• Ian Lance Taylor's avatar
    os: calling Fd disables the SetDeadline methods · 41534957
    Ian Lance Taylor authored
    The full truth seems too complicated to write in this method's doc, so
    I'm going with a simple half truth.
    
    The full truth is that Fd returns the descriptor in blocking mode,
    because that is historically how it worked, and existing programs
    would be surprised if the descriptor is suddenly non-blocking. On Unix
    systems whether a file is non-blocking or not is a property of the
    underlying file description, not of a particular file descriptor, so
    changing the returned descriptor to blocking mode also changes the
    existing File to blocking mode. Blocking mode works fine, althoug I/O
    operations now take up a thread. SetDeadline and friends rely on the
    runtime poller, and the runtime poller only works if the descriptor is
    non-blocking. So it's correct that calling Fd disables SetDeadline.
    The other half of the truth is that if the program is willing to work
    with a non-blocking descriptor, it could call
    syscall.SetNonblock(descriptor, true) to change the descriptor, and
    the original File, to non-blocking mode. At that point SetDeadline
    would start working again. I tried to write that in a way that is
    short and comprehensible but failed. Since deadlines mostly work on
    pipes, and there isn't much reason to call Fd on a pipe, and few
    people use SetDeadline, I decided to punt.
    
    Fixes #22934
    
    Change-Id: I2e49e036f0bcf71f5365193831696f9e4120527c
    Reviewed-on: https://go-review.googlesource.com/81636Reviewed-by: 's avatarBrad Fitzpatrick <bradfitz@golang.org>
    41534957
file_plan9.go 13.6 KB