run-command: use the async-signal-safe execv instead of execvp

Convert the function used to exec from 'execvp()' to 'execv()' as the (p)
variant of exec isn't async-signal-safe and has the potential to call malloc
during the path resolution it performs.  Instead we simply do the path
resolution ourselves during the preparation stage prior to forking.  There also
don't exist any portable (p) variants which also take in an environment to use
in the exec'd process.  This allows easy migration to using 'execve()' in a
future patch.

Also, as noted in [1], in the event of an ENOEXEC the (p) variants of
exec will attempt to execute the command by interpreting it with the
'sh' utility.  To maintain this functionality, if 'execv()' fails with
ENOEXEC, start_command will atempt to execute the command by
interpreting it with 'sh'.

[1] http://pubs.opengroup.org/onlinepubs/009695399/functions/exec.html

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -238,6 +238,12 @@ static void prepare_cmd(struct argv_array *out, const struct child_process *cmd)
 	if (!cmd->argv[0])
 		die("BUG: command is empty");
 
+	/*
+	 * Add SHELL_PATH so in the event exec fails with ENOEXEC we can
+	 * attempt to interpret the command with 'sh'.
+	 */
+	argv_array_push(out, SHELL_PATH);
+
 	if (cmd->git_cmd) {
 		argv_array_push(out, "git");
 		argv_array_pushv(out, cmd->argv);
@@ -246,6 +252,20 @@ static void prepare_cmd(struct argv_array *out, const struct child_process *cmd)
 	} else {
 		argv_array_pushv(out, cmd->argv);
 	}
+
+	/*
+	 * If there are no '/' characters in the command then perform a path
+	 * lookup and use the resolved path as the command to exec.  If there
+	 * are no '/' characters or if the command wasn't found in the path,
+	 * have exec attempt to invoke the command directly.
+	 */
+	if (!strchr(out->argv[1], '/')) {
+		char *program = locate_in_PATH(out->argv[1]);
+		if (program) {
+			free((char *)out->argv[1]);
+			out->argv[1] = program;
+		}
+	}
 }
 #endif
 
@@ -445,7 +465,15 @@ fail_pipe:
 			}
 		}
 
-		sane_execvp(argv.argv[0], (char *const *) argv.argv);
+		/*
+		 * Attempt to exec using the command and arguments starting at
+		 * argv.argv[1].  argv.argv[0] contains SHELL_PATH which will
+		 * be used in the event exec failed with ENOEXEC at which point
+		 * we will try to interpret the command using 'sh'.
+		 */
+		execv(argv.argv[1], (char *const *) argv.argv + 1);
+		if (errno == ENOEXEC)
+			execv(argv.argv[0], (char *const *) argv.argv);
 
 		if (errno == ENOENT) {
 			if (!cmd->silent_exec_failure)