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os: reject WriteAt if file opened in append mode

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WriteAt use pwrite syscall on *nix or WriteFile on Windows.

On Linux/Windows, these system calls always write to end of file in
append mode, regardless of offset parameter.

It is hard (maybe impossible) to make WriteAt work portably.

Making WriteAt returns an error if file is opened in append mode, we
guarantee to get consistent behavior between platforms, also prevent
user from accidently corrupting their data.

Fixes #30716

Change-Id: If83d935a22a29eed2ff8fe53d13d0b4798aa2b81
Reviewed-on: https://go-review.googlesource.com/c/go/+/166578
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
LE Manh Cuong 2019-03-11 13:58:20 +07:00 • committed by Ian Lance Taylor
parent 5ee2290420
commit c7f7f59368
6 changed files with 39 additions and 7 deletions
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1
src/os/file_unix.go
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@ -52,6 +52,7 @@ type file struct {
dirinfo *dirInfo // nil unless directory being read
nonblock bool // whether we set nonblocking mode
stdoutOrErr bool // whether this is stdout or stderr
appendMode bool // whether file is opened for appending
}
// Fd returns the integer Unix file descriptor referencing the open file.