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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>
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@ -112,7 +112,15 @@ func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err
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return 0, false, nil
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case nil:
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if n == 0 {
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// src is at EOF, which means we are done.
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// If we did not read any bytes at all,
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// then this file may be in a file system
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// where copy_file_range silently fails.
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// https://lore.kernel.org/linux-fsdevel/20210126233840.GG4626@dread.disaster.area/T/#m05753578c7f7882f6e9ffe01f981bc223edef2b0
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if written == 0 {
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return 0, false, nil
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}
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// Otherwise src is at EOF, which means
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// we are done.
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return written, true, nil
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}
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remain -= n
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@ -361,3 +361,35 @@ func (h *copyFileRangeHook) install() {
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func (h *copyFileRangeHook) uninstall() {
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*PollCopyFileRangeP = h.original
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}
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// On some kernels copy_file_range fails on files in /proc.
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func TestProcCopy(t *testing.T) {
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const cmdlineFile = "/proc/self/cmdline"
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cmdline, err := os.ReadFile(cmdlineFile)
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if err != nil {
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t.Skipf("can't read /proc file: %v", err)
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}
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in, err := os.Open(cmdlineFile)
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if err != nil {
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t.Fatal(err)
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}
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defer in.Close()
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outFile := filepath.Join(t.TempDir(), "cmdline")
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out, err := os.Create(outFile)
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if err != nil {
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t.Fatal(err)
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}
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if _, err := io.Copy(out, in); err != nil {
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t.Fatal(err)
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}
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if err := out.Close(); err != nil {
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t.Fatal(err)
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}
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copy, err := os.ReadFile(outFile)
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if err != nil {
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t.Fatal(err)
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}
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if !bytes.Equal(cmdline, copy) {
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t.Errorf("copy of %q got %q want %q\n", cmdlineFile, copy, cmdline)
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}
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}
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