win32: add a helper to run `git.exe` without a foreground window

On Windows, there are two kinds of executables, console ones and
non-console ones. Git's executables are all console ones.

When launching the former e.g. in a scheduled task, a CMD window pops
up. This is not what we want for the tasks installed via the `git
maintenance` command.

To work around this, let's introduce `headless-git.exe`, which is a
non-console program that does _not_ pop up any window. All it does is to
re-launch `git.exe`, suppressing that console window, passing through
all command-line arguments as-are.

Helped-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Helped-by: Yuyi Wang <Strawberry_Str@hotmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 115 insertions, 0 deletions

diff --git a/compat/win32/headless.c b/compat/win32/headless.c
new file mode 100644
index 0000000000..8b00dfe3bd
--- /dev/null
+++ b/compat/win32/headless.c
@@ -0,0 +1,115 @@
+/*
+ * headless Git - run Git without opening a console window on Windows
+ */
+
+#define STRICT
+#define WIN32_LEAN_AND_MEAN
+#define UNICODE
+#define _UNICODE
+#include <windows.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <wchar.h>
+
+/*
+ * If `dir` contains the path to a Git exec directory, extend `PATH` to
+ * include the corresponding `bin/` directory (which is where all those
+ * `.dll` files needed by `git.exe` are, on Windows).
+ */
+static int extend_path(wchar_t *dir, size_t dir_len)
+{
+	const wchar_t *suffix = L"\\libexec\\git-core";
+	size_t suffix_len = wcslen(suffix);
+	wchar_t *env;
+	DWORD len;
+
+	if (dir_len < suffix_len)
+		return 0;
+
+	dir_len -= suffix_len;
+	if (memcmp(dir + dir_len, suffix, suffix_len * sizeof(wchar_t)))
+		return 0;
+
+	len = GetEnvironmentVariableW(L"PATH", NULL, 0);
+	if (!len)
+		return 0;
+
+	env = _alloca((dir_len + 5 + len) * sizeof(wchar_t));
+	wcsncpy(env, dir, dir_len);
+	wcscpy(env + dir_len, L"\\bin;");
+	if (!GetEnvironmentVariableW(L"PATH", env + dir_len + 5, len))
+		return 0;
+
+	SetEnvironmentVariableW(L"PATH", env);
+	return 1;
+}
+
+int WINAPI wWinMain(_In_ HINSTANCE instance,
+		    _In_opt_ HINSTANCE previous_instance,
+		    _In_ LPWSTR command_line, _In_ int show)
+{
+	wchar_t git_command_line[32768];
+	size_t size = sizeof(git_command_line) / sizeof(wchar_t);
+	const wchar_t *needs_quotes = L"";
+	int slash = 0, i;
+
+	STARTUPINFO startup_info = {
+		.cb = sizeof(STARTUPINFO),
+		.dwFlags = STARTF_USESHOWWINDOW,
+		.wShowWindow = SW_HIDE,
+	};
+	PROCESS_INFORMATION process_info = { 0 };
+	DWORD creation_flags = CREATE_UNICODE_ENVIRONMENT |
+		CREATE_NEW_CONSOLE | CREATE_NO_WINDOW;
+	DWORD exit_code;
+
+	/* First, determine the full path of argv[0] */
+	for (i = 0; _wpgmptr[i]; i++)
+		if (_wpgmptr[i] == L' ')
+			needs_quotes = L"\"";
+		else if (_wpgmptr[i] == L'\\')
+			slash = i;
+
+	if (slash >= size - 11)
+		return 127; /* Too long path */
+
+	/* If it is in Git's exec path, add the bin/ directory to the PATH */
+	extend_path(_wpgmptr, slash);
+
+	/* Then, add the full path of `git.exe` as argv[0] */
+	i = swprintf_s(git_command_line, size, L"%ls%.*ls\\git.exe%ls",
+		       needs_quotes, slash, _wpgmptr, needs_quotes);
+	if (i < 0)
+		return 127; /* Too long path */
+
+	if (*command_line) {
+		/* Now, append the command-line arguments */
+		i = swprintf_s(git_command_line + i, size - i,
+			       L" %ls", command_line);
+		if (i < 0)
+			return 127;
+	}
+
+	startup_info.hStdInput = GetStdHandle(STD_INPUT_HANDLE);
+	startup_info.hStdOutput = GetStdHandle(STD_OUTPUT_HANDLE);
+	startup_info.hStdError = GetStdHandle(STD_ERROR_HANDLE);
+
+	if (!CreateProcess(NULL, /* infer argv[0] from the command line */
+			   git_command_line, /* modified command line */
+			   NULL, /* inherit process handles? */
+			   NULL, /* inherit thread handles? */
+			   FALSE, /* handles inheritable? */
+			   creation_flags,
+			   NULL, /* use this process' environment */
+			   NULL, /* use this process' working directory */
+			   &startup_info, &process_info))
+		return 129; /* could not start */
+	WaitForSingleObject(process_info.hProcess, INFINITE);
+	if (!GetExitCodeProcess(process_info.hProcess, &exit_code))
+		exit_code = 130; /* Could not determine exit code? */
+
+	CloseHandle(process_info.hProcess);
+	CloseHandle(process_info.hThread);
+
+	return (int)exit_code;
+}