Brown-bag fix on top of js/mingw-inherit-only-std-handles

Johannes "Hannes" Sixt pointed out that this patch series (which already made it to next) introduces a problem: when falling back to spawning the process without restricting the file handles, errno is not set accurately.

Sadly, the regression test failure observed by Hannes did not show up over here, nor in the PR builds (or, for that matter, the literally hundreds of CI builds that patch series underwent as part of Git for Windows' master branch). The cause was that errno is set to the expected ENOENT by another part of the code, too, that happens right before the calls to CreateProcessW(): the test whether a given path refers to a script that needs to be executed via an interpreter (such as sh.exe) obviously needs to open the file, and that obviously fails, setting errno = ENOENT!

I have currently no idea what function call could be responsible for re-setting errno to the reported ERANGE... But at least over here, when I partially apply this patch, the part that sets errno = 0;, t0061.2 fails for me, too.

Change since v2:

* We no longer map ERROR_SUCCESS to 0, but instead raise a bug message when the code is asked to handle the "No error" error.

Changes since v1:

* A copy/edit fail in the commit message was fixed.
* We now assign errno only when the call to CreateProcessW() failed.
* For good measure, we teach the err_win_to_posix() function to translate ERROR_SUCCESS into the errno value 0.

Johannes Schindelin (2):
mingw: do set errno correctly when trying to restrict handle
inheritance
mingw: translate ERROR_SUCCESS to errno = 0

 compat/mingw.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

base-commit: ac33519ddfa818f420b4ef5a09b4a7b3ac8adeb8
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-480%2Fdscho%2Fmingw-inherit-only-std-handles-set-errno-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-480/dscho/mingw-inherit-only-std-handles-set-errno-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/480

Range-diff vs v2:

 1:  280b6d08a4 = 1:  280b6d08a4 mingw: do set `errno` correctly when trying to restrict handle inheritance
 2:  c3dea00fb1 ! 2:  e04e1269b3 mingw: translate ERROR_SUCCESS to errno = 0

@@ -3,12 +3,15 @@
mingw: translate ERROR_SUCCESS to errno = 0

Johannes Sixt pointed out that the err_win_to_posix() function
- mishandles ERROR_SUCCESS. This commit fixes that.
+ mishandles ERROR_SUCCESS: it maps it to ENOSYS.

- Technically, we try to only assign errno to the corresponding value of
- GetLastError() (which translation is performed by that function) when
- a Win32 API call failed, so this change is purely defensive and is not
- expected to fix an existing bug in our code base.
+ The only purpose of this function is to map Win32 API errors to errno
+ ones, and there is actually no equivalent to ERROR_SUCCESS: the idea
+ of errno is that it will only be set in case of an error, and left
+ alone in case of success.
+
+ Therefore, as pointed out by Junio Hamano, it is a bug to call this
+ function when there was not even any error to map.

@@ -19,7 +22,7 @@
case ERROR_SHARING_BUFFER_EXCEEDED: error = ENFILE; break; case ERROR_SHARING_VIOLATION: error = EACCES; break; case ERROR_STACK_OVERFLOW: error = ENOMEM; break;
-+ case ERROR_SUCCESS: error = 0; break;
++ case ERROR_SUCCESS: BUG("err_win_to_posix() called without an error!");
case ERROR_SWAPERROR: error = ENOENT; break; case ERROR_TOO_MANY_MODULES: error = EMFILE; break; case ERROR_TOO_MANY_OPEN_FILES: error = EMFILE; break;

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