Merge branch 'redirect-std-handles'

This topic branch introduces a highly-experimental feature allowing to
override stdin/stdout/stderr by setting environment variables e.g. to
named pipes, solving a problem in highly multi-threaded applications
where inheritable handles could cause blocked Git operations.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/git.txt

@@ -709,6 +709,23 @@ of clones and fetches.
 	the background which do not want to cause lock contention with
 	other operations on the repository.  Defaults to `1`.
 
+`GIT_REDIRECT_STDIN`::
+`GIT_REDIRECT_STDOUT`::
+`GIT_REDIRECT_STDERR`::
+	(EXPERIMENTAL) Windows-only: allow redirecting the standard
+	input/output/error handles. This is particularly useful in
+	multi-threaded applications where the canonical way to pass
+	standard handles via `CreateProcess()` is not an option because
+	it would require the handles to be marked inheritable (and
+	consequently *every* spawned process would inherit them, possibly
+	blocking regular Git operations). The primary intended use case
+	is to use named pipes for communication.
++
+Two special values are supported: `off` will simply close the
+corresponding standard handle, and if `GIT_REDIRECT_STDERR` is
+`2>&1`, standard error will be redirected to the same handle as
+standard output.
+
 Discussion[[Discussion]]
 ------------------------
 

compat/mingw.c

@@ -2695,6 +2695,62 @@ static char *wcstoutfdup_startup(char *buffer, const wchar_t *wcs, size_t len)
 	return memcpy(malloc_startup(len), buffer, len);
 }
 
+static void maybe_redirect_std_handle(const wchar_t *key, DWORD std_id, int fd,
+				      DWORD desired_access, DWORD flags)
+{
+	DWORD create_flag = fd ? OPEN_ALWAYS : OPEN_EXISTING;
+	wchar_t buf[MAX_LONG_PATH];
+	DWORD max = ARRAY_SIZE(buf);
+	HANDLE handle;
+	DWORD ret = GetEnvironmentVariableW(key, buf, max);
+
+	if (!ret || ret >= max)
+		return;
+
+	/* make sure this does not leak into child processes */
+	SetEnvironmentVariableW(key, NULL);
+	if (!wcscmp(buf, L"off")) {
+		close(fd);
+		handle = GetStdHandle(std_id);
+		if (handle != INVALID_HANDLE_VALUE)
+			CloseHandle(handle);
+		return;
+	}
+	if (std_id == STD_ERROR_HANDLE && !wcscmp(buf, L"2>&1")) {
+		handle = GetStdHandle(STD_OUTPUT_HANDLE);
+		if (handle == INVALID_HANDLE_VALUE) {
+			close(fd);
+			handle = GetStdHandle(std_id);
+			if (handle != INVALID_HANDLE_VALUE)
+				CloseHandle(handle);
+		} else {
+			int new_fd = _open_osfhandle((intptr_t)handle, O_BINARY);
+			SetStdHandle(std_id, handle);
+			dup2(new_fd, fd);
+			/* do *not* close the new_fd: that would close stdout */
+		}
+		return;
+	}
+	handle = CreateFileW(buf, desired_access, 0, NULL, create_flag,
+			     flags, NULL);
+	if (handle != INVALID_HANDLE_VALUE) {
+		int new_fd = _open_osfhandle((intptr_t)handle, O_BINARY);
+		SetStdHandle(std_id, handle);
+		dup2(new_fd, fd);
+		close(new_fd);
+	}
+}
+
+static void maybe_redirect_std_handles(void)
+{
+	maybe_redirect_std_handle(L"GIT_REDIRECT_STDIN", STD_INPUT_HANDLE, 0,
+				  GENERIC_READ, FILE_ATTRIBUTE_NORMAL);
+	maybe_redirect_std_handle(L"GIT_REDIRECT_STDOUT", STD_OUTPUT_HANDLE, 1,
+				  GENERIC_WRITE, FILE_ATTRIBUTE_NORMAL);
+	maybe_redirect_std_handle(L"GIT_REDIRECT_STDERR", STD_ERROR_HANDLE, 2,
+				  GENERIC_WRITE, FILE_FLAG_NO_BUFFERING);
+}
+
 void mingw_startup(void)
 {
 	int i, maxlen, argc;
@@ -2702,6 +2758,8 @@ void mingw_startup(void)
 	wchar_t **wenv, **wargv;
 	_startupinfo si;
 
+	maybe_redirect_std_handles();
+
 	/* get wide char arguments and environment */
 	si.newmode = 0;
 	if (__wgetmainargs(&argc, &wargv, &wenv, _CRT_glob, &si) < 0)

t/t0001-init.sh

@@ -453,4 +453,16 @@ test_expect_success 're-init from a linked worktree' '
 	)
 '
 
+test_expect_success MINGW 'redirect std handles' '
+	GIT_REDIRECT_STDOUT=output.txt git rev-parse --git-dir &&
+	test .git = "$(cat output.txt)" &&
+	test -z "$(GIT_REDIRECT_STDOUT=off git rev-parse --git-dir)" &&
+	test_must_fail env \
+		GIT_REDIRECT_STDOUT=output.txt \
+		GIT_REDIRECT_STDERR="2>&1" \
+		git rev-parse --git-dir --verify refs/invalid &&
+	printf ".git\nfatal: Needed a single revision\n" >expect &&
+	test_cmp expect output.txt
+'
+
 test_done