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>

Makefile

@@ -2654,6 +2654,13 @@ compat/nedmalloc/nedmalloc.sp compat/nedmalloc/nedmalloc.o: EXTRA_CPPFLAGS = \
 compat/nedmalloc/nedmalloc.sp: SP_EXTRA_FLAGS += -Wno-non-pointer-null
 endif
 
+headless-git.o: compat/win32/headless.c GIT-CFLAGS
+	$(QUIET_CC)$(CC) $(ALL_CFLAGS) $(COMPAT_CFLAGS) \
+		-fno-stack-protector -o $@ -c -Wall -Wwrite-strings $<
+
+headless-git$X: headless-git.o git.res GIT-LDFLAGS
+	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) $(ALL_LDFLAGS) -mwindows -o $@ $< git.res
+
 git-%$X: %.o GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(LIBS)
 
@@ -3410,6 +3417,7 @@ clean: profile-clean coverage-clean cocciclean
 	$(RM) po/git.pot po/git-core.pot
 	$(RM) *.res
 	$(RM) $(OBJECTS)
+	$(RM) headless-git.o
 	$(RM) $(LIB_FILE) $(XDIFF_LIB) $(REFTABLE_LIB) $(REFTABLE_TEST_LIB)
 	$(RM) $(ALL_PROGRAMS) $(SCRIPT_LIB) $(BUILT_INS) git$X
 	$(RM) $(TEST_PROGRAMS)
@@ -3439,6 +3447,7 @@ endif
 	$(RM) GIT-SCRIPT-DEFINES GIT-PERL-DEFINES GIT-PERL-HEADER GIT-PYTHON-VARS
 ifdef MSVC
 	$(RM) $(patsubst %.o,%.o.pdb,$(OBJECTS))
+	$(RM) headless-git.o.pdb
 	$(RM) $(patsubst %.exe,%.pdb,$(OTHER_PROGRAMS))
 	$(RM) $(patsubst %.exe,%.ilk,$(OTHER_PROGRAMS))
 	$(RM) $(patsubst %.exe,%.iobj,$(OTHER_PROGRAMS))

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;
+}

config.mak.uname

@@ -527,6 +527,8 @@ else
 endif
 	X = .exe
 
+	EXTRA_PROGRAMS += headless-git$X
+
 compat/msvc.o: compat/msvc.c compat/mingw.c GIT-CFLAGS
 endif
 ifeq ($(uname_S),Interix)
@@ -680,6 +682,7 @@ ifeq ($(uname_S),MINGW)
 	RC = windres -O coff
 	NATIVE_CRLF = YesPlease
 	X = .exe
+	EXTRA_PROGRAMS += headless-git$X
 ifneq (,$(wildcard ../THIS_IS_MSYSGIT))
 	htmldir = doc/git/html/
 	prefix =

contrib/buildsystems/CMakeLists.txt

@@ -734,6 +734,15 @@ if(WIN32)
 	else()
 		message(FATAL_ERROR "Unhandled compiler: ${CMAKE_C_COMPILER_ID}")
 	endif()
+
+	add_executable(headless-git ${CMAKE_SOURCE_DIR}/compat/win32/headless.c)
+	if(CMAKE_C_COMPILER_ID STREQUAL "GNU" OR CMAKE_C_COMPILER_ID STREQUAL "Clang")
+		target_link_options(headless-git PUBLIC -municode -Wl,-subsystem,windows)
+	elseif(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
+		target_link_options(headless-git PUBLIC /NOLOGO /ENTRY:wWinMainCRTStartup /SUBSYSTEM:WINDOWS)
+	else()
+		message(FATAL_ERROR "Unhandled compiler: ${CMAKE_C_COMPILER_ID}")
+	endif()
 elseif(UNIX)
 	target_link_libraries(common-main pthread rt)
 endif()

contrib/buildsystems/Generators/Vcxproj.pm

@@ -76,7 +76,7 @@ sub createProject {
 
     my $libs_release = "\n    ";
     my $libs_debug = "\n    ";
-    if (!$static_library) {
+    if (!$static_library && $name ne 'headless-git') {
       $libs_release = join(";", sort(grep /^(?!libgit\.lib|xdiff\/lib\.lib|vcs-svn\/lib\.lib|reftable\/libreftable\.lib)/, @{$$build_structure{"$prefix${name}_LIBS"}}));
       $libs_debug = $libs_release;
       $libs_debug =~ s/zlib\.lib/zlibd\.lib/g;
@@ -254,7 +254,7 @@ EOM
     print F << "EOM";
   </ItemGroup>
 EOM
-    if (!$static_library || $target =~ 'vcs-svn' || $target =~ 'xdiff') {
+    if ((!$static_library || $target =~ 'vcs-svn' || $target =~ 'xdiff') && !($name =~ /headless-git/)) {
       my $uuid_libgit = $$build_structure{"LIBS_libgit_GUID"};
       my $uuid_libreftable = $$build_structure{"LIBS_reftable/libreftable_GUID"};
       my $uuid_xdiff_lib = $$build_structure{"LIBS_xdiff/lib_GUID"};

contrib/buildsystems/engine.pl

@@ -370,6 +370,7 @@ sub handleLinkLine
 #    exit(1);
     foreach (@objfiles) {
         my $sourcefile = $_;
+        $sourcefile =~ s/^headless-git\.o$/compat\/win32\/headless.c/;
         $sourcefile =~ s/\.o$/.c/;
         $sourcefile =~ s/\.res$/.rc/;
         push(@sources, $sourcefile);