Re: [PATCH 1/1] mingw: do set `errno` correctly when trying to restrict handle inheritance

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Hannes,

On Sat, 30 Nov 2019, Johannes Sixt wrote:

> Am 29.11.19 um 22:44 schrieb Johannes Schindelin via GitGitGadget:
> > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> >
> > In 9a780a384de (mingw: spawned processes need to inherit only standard
> > handles, 2019-11-22), we taught the Windows-specific part to restrict
> > which file handles are passed on to the spawned processes.
> >
> > Since this logic seemed to be a bit fragile across Windows versions (we
> > _still_ support Windows Vista in Git for Windows, for example), a
> > fall-back was added to try spawning the process again, this time without
> > restricting which file handles are to be inherited by the spawned
> > process.
> >
> > In the common case (i.e. when the process could not be spawned for
> > reasons _other_ than the file handle inheritance), the fall-back attempt
> > would still fail, of course.
> >
> > Crucially, one thing we missed in that code path was to set `errno`
> > appropriately.
> >
> > This should have been caught by t0061.2 which expected `errno` to be
> > `ENOENT` after trying to start a process for a non-existing executable,
> > but `errno` was set to `ENOENT` prior to the `CreateProcessW()` call:
> > while looking for the config settings for trace2, Git tries to access
> > `xdg_config` and `user_config` via `access_or_die()`, and as neither of
> > those config files exists when running the test case (because in Git's
> > test suite, `HOME` points to the test directory), the `errno` has the
> > expected value, but for the wrong reasons.
> >
> > Let's fix that by making sure that `errno` is set correctly.
> >
> > It would be nice if we could somehow fix t0061 to make sure that this
> > does not regress again. One approach that seemed like it should work,
> > but did not, was to set `errno` to 0 in the test helper that is used by
> > t0061.2.
> >
> > However, when `mingw_spawnvpe()` wants to see whether the file in
> > question is a script, it calls `parse_interpreter()`, which in turn
> > tries to `open()` the file.0/compat/mingw.c#L1134. Obviously,
>
> Copy-and-paste garbage?

Yes :-(

> > this call fails, and sets `errno` to `ENOENT`, deep inside the call
> > chain started from that test helper.
> >
> > Instead, we force re-set `errno` at the beginning of the function
> > `mingw_spawnve_fd()`, which _should_ be safe given that callers of that
> > function will want to look at `errno` if -1 was returned. And if that
> > `errno` is 0 ("No error"), regression tests like t0061.2 will kick in.
> >
> > Reported-by: Johannes Sixt <j6t@kdbg.org>
> > Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> > ---
> >  compat/mingw.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/compat/mingw.c b/compat/mingw.c
> > index 2b6eca2f56..bb4eb4211a 100644
> > --- a/compat/mingw.c
> > +++ b/compat/mingw.c
> > @@ -1423,6 +1423,9 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
> >  	const char *(*quote_arg)(const char *arg) =
> >  		is_msys2_sh(*argv) ? quote_arg_msys2 : quote_arg_msvc;
> >
> > +	/* Make sure to override previous errors, if any */
> > +	errno = 0;
> > +
> >  	if (restrict_handle_inheritance < 0)
> >  		restrict_handle_inheritance = core_restrict_inherited_handles;
> >  	/*
> > @@ -1580,8 +1583,8 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
> >  		ret = CreateProcessW(*wcmd ? wcmd : NULL, wargs, NULL, NULL,
> >  				     TRUE, flags, wenvblk, dir ? wdir : NULL,
> >  				     &si.StartupInfo, &pi);
> > +		errno = err_win_to_posix(GetLastError());
>
> I think this should be protected by 'if (!ret)' because
> err_win_to_posix() does not handle ERROR_SUCCESS and turns it into
> ENOSYS. It's not that bad because in the case of success we do not
> guarantee any value of errno anyway.

Ah, that's the reason!

Are you building with `NO_GETTEXT` perchance? I ask because gettext
overrides `vsnprintf()` with their own version, which is obviously quite
different from the version MSVCRT provides... and the former version is
what I use, and what all those CI/PR builds use.

> >  		if (ret && buf.len) {
> > -			errno = err_win_to_posix(GetLastError());
> >  			warning("failed to restrict file handles (%ld)\n\n%s",
> >  				err, buf.buf);
> >  		}
> >
>
> That said, this fixes the failure.

Thanks,
Dscho