+					t.Fatalf("%s: transferred %d, want %d", testName, n, len(data))
+				}
+				if !hook.called {
+					t.Fatalf("%s: should have called the hook", testName)
+				}
+				mustSeekStart(t, dst)
+				mustContainData(t, dst, data)
 			}
-			if n != int64(len(data)) {
-				t.Fatalf("transferred %d, want %d", n, len(data))
-			}
-			if !hook.called {
-				t.Fatalf("should have called poll.CopyFileRange")
-			}
-			mustSeekStart(t, dst)
-			mustContainData(t, dst, data)
 		})
 	})
 	t.Run("Nil", func(t *testing.T) {
@@ -480,8 +517,16 @@ func testSpliceToTTY(t *testing.T, proto string, size int64) {
 }
 
 func testCopyFileRange(t *testing.T, size int64, limit int64) {
-	dst, src, data, hook := newCopyFileRangeTest(t, size)
+	dst, src, data, hook, name := newCopyFileRangeTest(t, size)
+	testCopyFile(t, dst, src, data, hook, limit, name)
+}
 
+func testSendfileOverCopyFileRange(t *testing.T, size int64, limit int64) {
+	dst, src, data, hook, name := newSendfileOverCopyFileRangeTest(t, size)
+	testCopyFile(t, dst, src, data, hook, limit, name)
+}
+
+func testCopyFile(t *testing.T, dst, src *File, data []byte, hook *copyFileHook, limit int64, testName string) {
 	// If we have a limit, wrap the reader.
 	var (
 		realsrc io.Reader
@@ -498,22 +543,22 @@ func testCopyFileRange(t *testing.T, size int64, limit int64) {
 	}
 
 	// Now call ReadFrom (through io.Copy), which will hopefully call
-	// poll.CopyFileRange.
+	// poll.CopyFileRange or poll.SendFile.
 	n, err := io.Copy(dst, realsrc)
 	if err != nil {
-		t.Fatal(err)
+		t.Fatalf("%s: %v", testName, err)
 	}
 
-	// If we didn't have a limit, we should have called poll.CopyFileRange
+	// If we didn't have a limit or had a positive limit, we should have called
-	// with the right file descriptor arguments.
+	// poll.CopyFileRange or poll.SendFile with the right file descriptor arguments.
-	if limit > 0 && !hook.called {
+	if limit != 0 && !hook.called {
-		t.Fatal("never called poll.CopyFileRange")
+		t.Fatalf("%s: never called the hook", testName)
 	}
 	if hook.called && hook.dstfd != int(dst.Fd()) {
-		t.Fatalf("wrong destination file descriptor: got %d, want %d", hook.dstfd, dst.Fd())
+		t.Fatalf("%s: wrong destination file descriptor: got %d, want %d", testName, hook.dstfd, dst.Fd())
 	}
 	if hook.called && hook.srcfd != int(src.Fd()) {
-		t.Fatalf("wrong source file descriptor: got %d, want %d", hook.srcfd, src.Fd())
+		t.Fatalf("%s: wrong source file descriptor: got %d, want %d", testName, hook.srcfd, src.Fd())
 	}
 
 	// Check that the offsets after the transfer make sense, that the size
@@ -521,20 +566,20 @@ func testCopyFileRange(t *testing.T, size int64, limit int64) {
 	// file contains exactly the bytes we expect it to contain.
 	dstoff, err := dst.Seek(0, io.SeekCurrent)
 	if err != nil {
-		t.Fatal(err)
+		t.Fatalf("%s: %v", testName, err)
 	}
 	srcoff, err := src.Seek(0, io.SeekCurrent)
 	if err != nil {
-		t.Fatal(err)
+		t.Fatalf("%s: %v", testName, err)
 	}
 	if dstoff != srcoff {
-		t.Errorf("offsets differ: dstoff = %d, srcoff = %d", dstoff, srcoff)
+		t.Errorf("%s: offsets differ: dstoff = %d, srcoff = %d", testName, dstoff, srcoff)
 	}
 	if dstoff != int64(len(data)) {
-		t.Errorf("dstoff = %d, want %d", dstoff, len(data))
+		t.Errorf("%s: dstoff = %d, want %d", testName, dstoff, len(data))
 	}
 	if n != int64(len(data)) {
-		t.Errorf("short ReadFrom: wrote %d bytes, want %d", n, len(data))
+		t.Errorf("%s: short ReadFrom: wrote %d bytes, want %d", testName, n, len(data))
 	}
 	mustSeekStart(t, dst)
 	mustContainData(t, dst, data)
@@ -542,47 +587,53 @@ func testCopyFileRange(t *testing.T, size int64, limit int64) {
 	// If we had a limit, check that it was updated.
 	if lr != nil {
 		if want := limit - n; lr.N != want {
-			t.Fatalf("didn't update limit correctly: got %d, want %d", lr.N, want)
+			t.Fatalf("%s: didn't update limit correctly: got %d, want %d", testName, lr.N, want)
 		}
 	}
 }
 
 // newCopyFileRangeTest initializes a new test for copy_file_range.
 //
-// It creates source and destination files, and populates the source file
+// It hooks package os' call to poll.CopyFileRange and returns the hook,
-// with random data of the specified size. It also hooks package os' call
+// so it can be inspected.
-// to poll.CopyFileRange and returns the hook so it can be inspected.
+func newCopyFileRangeTest(t *testing.T, size int64) (dst, src *File, data []byte, hook *copyFileHook, name string) {
-func newCopyFileRangeTest(t *testing.T, size int64) (dst, src *File, data []byte, hook *copyFileRangeHook) {
+	t.Helper()
+	name = "newCopyFileRangeTest"
+
+	dst, src, data = newCopyFileTest(t, size)
+	hook, _ = hookCopyFileRange(t)
+
+	return
+}
+
+// newSendFileTest initializes a new test for sendfile over copy_file_range.
+// It hooks package os' call to poll.SendFile and returns the hook,
+// so it can be inspected.
+func newSendfileOverCopyFileRangeTest(t *testing.T, size int64) (dst, src *File, data []byte, hook *copyFileHook, name string) {
 	t.Helper()
 
-	hook = hookCopyFileRange(t)
+	name = "newSendfileOverCopyFileRangeTest"
-	tmp := t.TempDir()
 
-	src, err := Create(filepath.Join(tmp, "src"))
+	dst, src, data = newCopyFileTest(t, size)
-	if err != nil {
+	hook, _ = hookSendFileOverCopyFileRange(t)
-		t.Fatal(err)
-	}
-	t.Cleanup(func() { src.Close() })
 
-	dst, err = Create(filepath.Join(tmp, "dst"))
+	return
+}
+
+// newCopyFileTest initializes a new test for copying data between files.
+// It creates source and destination files, and populates the source file
+// with random data of the specified size, then rewind it, so it can be
+// consumed by copy_file_range(2) or sendfile(2).
+func newCopyFileTest(t *testing.T, size int64) (dst, src *File, data []byte) {
+	src, data = createTempFile(t, "test-copy_file_range-sendfile-src", size)
+
+	dst, err := CreateTemp(t.TempDir(), "test-copy_file_range-sendfile-dst")
 	if err != nil {
 		t.Fatal(err)
 	}
 	t.Cleanup(func() { dst.Close() })
 
-	// Populate the source file with data, then rewind it, so it can be
+	return
-	// consumed by copy_file_range(2).
-	prng := rand.New(rand.NewSource(time.Now().Unix()))
-	data = make([]byte, size)
-	prng.Read(data)
-	if _, err := src.Write(data); err != nil {
-		t.Fatal(err)
-	}
-	if _, err := src.Seek(0, io.SeekStart); err != nil {
-		t.Fatal(err)
-	}
-
-	return dst, src, data, hook
 }
 
 // newSpliceFileTest initializes a new test for splice.
@@ -642,40 +693,58 @@ func mustSeekStart(t *testing.T, f *File) {
 	}
 }
 
-func hookCopyFileRange(t *testing.T) *copyFileRangeHook {
+func hookCopyFileRange(t *testing.T) (hook *copyFileHook, name string) {
-	h := new(copyFileRangeHook)
+	name = "hookCopyFileRange"
-	h.install()
+
-	t.Cleanup(h.uninstall)
+	hook = new(copyFileHook)
-	return h
+	orig := *PollCopyFileRangeP
+	t.Cleanup(func() {
+		*PollCopyFileRangeP = orig
+	})
+	*PollCopyFileRangeP = func(dst, src *poll.FD, remain int64) (int64, bool, error) {
+		hook.called = true
+		hook.dstfd = dst.Sysfd
+		hook.srcfd = src.Sysfd
+		hook.written, hook.handled, hook.err = orig(dst, src, remain)
+		return hook.written, hook.handled, hook.err
+	}
+	return
 }
 
-type copyFileRangeHook struct {
+func hookSendFileOverCopyFileRange(t *testing.T) (hook *copyFileHook, name string) {
+	name = "hookSendFileOverCopyFileRange"
+
+	// Disable poll.CopyFileRange to force the fallback to poll.SendFile.
+	originalCopyFileRange := *PollCopyFileRangeP
+	*PollCopyFileRangeP = func(dst, src *poll.FD, remain int64) (written int64, handled bool, err error) {
+		return 0, false, nil
+	}
+
+	hook = new(copyFileHook)
+	orig := poll.TestHookDidSendFile
+	t.Cleanup(func() {
+		*PollCopyFileRangeP = originalCopyFileRange
+		poll.TestHookDidSendFile = orig
+	})
+	poll.TestHookDidSendFile = func(dstFD *poll.FD, src int, written int64, err error, handled bool) {
+		hook.called = true
+		hook.dstfd = dstFD.Sysfd
+		hook.srcfd = src
+		hook.written = written
+		hook.err = err
+		hook.handled = handled
+	}
+	return
+}
+
+type copyFileHook struct {
 	called bool
 	dstfd  int
 	srcfd  int
-	remain int64
 
 	written int64
 	handled bool
 	err     error
-
-	original func(dst, src *poll.FD, remain int64) (int64, bool, error)
-}
-
-func (h *copyFileRangeHook) install() {
-	h.original = *PollCopyFileRangeP
-	*PollCopyFileRangeP = func(dst, src *poll.FD, remain int64) (int64, bool, error) {
-		h.called = true
-		h.dstfd = dst.Sysfd
-		h.srcfd = src.Sysfd
-		h.remain = remain
-		h.written, h.handled, h.err = h.original(dst, src, remain)
-		return h.written, h.handled, h.err
-	}
-}
-
-func (h *copyFileRangeHook) uninstall() {
-	*PollCopyFileRangeP = h.original
 }
 
 func hookSpliceFile(t *testing.T) *spliceFileHook {

src/os/writeto_linux_test.go

@@ -102,7 +102,7 @@ func newSendFileTest(t *testing.T, proto string, size int64) (net.Conn, *File, n
 	hook := hookSendFile(t)
 
 	client, server := createSocketPair(t, proto)
-	tempFile, data := createTempFile(t, size)
+	tempFile, data := createTempFile(t, "writeto-sendfile-to-socket", size)
 
 	return client, tempFile, server, data, hook
 }
@@ -134,8 +134,8 @@ type sendFileHook struct {
 	err     error
 }
 
-func createTempFile(t *testing.T, size int64) (*File, []byte) {
+func createTempFile(t *testing.T, name string, size int64) (*File, []byte) {
-	f, err := CreateTemp(t.TempDir(), "writeto-sendfile-to-socket")
+	f, err := CreateTemp(t.TempDir(), name)
 	if err != nil {
 		t.Fatalf("failed to create temporary file: %v", err)
 	}

src/os/zero_copy_linux.go

@@ -49,16 +49,17 @@ func (f *File) writeTo(w io.Writer) (written int64, handled bool, err error) {
 }
 
 func (f *File) readFrom(r io.Reader) (written int64, handled bool, err error) {
-	// Neither copy_file_range(2) nor splice(2) supports destinations opened with
+	// Neither copy_file_range(2)/sendfile(2) nor splice(2) supports destinations opened with
 	// O_APPEND, so don't bother to try zero-copy with these system calls.
 	//
 	// Visit https://man7.org/linux/man-pages/man2/copy_file_range.2.html#ERRORS and
+	// https://man7.org/linux/man-pages/man2/sendfile.2.html#ERRORS and
 	// https://man7.org/linux/man-pages/man2/splice.2.html#ERRORS for details.
 	if f.appendMode {
 		return 0, false, nil
 	}
 
-	written, handled, err = f.copyFileRange(r)
+	written, handled, err = f.copyFile(r)
 	if handled {
 		return
 	}
@@ -95,7 +96,7 @@ func (f *File) spliceToFile(r io.Reader) (written int64, handled bool, err error
 	return written, handled, wrapSyscallError("splice", err)
 }
 
-func (f *File) copyFileRange(r io.Reader) (written int64, handled bool, err error) {
+func (f *File) copyFile(r io.Reader) (written int64, handled bool, err error) {
 	var (
 		remain int64
 		lr     *io.LimitedReader
@@ -124,7 +125,44 @@ func (f *File) copyFileRange(r io.Reader) (written int64, handled bool, err erro
 	if lr != nil {
 		lr.N -= written
 	}
-	return written, handled, wrapSyscallError("copy_file_range", err)
+
+	if handled {
+		return written, handled, wrapSyscallError("copy_file_range", err)
+	}
+
+	// If fd_in and fd_out refer to the same file and the source and target ranges overlap,
+	// copy_file_range(2) just returns EINVAL error. poll.CopyFileRange will ignore that
+	// error and act like it didn't call copy_file_range(2). Then the caller will fall back
+	// to generic copy, which results in doubling the content in the file.
+	// By contrast, sendfile(2) allows this kind of overlapping and works like a memmove,
+	// in this case the file content will remain the same after copying, which is not what we want.
+	// Thus, we just bail out here and leave it to generic copy when it's a file copying itself.
+	if f.pfd.Sysfd == src.pfd.Sysfd {
+		return 0, false, nil
+	}
+
+	sc, err := src.SyscallConn()
+	if err != nil {
+		return
+	}
+
+	// We can employ sendfile(2) when copy_file_range(2) fails to handle the copy.
+	// sendfile(2) enabled file-to-file copying since Linux 2.6.33 and Go requires
+	// Linux 3.17 or later, so we're good to go.
+	// Check out https://man7.org/linux/man-pages/man2/sendfile.2.html#DESCRIPTION for more details.
+	rerr := sc.Read(func(fd uintptr) bool {
+		written, err, handled = poll.SendFile(&f.pfd, int(fd), remain)
+		return true
+	})
+	if lr != nil {
+		lr.N -= written
+	}
+
+	if err == nil {
+		err = rerr
+	}
+
+	return written, handled, wrapSyscallError("sendfile", err)
 }
 
 // getPollFDAndNetwork tries to get the poll.FD and network type from the given interface