internal/poll: if copy_file_range returns 0, assume it failed

On current Linux kernels copy_file_range does not correctly handle
files in certain special file systems, such as /proc. For those file
systems it fails to copy any data and returns zero. This breaks Go's
io.Copy for those files.

Fix the problem by assuming that if copy_file_range returns 0 the
first time it is called on a file, that that file is not supported.
In that case fall back to just using read. This will force an extra
system call when using io.Copy to copy a zero-sized normal file,
but at least it will work correctly.

For #36817
Fixes #44272

Change-Id: I02e81872cb70fda0ce5485e2ea712f219132e614
Reviewed-on: https://go-review.googlesource.com/c/go/+/291989
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/internal/poll/copy_file_range_linux.go

@@ -112,7 +112,15 @@ func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err
 			return 0, false, nil
 		case nil:
 			if n == 0 {
-				// src is at EOF, which means we are done.
+				// If we did not read any bytes at all,
+				// then this file may be in a file system
+				// where copy_file_range silently fails.
+				// https://lore.kernel.org/linux-fsdevel/20210126233840.GG4626@dread.disaster.area/T/#m05753578c7f7882f6e9ffe01f981bc223edef2b0
+				if written == 0 {
+					return 0, false, nil
+				}
+				// Otherwise src is at EOF, which means
+				// we are done.
 				return written, true, nil
 			}
 			remain -= n

src/os/readfrom_linux_test.go

@@ -361,3 +361,35 @@ func (h *copyFileRangeHook) install() {
 func (h *copyFileRangeHook) uninstall() {
 	*PollCopyFileRangeP = h.original
 }
+
+// On some kernels copy_file_range fails on files in /proc.
+func TestProcCopy(t *testing.T) {
+	const cmdlineFile = "/proc/self/cmdline"
+	cmdline, err := os.ReadFile(cmdlineFile)
+	if err != nil {
+		t.Skipf("can't read /proc file: %v", err)
+	}
+	in, err := os.Open(cmdlineFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer in.Close()
+	outFile := filepath.Join(t.TempDir(), "cmdline")
+	out, err := os.Create(outFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := io.Copy(out, in); err != nil {
+		t.Fatal(err)
+	}
+	if err := out.Close(); err != nil {
+		t.Fatal(err)
+	}
+	copy, err := os.ReadFile(outFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(cmdline, copy) {
+		t.Errorf("copy of %q got %q want %q\n", cmdlineFile, copy, cmdline)
+	}
+}