start_command: do not clobber cmd->env on Windows code path

Previously, it would not be possible to call start_command twice for the
same struct child_process that has env set.

The fix is achieved by moving the loop that modifies the environment block
into a helper function. This also allows us to make two other helper
functions static.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -824,7 +824,7 @@ void mingw_execvp(const char *cmd, char *const *argv)
 	free_path_split(path);
 }
 
-char **copy_environ()
+static char **copy_environ(void)
 {
 	char **env;
 	int i = 0;
@@ -861,7 +861,7 @@ static int lookup_env(char **env, const char *name, size_t nmln)
 /*
  * If name contains '=', then sets the variable, otherwise it unsets it
  */
-char **env_setenv(char **env, const char *name)
+static char **env_setenv(char **env, const char *name)
 {
 	char *eq = strchrnul(name, '=');
 	int i = lookup_env(env, name, eq-name);
@@ -886,6 +886,18 @@ char **env_setenv(char **env, const char *name)
 	return env;
 }
 
+/*
+ * Copies global environ and adjusts variables as specified by vars.
+ */
+char **make_augmented_environ(const char *const *vars)
+{
+	char **env = copy_environ();
+
+	while (*vars)
+		env = env_setenv(env, *vars++);
+	return env;
+}
+
 /* this is the first function to call into WS_32; initialize it */
 #undef gethostbyname
 struct hostent *mingw_gethostbyname(const char *host)

compat/mingw.h

@@ -222,9 +222,8 @@ void mingw_open_html(const char *path);
  * helpers
  */
 
-char **copy_environ(void);
+char **make_augmented_environ(const char *const *vars);
 void free_environ(char **env);
-char **env_setenv(char **env, const char *name);
 
 /*
  * A replacement of main() that ensures that argv[0] has a path

run-command.c

@@ -173,11 +173,8 @@ int start_command(struct child_process *cmd)
 
 	if (cmd->dir)
 		die("chdir in start_command() not implemented");
-	if (cmd->env) {
-		env = copy_environ();
-		for (; *cmd->env; cmd->env++)
-			env = env_setenv(env, *cmd->env);
-	}
+	if (cmd->env)
+		env = make_augmented_environ(cmd->env);
 
 	if (cmd->git_cmd) {
 		cmd->argv = prepare_git_cmd(cmd->argv);