run-command: add an opt-in posix_spawn backend on macOS

Platforms that can start child programs but cannot duplicate the parent
process cannot use the POSIX start_command() path. It always calls
fork(), even when the platform supplies a spawn API.

Add USE_POSIX_SPAWN as an opt-in macOS prototype. Keep the existing fork
backend as the default. Reuse command preparation, environment deltas
and child cleanup, and translate the child setup into spawn file
actions and attributes. Preserve the ENOEXEC shell fallback and the
synchronous error reporting expected by start_command() callers.

The macOS implementation provides a concrete place to discuss the
backend boundary before adding another target. It relies on macOS
spawn extensions for cwd and descriptor inheritance. It is not a WASM
port: NO_PTHREADS start_async() still calls fork(), and no wasm child
process has been run with this change.

The RFC question is whether a posix_spawn backend is the right direction
for platforms without fork, or whether Git would prefer a small platform
hook like the existing Windows launch path.

With the opt-in backend, t0061 preloads a macOS test library that makes
fork() return ENOSYS. A canary requires that failure before the launch,
cwd, environment, pipe, error and fallback cases run. The tests call
binaries directly because macOS strips DYLD_* from shell wrappers.

On macOS arm64, DEVELOPER=1 builds and t0061 pass with both backends:
26 cases, with three MINGW skips for spawn and an additional canary
skip for the default backend. Without the test library, the canary
fails as expected. A separate external-alias control also fails on the
default backend when fork() returns ENOSYS.

AI assistance: GPT-6.1 Sol and GPT-6 Luna through the Codex harness.

Signed-off-by: Yannic Charlon <justyannicc.8tlny@simplelogin.com>

Author: Yannic Charlon

GIT-BUILD-OPTIONS.in

@@ -44,5 +44,6 @@ TEST_OUTPUT_DIRECTORY=@TEST_OUTPUT_DIRECTORY@
 TEST_SHELL_PATH=@TEST_SHELL_PATH@
 USE_GETTEXT_SCHEME=@USE_GETTEXT_SCHEME@
 USE_LIBPCRE2=@USE_LIBPCRE2@
+USE_POSIX_SPAWN=@USE_POSIX_SPAWN@
 WITH_BREAKING_CHANGES=@WITH_BREAKING_CHANGES@
 X=@X@

Makefile

@@ -149,6 +149,9 @@ include shared.mak
 #
 # Define NO_PTHREADS if you do not have or do not want to use Pthreads.
 #
+# Define USE_POSIX_SPAWN to use posix_spawn() for start_command().  This is
+# currently supported on macOS only.
+#
 # Define NO_PREAD if you have a problem with pread() system call (e.g.
 # cygwin1.dll before v1.5.22).
 #
@@ -889,6 +892,10 @@ TEST_PROGRAMS_NEED_X += test-tool
 
 TEST_PROGRAMS = $(patsubst %,t/helper/%$X,$(TEST_PROGRAMS_NEED_X))
 
+ifdef USE_POSIX_SPAWN
+TEST_HELPER_LIBS += t/helper/test-fork-interposer.dylib
+endif
+
 # List built-in command $C whose implementation cmd_$C() is not in
 # builtin/$C.o but is linked in as part of some other command.
 BUILT_INS += $(patsubst builtin/%.o,git-%$X,$(BUILTIN_OBJS))
@@ -2247,6 +2254,10 @@ else
 	EXTLIBS += $(PTHREAD_LIBS)
 endif
 
+ifdef USE_POSIX_SPAWN
+	BASIC_CFLAGS += -DUSE_POSIX_SPAWN
+endif
+
 ifdef HAVE_PATHS_H
 	BASIC_CFLAGS += -DHAVE_PATHS_H
 endif
@@ -3387,6 +3398,7 @@ GIT-BUILD-OPTIONS: FORCE
 		-e "s|@TEST_SHELL_PATH@|\'$(TEST_SHELL_PATH_SQ)\'|" \
 		-e "s|@USE_GETTEXT_SCHEME@|\'$(USE_GETTEXT_SCHEME)\'|" \
 		-e "s|@USE_LIBPCRE2@|\'$(USE_LIBPCRE2)\'|" \
+		-e "s|@USE_POSIX_SPAWN@|\'$(USE_POSIX_SPAWN)\'|" \
 		-e "s|@WITH_BREAKING_CHANGES@|\'$(WITH_BREAKING_CHANGES)\'|" \
 		-e "s|@X@|\'$(X)\'|" \
 		GIT-BUILD-OPTIONS.in >$@+
@@ -3408,7 +3420,7 @@ endif
 
 test_bindir_programs := $(patsubst %,bin-wrappers/%,$(BINDIR_PROGRAMS_NEED_X) $(BINDIR_PROGRAMS_NO_X) $(TEST_PROGRAMS_NEED_X))
 
-all:: $(TEST_PROGRAMS) $(test_bindir_programs) $(UNIT_TEST_PROGS) $(CLAR_TEST_PROG)
+all:: $(TEST_PROGRAMS) $(TEST_HELPER_LIBS) $(test_bindir_programs) $(UNIT_TEST_PROGS) $(CLAR_TEST_PROG)
 
 $(test_bindir_programs): bin-wrappers/%: bin-wrappers/wrap-for-bin.sh
 	$(QUIET_GEN)sed -e '1s|#!.*/sh|#!$(SHELL_PATH_SQ)|' \
@@ -3442,11 +3454,16 @@ perf: all
 
 .PRECIOUS: $(TEST_OBJS)
 
-t/helper/test-tool$X: $(patsubst %,t/helper/%,$(TEST_BUILTINS_OBJS)) $(UNIT_TEST_DIR)/test-lib.o
+t/helper/test-tool$X: $(patsubst %,t/helper/%,$(TEST_BUILTINS_OBJS)) $(UNIT_TEST_DIR)/test-lib.o $(TEST_HELPER_LIBS)
 
 t/helper/test-%$X: t/helper/test-%.o GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(filter %.a,$^) $(LIBS)
 
+ifdef USE_POSIX_SPAWN
+t/helper/test-fork-interposer.dylib: t/helper/test-fork-interposer.c
+	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -dynamiclib -o $@ $(ALL_LDFLAGS) $<
+endif
+
 check-sha1:: t/helper/test-tool$X
 	t/helper/test-sha1.sh
 
@@ -3918,7 +3935,7 @@ clean: profile-clean coverage-clean cocciclean
 	$(RM) headless-git.o
 	$(RM) $(LIB_FILE)
 	$(RM) $(ALL_PROGRAMS) $(SCRIPT_LIB) $(BUILT_INS) $(OTHER_PROGRAMS)
-	$(RM) $(TEST_PROGRAMS)
+	$(RM) $(TEST_PROGRAMS) t/helper/test-fork-interposer.dylib
 	$(RM) $(FUZZ_PROGRAMS)
 	$(RM) $(SP_OBJ)
 	$(RM) $(HCC)

run-command.c

@@ -18,6 +18,28 @@
 #include "packfile.h"
 #include "compat/nonblock.h"
 
+#ifdef USE_POSIX_SPAWN
+#include <spawn.h>
+#include <AvailabilityMacros.h>
+
+#ifndef __APPLE__
+#error "USE_POSIX_SPAWN currently requires the macOS spawn file actions"
+#endif
+
+static int spawn_add_fchdir(posix_spawn_file_actions_t *actions, int fd)
+{
+#if MAC_OS_X_VERSION_MIN_REQUIRED >= 260000
+	return posix_spawn_file_actions_addfchdir(actions, fd);
+#else
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+	int error = posix_spawn_file_actions_addfchdir_np(actions, fd);
+#pragma clang diagnostic pop
+	return error;
+#endif
+}
+#endif
+
 void child_process_init(struct child_process *child)
 {
 	struct child_process blank = CHILD_PROCESS_INIT;
@@ -309,7 +331,9 @@ static const char **prepare_shell_cmd(struct strvec *out, const char **argv)
 }
 
 #ifndef GIT_WINDOWS_NATIVE
+#ifndef USE_POSIX_SPAWN
 static int child_notifier = -1;
+#endif
 
 enum child_errcode {
 	CHILD_ERR_CHDIR,
@@ -325,6 +349,7 @@ struct child_err {
 	int syserr; /* errno */
 };
 
+#ifndef USE_POSIX_SPAWN
 static void child_die(enum child_errcode err)
 {
 	struct child_err buf;
@@ -373,6 +398,7 @@ static void NORETURN child_die_fn(const char *err UNUSED, va_list params UNUSED)
 	xwrite(2, msg, sizeof(msg) - 1);
 	_exit(2);
 }
+#endif
 
 /* this runs in the parent process */
 static void child_err_spew(struct child_process *cmd, struct child_err *cerr)
@@ -496,6 +522,189 @@ static char **prep_childenv(const char *const *deltaenv)
 	return childenv;
 }
 
+#ifdef USE_POSIX_SPAWN
+static int spawn_add_dup2(posix_spawn_file_actions_t *actions, int from,
+			  int to, unsigned char stdio_touched[3])
+{
+	int error = posix_spawn_file_actions_adddup2(actions, from, to);
+	if (!error && to < 3)
+		stdio_touched[to] = 1;
+	return error;
+}
+
+static int spawn_add_close(posix_spawn_file_actions_t *actions, int fd,
+			   unsigned char stdio_touched[3])
+{
+	int error = posix_spawn_file_actions_addclose(actions, fd);
+	if (!error && fd < 3)
+		stdio_touched[fd] = 1;
+	return error;
+}
+
+static int spawn_add_open(posix_spawn_file_actions_t *actions, int fd,
+			  unsigned char stdio_touched[3])
+{
+	int error = posix_spawn_file_actions_addopen(actions, fd, "/dev/null",
+						     O_RDWR, 0);
+	if (!error && fd < 3)
+		stdio_touched[fd] = 1;
+	return error;
+}
+
+static pid_t spawn_command(const struct child_process *cmd, const char **argv,
+			   char **childenv, int need_in, int fdin[2],
+			   int need_out, int fdout[2],
+			   int need_err, int fderr[2],
+			   enum child_errcode *error_kind)
+{
+	posix_spawn_file_actions_t actions;
+	posix_spawnattr_t attr;
+	const posix_spawnattr_t *attrp = NULL;
+	unsigned char stdio_touched[3] = { 0 };
+	int actions_initialized = 0, attr_initialized = 0;
+	sigset_t signal_mask, signal_defaults;
+	struct sigaction sa;
+	short flags = POSIX_SPAWN_SETSIGMASK | POSIX_SPAWN_SETSIGDEF;
+	int error;
+	pid_t pid = -1;
+	int dir_fd = -1;
+	int sig, fd;
+
+	*error_kind = CHILD_ERR_ERRNO;
+
+#define ADD_SPAWN_ACTION(action) \
+	do { \
+		error = (action); \
+		if (error) \
+			goto done; \
+	} while (0)
+
+	error = posix_spawn_file_actions_init(&actions);
+	if (error)
+		goto done;
+	actions_initialized = 1;
+	error = posix_spawnattr_init(&attr);
+	if (error)
+		goto done;
+	attr_initialized = 1;
+	attrp = &attr;
+
+#ifdef NO_PTHREADS
+	if (sigprocmask(SIG_SETMASK, NULL, &signal_mask) < 0) {
+		error = errno;
+		goto done;
+	}
+#else
+	error = pthread_sigmask(SIG_SETMASK, NULL, &signal_mask);
+	if (error)
+		goto done;
+#endif
+	if (sigemptyset(&signal_defaults) < 0) {
+		error = errno;
+		goto done;
+	}
+	for (sig = 1; sig < NSIG; sig++) {
+		if (!sigaction(sig, NULL, &sa) && sa.sa_handler != SIG_IGN)
+			sigaddset(&signal_defaults, sig);
+	}
+	ADD_SPAWN_ACTION(posix_spawnattr_setsigmask(&attr, &signal_mask));
+	ADD_SPAWN_ACTION(posix_spawnattr_setsigdefault(&attr,
+							 &signal_defaults));
+
+	if (cmd->no_stdin)
+		ADD_SPAWN_ACTION(spawn_add_open(&actions, 0, stdio_touched));
+	else if (need_in) {
+		ADD_SPAWN_ACTION(spawn_add_dup2(&actions, fdin[0], 0,
+						 stdio_touched));
+		ADD_SPAWN_ACTION(spawn_add_close(&actions, fdin[0], stdio_touched));
+		ADD_SPAWN_ACTION(spawn_add_close(&actions, fdin[1], stdio_touched));
+	} else if (cmd->in) {
+		ADD_SPAWN_ACTION(spawn_add_dup2(&actions, cmd->in, 0,
+						 stdio_touched));
+		ADD_SPAWN_ACTION(spawn_add_close(&actions, cmd->in, stdio_touched));
+	}
+
+	if (cmd->no_stderr)
+		ADD_SPAWN_ACTION(spawn_add_open(&actions, 2, stdio_touched));
+	else if (need_err) {
+		ADD_SPAWN_ACTION(spawn_add_dup2(&actions, fderr[1], 2,
+						 stdio_touched));
+		ADD_SPAWN_ACTION(spawn_add_close(&actions, fderr[0], stdio_touched));
+		ADD_SPAWN_ACTION(spawn_add_close(&actions, fderr[1], stdio_touched));
+	} else if (cmd->err > 1) {
+		ADD_SPAWN_ACTION(spawn_add_dup2(&actions, cmd->err, 2,
+						 stdio_touched));
+		ADD_SPAWN_ACTION(spawn_add_close(&actions, cmd->err, stdio_touched));
+	}
+
+	if (cmd->no_stdout)
+		ADD_SPAWN_ACTION(spawn_add_open(&actions, 1, stdio_touched));
+	else if (cmd->stdout_to_stderr)
+		ADD_SPAWN_ACTION(spawn_add_dup2(&actions, 2, 1, stdio_touched));
+	else if (need_out) {
+		ADD_SPAWN_ACTION(spawn_add_dup2(&actions, fdout[1], 1,
+						 stdio_touched));
+		ADD_SPAWN_ACTION(spawn_add_close(&actions, fdout[0], stdio_touched));
+		ADD_SPAWN_ACTION(spawn_add_close(&actions, fdout[1], stdio_touched));
+	} else if (cmd->out > 1) {
+		ADD_SPAWN_ACTION(spawn_add_dup2(&actions, cmd->out, 1,
+						 stdio_touched));
+		ADD_SPAWN_ACTION(spawn_add_close(&actions, cmd->out, stdio_touched));
+	}
+
+	if (cmd->dir) {
+		dir_fd = open(cmd->dir, O_SEARCH | O_CLOEXEC);
+		if (dir_fd < 0) {
+			*error_kind = CHILD_ERR_CHDIR;
+			error = errno;
+			goto done;
+		}
+		ADD_SPAWN_ACTION(posix_spawn_file_actions_addinherit_np(
+					&actions, dir_fd));
+		ADD_SPAWN_ACTION(spawn_add_fchdir(&actions, dir_fd));
+		ADD_SPAWN_ACTION(posix_spawn_file_actions_addclose(&actions,
+							 dir_fd));
+	}
+
+	if (cmd->close_fd_above_stderr) {
+		flags |= POSIX_SPAWN_CLOEXEC_DEFAULT;
+
+		for (fd = 0; fd < 3; fd++) {
+			int fd_flags;
+			if (stdio_touched[fd])
+				continue;
+			fd_flags = fcntl(fd, F_GETFD);
+			if (fd_flags < 0 || (fd_flags & FD_CLOEXEC))
+				continue;
+			ADD_SPAWN_ACTION(posix_spawn_file_actions_addinherit_np(
+					&actions, fd));
+		}
+	}
+	ADD_SPAWN_ACTION(posix_spawnattr_setflags(&attr, flags));
+
+	error = posix_spawn(&pid, argv[1], &actions, attrp,
+			    (char *const *)argv + 1, childenv);
+	if (error == ENOEXEC)
+		error = posix_spawn(&pid, argv[0], &actions, attrp,
+				    (char *const *)argv, childenv);
+	if (error)
+		pid = -1;
+
+done:
+	if (attr_initialized)
+		posix_spawnattr_destroy(&attr);
+	if (actions_initialized)
+		posix_spawn_file_actions_destroy(&actions);
+	if (dir_fd >= 0)
+		close(dir_fd);
+#undef ADD_SPAWN_ACTION
+	if (error)
+		errno = error;
+	return pid;
+}
+#endif
+
+#ifndef USE_POSIX_SPAWN
 struct atfork_state {
 #ifndef NO_PTHREADS
 	int cs;
@@ -545,6 +754,7 @@ static void atfork_parent(struct atfork_state *as)
 		"restoring signal mask");
 #endif
 }
+#endif
 
 #endif /* GIT_WINDOWS_NATIVE */
 
@@ -746,7 +956,37 @@ int start_command(struct child_process *cmd)
 		odb_close(cmd->odb_to_close);
 
 #ifndef GIT_WINDOWS_NATIVE
+#ifdef USE_POSIX_SPAWN
 {
+	struct strvec argv = STRVEC_INIT;
+	struct child_err spawn_error;
+	char **childenv;
+
+	if (prepare_cmd(&argv, cmd) < 0) {
+		failed_errno = errno;
+		cmd->pid = -1;
+		if (!cmd->silent_exec_failure)
+			error_errno("cannot run %s", cmd->args.v[0]);
+	} else {
+		trace_argv_printf(&argv.v[1], "trace: start_command:");
+		childenv = prep_childenv(cmd->env.v);
+		cmd->pid = spawn_command(cmd, argv.v, childenv,
+					 need_in, fdin, need_out, fdout,
+					 need_err, fderr, &spawn_error.err);
+		failed_errno = errno;
+		if (cmd->pid < 0) {
+			spawn_error.syserr = failed_errno;
+			if (!cmd->silent_exec_failure || failed_errno != ENOENT ||
+			    spawn_error.err == CHILD_ERR_CHDIR)
+				child_err_spew(cmd, &spawn_error);
+		} else if (cmd->clean_on_exit)
+			mark_child_for_cleanup(cmd->pid, cmd);
+		free(childenv);
+	}
+	strvec_clear(&argv);
+}
+#else
+	{
 	int notify_pipe[2];
 	int null_fd = -1;
 	char **childenv;
@@ -905,8 +1145,10 @@ int start_command(struct child_process *cmd)
 		close(null_fd);
 	strvec_clear(&argv);
 	free(childenv);
-}
 end_of_spawn:
+;
+}
+#endif
 
 #else
 {

t/helper/.gitignore

@@ -1,2 +1,3 @@
 /test-tool
 /test-fake-ssh
+/test-fork-interposer.dylib