Clean up use_posix_spawn

Switch from a global variable to a real function. Make the value atomic.
Clean up handle_fish_use_posix_spawn_change().

src/env.h

@@ -309,7 +309,7 @@ class env_stack_t final : public environment_t {
     static env_stack_t &globals();
 };
 
-extern bool g_use_posix_spawn;
+bool get_use_posix_spawn();
 
 extern bool term_has_xn;  // does the terminal have the "eat_newline_glitch"
 

src/env_dispatch.cpp

@@ -260,11 +260,18 @@ static void handle_curses_change(const environment_t &vars) {
     init_curses(vars);
 }
 
+/// Whether to use posix_spawn when possible.
+static relaxed_atomic_bool_t g_use_posix_spawn{false};
+
+bool get_use_posix_spawn() { return g_use_posix_spawn; }
+
 static void handle_fish_use_posix_spawn_change(const environment_t &vars) {
-    // note this defaults to true
-    auto use_posix_spawn = vars.get(L"fish_use_posix_spawn");
-    g_use_posix_spawn =
-        use_posix_spawn.missing_or_empty() ? true : bool_from_string(use_posix_spawn->as_string());
+    // Note if the variable is missing or empty, we default to true.
+    if (auto var = vars.get(L"fish_use_posix_spawn")) {
+        g_use_posix_spawn = var->empty() || bool_from_string(var->as_string());
+    } else {
+        g_use_posix_spawn = true;
+    }
 }
 
 /// Allow the user to override the limit on how much data the `read` command will process.
@@ -570,9 +577,6 @@ static void init_locale(const environment_t &vars) {
 /// Returns true if we think the terminal supports setting its title.
 bool term_supports_setting_title() { return can_set_term_title; }
 
-/// Miscellaneous variables.
-bool g_use_posix_spawn = false;
-
 // Limit `read` to 100 MiB (bytes not wide chars) by default. This can be overridden by the
 // fish_read_limit variable.
 size_t read_byte_limit = 100 * 1024 * 1024;

src/exec.cpp

@@ -207,6 +207,9 @@ bool is_thompson_shell_script(const char *path) {
 // we use fork(), we can call tcsetpgrp after the fork, before the exec, and avoid the race).
 static bool can_use_posix_spawn_for_job(const std::shared_ptr<job_t> &job,
                                         const dup2_list_t &dup2s) {
+    // Is it globally disabled?
+    if (!get_use_posix_spawn()) return false;
+
     // Hack - do not use posix_spawn if there are self-fd redirections.
     // For example if you were to write:
     //   cmd 6< /dev/null
@@ -545,8 +548,7 @@ static launch_result_t exec_external_command(parser_t &parser, const std::shared
 
 #if FISH_USE_POSIX_SPAWN
     // Prefer to use posix_spawn, since it's faster on some systems like OS X.
-    bool use_posix_spawn = g_use_posix_spawn && can_use_posix_spawn_for_job(j, dup2s);
-    if (use_posix_spawn) {
+    if (can_use_posix_spawn_for_job(j, dup2s)) {
         s_fork_count++;  // spawn counts as a fork+exec
 
         posix_spawner_t spawner(j.get(), dup2s);