[PATCH 1/3] mingw: add experimental feature to redirect standard handles

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

On Windows, file handles need to be marked inheritable when they need to
be used as standard input/output/error handles for a newly spawned
process. The problem with that, of course, is that the "inheritable" flag
is global and therefore can wreak havoc with highly multi-threaded
applications: other spawned processes will *also* inherit those file
handles, despite having *other* input/output/error handles, and never
close the former handles because they do not know about them.

Let's introduce a set of environment variables (GIT_REDIRECT_STDIN and
friends) that point to files, or even better, named pipes and that are
used by the spawned Git process. This helps work around above-mentioned
issue: those named pipes will be opened in a non-inheritable way upon
startup, and no handles are passed around.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 compat/mingw.c  | 43 +++++++++++++++++++++++++++++++++++++++++++
 t/t0001-init.sh |  6 ++++++
 2 files changed, 49 insertions(+)

diff --git a/compat/mingw.c b/compat/mingw.c
index 8b6fa0db446..6c6c7795a70 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -2139,6 +2139,47 @@ 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_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;
+	}
+	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;
@@ -2146,6 +2187,8 @@ 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)
diff --git a/t/t0001-init.sh b/t/t0001-init.sh
index 86c1a51654f..0fd2fc45385 100755
--- a/t/t0001-init.sh
+++ b/t/t0001-init.sh
@@ -453,4 +453,10 @@ 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_done
-- 
2.15.0.windows.1