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

> 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,

> 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());

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