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os: document and emphasize a potential misuse of File.Fd

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This CL revises the document of File.Fd that explicitly points
its user to runtime.SetFinalizer where contains the information
that a file descriptor could be closed in a finalizer and therefore
causes a failure in syscall.Write if runtime.KeepAlive is not invoked.

The CL also suggests an alternative of File.Fd towards File.SyscallConn.

Fixes #41505

Change-Id: I6816f0157add48b649bf1fb793cf19dcea6894b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/256899
Reviewed-by: Rob Pike <r@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Changkun Ou 2020-09-24 08:57:00 +02:00 • committed by Rob Pike
parent 6f02578f9c
commit 7bb6fed9b5
4 changed files with 28 additions and 15 deletions
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9
src/os/file_unix.go
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@ -62,8 +62,13 @@ type file struct {
}
// Fd returns the integer Unix file descriptor referencing the open file.
// The file descriptor is valid only until f.Close is called or f is garbage collected.
// On Unix systems this will cause the SetDeadline methods to stop working.
// If f is closed, the file descriptor becomes invalid.
// If f is garbage collected, a finalizer may close the file descriptor,
// making it invalid; see runtime.SetFinalizer for more information on when
// a finalizer might be run. On Unix systems this will cause the SetDeadline
// methods to stop working.
//
// As an alternative, see the f.SyscallCon method.
func (f *File) Fd() uintptr {
if f == nil {
return ^(uintptr(0))