ui: swallow EBADF on stderr
ui.write_err already swallows EPIPE and EIO if a write to stderr fails.
On Mac OS X at least, a write to a closed file descriptor results in
EBADF. Before this patch, hg would exit with status 1 if a write to
stderr failed during startup (e.g. while trying to print a warning about
not finding an extension):
$ ./hg --config extensions.foo= version 2>&-; echo $?
1
With this patch, it correctly swallows stderr and continues to run the
command:
$ ./hg --config extensions.foo= version 2>&-
Mercurial Distributed SCM (version 2.1)
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