run-command: prepare child environment before forking

In order to avoid allocation between 'fork()' and 'exec()' prepare the
environment to be used in the child process prior to forking.

Switch to using 'execve()' so that the construct child environment can
used in the exec'd process.

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

Author: bmwill

run-command.c

@@ -267,6 +267,55 @@ static void prepare_cmd(struct argv_array *out, const struct child_process *cmd)
 		}
 	}
 }
+
+static char **prep_childenv(const char *const *deltaenv)
+{
+	extern char **environ;
+	char **childenv;
+	struct string_list env = STRING_LIST_INIT_DUP;
+	struct strbuf key = STRBUF_INIT;
+	const char *const *p;
+	int i;
+
+	/* Construct a sorted string list consisting of the current environ */
+	for (p = (const char *const *) environ; p && *p; p++) {
+		const char *equals = strchr(*p, '=');
+
+		if (equals) {
+			strbuf_reset(&key);
+			strbuf_add(&key, *p, equals - *p);
+			string_list_append(&env, key.buf)->util = (void *) *p;
+		} else {
+			string_list_append(&env, *p)->util = (void *) *p;
+		}
+	}
+	string_list_sort(&env);
+
+	/* Merge in 'deltaenv' with the current environ */
+	for (p = deltaenv; p && *p; p++) {
+		const char *equals = strchr(*p, '=');
+
+		if (equals) {
+			/* ('key=value'), insert or replace entry */
+			strbuf_reset(&key);
+			strbuf_add(&key, *p, equals - *p);
+			string_list_insert(&env, key.buf)->util = (void *) *p;
+		} else {
+			/* otherwise ('key') remove existing entry */
+			string_list_remove(&env, *p, 0);
+		}
+	}
+
+	/* Create an array of 'char *' to be used as the childenv */
+	childenv = xmalloc((env.nr + 1) * sizeof(char *));
+	for (i = 0; i < env.nr; i++)
+		childenv[i] = env.items[i].util;
+	childenv[env.nr] = NULL;
+
+	string_list_clear(&env, 0);
+	strbuf_release(&key);
+	return childenv;
+}
 #endif
 
 static inline void set_cloexec(int fd)
@@ -395,12 +444,14 @@ int start_command(struct child_process *cmd)
 #ifndef GIT_WINDOWS_NATIVE
 {
 	int notify_pipe[2];
+	char **childenv;
 	struct argv_array argv = ARGV_ARRAY_INIT;
 
 	if (pipe(notify_pipe))
 		notify_pipe[0] = notify_pipe[1] = -1;
 
 	prepare_cmd(&argv, cmd);
+	childenv = prep_childenv(cmd->env);
 
 	cmd->pid = fork();
 	failed_errno = errno;
@@ -456,24 +507,18 @@ int start_command(struct child_process *cmd)
 		if (cmd->dir && chdir(cmd->dir))
 			die_errno("exec '%s': cd to '%s' failed", cmd->argv[0],
 			    cmd->dir);
-		if (cmd->env) {
-			for (; *cmd->env; cmd->env++) {
-				if (strchr(*cmd->env, '='))
-					putenv((char *)*cmd->env);
-				else
-					unsetenv(*cmd->env);
-			}
-		}
 
 		/*
 		 * 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);
+		execve(argv.argv[1], (char *const *) argv.argv + 1,
+		       (char *const *) childenv);
 		if (errno == ENOEXEC)
-			execv(argv.argv[0], (char *const *) argv.argv);
+			execve(argv.argv[0], (char *const *) argv.argv,
+			       (char *const *) childenv);
 
 		if (errno == ENOENT) {
 			if (!cmd->silent_exec_failure)
@@ -509,6 +554,7 @@ int start_command(struct child_process *cmd)
 	close(notify_pipe[0]);
 
 	argv_array_clear(&argv);
+	free(childenv);
 }
 #else
 {