run-command API: remove "argv" member, always use "args"

Remove the "argv" member from the run-command API, ever since "args"
was added in c460c0ecdc (run-command: store an optional argv_array,
2014-05-15) being able to provide either "argv" or "args" has led to
some confusion and bugs.

If we hadn't gone in that direction and only had an "argv" our
problems wouldn't have been solved either, as noted in [1] (and in the
documentation amended here) it comes with inherent memory management
issues: The caller would have to hang on to the "argv" until the
run-command API was finished. If the "argv" was an argument to main()
this wasn't an issue, but if it it was manually constructed using the
API might be painful.

We also have a recent report[2] of a user of the API segfaulting,
which is a direct result of it being complex to use. This commit
addresses the root cause of that bug.

This change is larger than I'd like, but there's no easy way to avoid
it that wouldn't involve even more verbose intermediate steps. We use
the "argv" as the source of truth over the "args", so we need to
change all parts of run-command.[ch] itself, as well as the trace2
logging at the same time.

The resulting Windows-specific code in start_command() is a bit nasty,
as we're now assigning to a strvec's "v" member, instead of to our own
"argv". There was a suggestion of some alternate approaches in reply
to an earlier version of this commit[3], but let's leave larger a
larger and needless refactoring of this code for now.

1. http://lore.kernel.org/git/YT6BnnXeAWn8BycF@coredump.intra.peff.net
2. https://lore.kernel.org/git/20211120194048.12125-1-ematsumiya@suse.de/
3. https://lore.kernel.org/git/patch-5.5-ea1011f7473-20211122T153605Z-avarab@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -380,7 +380,7 @@ static void child_err_spew(struct child_process *cmd, struct child_err *cerr)
 	switch (cerr->err) {
 	case CHILD_ERR_CHDIR:
 		error_errno("exec '%s': cd to '%s' failed",
-			    cmd->argv[0], cmd->dir);
+			    cmd->args.v[0], cmd->dir);
 		break;
 	case CHILD_ERR_DUP2:
 		error_errno("dup2() in child failed");
@@ -392,12 +392,12 @@ static void child_err_spew(struct child_process *cmd, struct child_err *cerr)
 		error_errno("sigprocmask failed restoring signals");
 		break;
 	case CHILD_ERR_ENOENT:
-		error_errno("cannot run %s", cmd->argv[0]);
+		error_errno("cannot run %s", cmd->args.v[0]);
 		break;
 	case CHILD_ERR_SILENT:
 		break;
 	case CHILD_ERR_ERRNO:
-		error_errno("cannot exec '%s'", cmd->argv[0]);
+		error_errno("cannot exec '%s'", cmd->args.v[0]);
 		break;
 	}
 	set_error_routine(old_errfn);
@@ -405,7 +405,7 @@ static void child_err_spew(struct child_process *cmd, struct child_err *cerr)
 
 static int prepare_cmd(struct strvec *out, const struct child_process *cmd)
 {
-	if (!cmd->argv[0])
+	if (!cmd->args.v[0])
 		BUG("command is empty");
 
 	/*
@@ -415,11 +415,11 @@ static int prepare_cmd(struct strvec *out, const struct child_process *cmd)
 	strvec_push(out, SHELL_PATH);
 
 	if (cmd->git_cmd) {
-		prepare_git_cmd(out, cmd->argv);
+		prepare_git_cmd(out, cmd->args.v);
 	} else if (cmd->use_shell) {
-		prepare_shell_cmd(out, cmd->argv);
+		prepare_shell_cmd(out, cmd->args.v);
 	} else {
-		strvec_pushv(out, cmd->argv);
+		strvec_pushv(out, cmd->args.v);
 	}
 
 	/*
@@ -663,7 +663,7 @@ static void trace_run_command(const struct child_process *cp)
 		trace_add_env(&buf, cp->env);
 	if (cp->git_cmd)
 		strbuf_addstr(&buf, " git");
-	sq_quote_argv_pretty(&buf, cp->argv);
+	sq_quote_argv_pretty(&buf, cp->args.v);
 
 	trace_printf("%s", buf.buf);
 	strbuf_release(&buf);
@@ -676,8 +676,6 @@ int start_command(struct child_process *cmd)
 	int failed_errno;
 	char *str;
 
-	if (!cmd->argv)
-		cmd->argv = cmd->args.v;
 	if (!cmd->env)
 		cmd->env = cmd->env_array.v;
 
@@ -729,7 +727,7 @@ int start_command(struct child_process *cmd)
 			str = "standard error";
 fail_pipe:
 			error("cannot create %s pipe for %s: %s",
-				str, cmd->argv[0], strerror(failed_errno));
+				str, cmd->args.v[0], strerror(failed_errno));
 			child_process_clear(cmd);
 			errno = failed_errno;
 			return -1;
@@ -758,7 +756,7 @@ fail_pipe:
 		failed_errno = errno;
 		cmd->pid = -1;
 		if (!cmd->silent_exec_failure)
-			error_errno("cannot run %s", cmd->argv[0]);
+			error_errno("cannot run %s", cmd->args.v[0]);
 		goto end_of_spawn;
 	}
 
@@ -868,7 +866,7 @@ fail_pipe:
 	}
 	atfork_parent(&as);
 	if (cmd->pid < 0)
-		error_errno("cannot fork() for %s", cmd->argv[0]);
+		error_errno("cannot fork() for %s", cmd->args.v[0]);
 	else if (cmd->clean_on_exit)
 		mark_child_for_cleanup(cmd->pid, cmd);
 
@@ -885,7 +883,7 @@ fail_pipe:
 		 * At this point we know that fork() succeeded, but exec()
 		 * failed. Errors have been reported to our stderr.
 		 */
-		wait_or_whine(cmd->pid, cmd->argv[0], 0);
+		wait_or_whine(cmd->pid, cmd->args.v[0], 0);
 		child_err_spew(cmd, &cerr);
 		failed_errno = errno;
 		cmd->pid = -1;
@@ -902,7 +900,7 @@ end_of_spawn:
 #else
 {
 	int fhin = 0, fhout = 1, fherr = 2;
-	const char **sargv = cmd->argv;
+	const char **sargv = cmd->args.v;
 	struct strvec nargv = STRVEC_INIT;
 
 	if (cmd->no_stdin)
@@ -929,20 +927,20 @@ end_of_spawn:
 		fhout = dup(cmd->out);
 
 	if (cmd->git_cmd)
-		cmd->argv = prepare_git_cmd(&nargv, cmd->argv);
+		cmd->args.v = prepare_git_cmd(&nargv, sargv);
 	else if (cmd->use_shell)
-		cmd->argv = prepare_shell_cmd(&nargv, cmd->argv);
+		cmd->args.v = prepare_shell_cmd(&nargv, sargv);
 
-	cmd->pid = mingw_spawnvpe(cmd->argv[0], cmd->argv, (char**) cmd->env,
+	cmd->pid = mingw_spawnvpe(cmd->args.v[0], cmd->args.v, (char**) cmd->env,
 			cmd->dir, fhin, fhout, fherr);
 	failed_errno = errno;
 	if (cmd->pid < 0 && (!cmd->silent_exec_failure || errno != ENOENT))
-		error_errno("cannot spawn %s", cmd->argv[0]);
+		error_errno("cannot spawn %s", cmd->args.v[0]);
 	if (cmd->clean_on_exit && cmd->pid >= 0)
 		mark_child_for_cleanup(cmd->pid, cmd);
 
 	strvec_clear(&nargv);
-	cmd->argv = sargv;
+	cmd->args.v = sargv;
 	if (fhin != 0)
 		close(fhin);
 	if (fhout != 1)
@@ -992,7 +990,7 @@ end_of_spawn:
 
 int finish_command(struct child_process *cmd)
 {
-	int ret = wait_or_whine(cmd->pid, cmd->argv[0], 0);
+	int ret = wait_or_whine(cmd->pid, cmd->args.v[0], 0);
 	trace2_child_exit(cmd, ret);
 	child_process_clear(cmd);
 	invalidate_lstat_cache();
@@ -1001,7 +999,7 @@ int finish_command(struct child_process *cmd)
 
 int finish_command_in_signal(struct child_process *cmd)
 {
-	int ret = wait_or_whine(cmd->pid, cmd->argv[0], 1);
+	int ret = wait_or_whine(cmd->pid, cmd->args.v[0], 1);
 	trace2_child_exit(cmd, ret);
 	return ret;
 }

run-command.h

@@ -44,21 +44,17 @@
 struct child_process {
 
 	/**
-	 * The .argv member is set up as an array of string pointers (NULL
-	 * terminated), of which .argv[0] is the program name to run (usually
-	 * without a path). If the command to run is a git command, set argv[0] to
-	 * the command name without the 'git-' prefix and set .git_cmd = 1.
+	 * The .args is a `struct strvec', use that API to manipulate
+	 * it, e.g. strvec_pushv() to add an existing "const char **"
+	 * vector.
 	 *
-	 * Note that the ownership of the memory pointed to by .argv stays with the
-	 * caller, but it should survive until `finish_command` completes. If the
-	 * .argv member is NULL, `start_command` will point it at the .args
-	 * `strvec` (so you may use one or the other, but you must use exactly
-	 * one). The memory in .args will be cleaned up automatically during
+	 * If the command to run is a git command, set the first
+	 * element in the strvec to the command name without the
+	 * 'git-' prefix and set .git_cmd = 1.
+	 *
+	 * The memory in .args will be cleaned up automatically during
 	 * `finish_command` (or during `start_command` when it is unsuccessful).
-	 *
 	 */
-	const char **argv;
-
 	struct strvec args;
 	struct strvec env_array;
 	pid_t pid;

sub-process.c

@@ -187,7 +187,7 @@ static int handshake_capabilities(struct child_process *process,
 				*supported_capabilities |= capabilities[i].flag;
 		} else {
 			die("subprocess '%s' requested unsupported capability '%s'",
-			    process->argv[0], p);
+			    process->args.v[0], p);
 		}
 	}
 

trace2/tr2_tgt_event.c

@@ -354,7 +354,7 @@ static void fn_child_start_fl(const char *file, int line,
 	jw_object_inline_begin_array(&jw, "argv");
 	if (cmd->git_cmd)
 		jw_array_string(&jw, "git");
-	jw_array_argv(&jw, cmd->argv);
+	jw_array_argv(&jw, cmd->args.v);
 	jw_end(&jw);
 	jw_end(&jw);
 

trace2/tr2_tgt_normal.c

@@ -232,7 +232,7 @@ static void fn_child_start_fl(const char *file, int line,
 	strbuf_addch(&buf_payload, ' ');
 	if (cmd->git_cmd)
 		strbuf_addstr(&buf_payload, "git ");
-	sq_append_quote_argv_pretty(&buf_payload, cmd->argv);
+	sq_append_quote_argv_pretty(&buf_payload, cmd->args.v);
 
 	normal_io_write_fl(file, line, &buf_payload);
 	strbuf_release(&buf_payload);

trace2/tr2_tgt_perf.c

@@ -335,10 +335,10 @@ static void fn_child_start_fl(const char *file, int line,
 	strbuf_addstr(&buf_payload, " argv:[");
 	if (cmd->git_cmd) {
 		strbuf_addstr(&buf_payload, "git");
-		if (cmd->argv[0])
+		if (cmd->args.nr)
 			strbuf_addch(&buf_payload, ' ');
 	}
-	sq_append_quote_argv_pretty(&buf_payload, cmd->argv);
+	sq_append_quote_argv_pretty(&buf_payload, cmd->args.v);
 	strbuf_addch(&buf_payload, ']');
 
 	perf_io_write_fl(file, line, event_name, NULL, &us_elapsed_absolute,