[PATCH v2 3/6] run-command: prepare child environment before forking

[Brandon Williams <bmwill@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 run-command.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 74 insertions(+), 9 deletions(-)

diff --git a/run-command.c b/run-command.c
index 9ee9fde97..5e2a03145 100644
--- a/run-command.c
+++ b/run-command.c
@@ -261,6 +261,75 @@ static void prepare_cmd(struct argv_array *out, const struct child_process *cmd)
 		}
 	}
 }
+
+static int env_isequal(const char *e1, const char *e2)
+{
+	for (;;) {
+		char c1 = *e1++;
+		char c2 = *e2++;
+		c1 = (c1 == '=') ? '\0' : tolower(c1);
+		c2 = (c2 == '=') ? '\0' : tolower(c2);
+
+		if (c1 != c2)
+			return 0;
+		if (c1 == '\0')
+			return 1;
+	}
+}
+
+static int searchenv(char **env, const char *name)
+{
+	int pos = 0;
+
+	for (; env[pos]; pos++)
+		if (env_isequal(env[pos], name))
+			break;
+
+	return pos;
+}
+
+static int do_putenv(char **env, int env_nr, const char *name)
+{
+	int pos = searchenv(env, name);
+
+	if (strchr(name, '=')) {
+		/* ('key=value'), insert of replace entry */
+		if (pos >= env_nr)
+			env_nr++;
+		env[pos] = (char *) name;
+	} else if (pos < env_nr) {
+		/* otherwise ('key') remove existing entry */
+		env_nr--;
+		memmove(&env[pos], &env[pos + 1],
+			(env_nr - pos) * sizeof(char *));
+		env[env_nr] = NULL;
+	}
+
+	return env_nr;
+}
+
+static char **prep_childenv(const char *const *deltaenv)
+{
+	char **childenv;
+	int childenv_nr = 0, childenv_alloc = 0;
+	int i;
+
+	for (i = 0; environ[i]; i++)
+		childenv_nr++;
+	for (i = 0; deltaenv && deltaenv[i]; i++)
+		childenv_alloc++;
+	/* Add one for the NULL termination */
+	childenv_alloc += childenv_nr + 1;
+
+	childenv = xcalloc(childenv_alloc, sizeof(char *));
+	memcpy(childenv, environ, childenv_nr * sizeof(char *));
+
+	/* merge in deltaenv */
+	for (i = 0; deltaenv && deltaenv[i]; i++)
+		childenv_nr = do_putenv(childenv, childenv_nr, deltaenv[i]);
+
+	return childenv;
+}
 #endif
 
 static inline void set_cloexec(int fd)
@@ -389,12 +458,14 @@ 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;
@@ -450,16 +521,9 @@ 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);
-			}
-		}
 
-		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)
@@ -495,6 +559,7 @@ int start_command(struct child_process *cmd)
 	close(notify_pipe[0]);
 
 	argv_array_clear(&argv);
+	free(childenv);
 }
 #else
 {
-- 
2.12.2.762.g0e3151a226-goog