Further fix the issue and add the assert that'd have prevented it.

Surprise: because FISH_USE_POSIX_SPAWN was from postfork.h, we
also were disabling things when we don't want to as well.

src/env.h

@@ -298,8 +298,6 @@ class env_stack_t final : public environment_t {
     static env_stack_t &globals();
 };
 
-bool get_use_posix_spawn();
-
 extern bool term_has_xn;  // does the terminal have the "eat_newline_glitch"
 
 /// Returns true if we think the terminal supports setting its title.

src/env_dispatch.cpp

@@ -43,6 +43,7 @@
 #include "input_common.h"
 #include "maybe.h"
 #include "output.h"
+#include "postfork.h"
 #include "proc.h"
 #include "reader.h"
 #include "screen.h"
@@ -268,10 +269,13 @@ static constexpr bool allow_use_posix_spawn() {
 #endif
 }
 
-/// 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; }
+bool get_use_posix_spawn() {
+    assert(allow_use_posix_spawn() && "get_use_posix_spawn() called but not allowed");
+    return g_use_posix_spawn;
+}
 
+/// Whether to use posix_spawn when possible.
 static void handle_fish_use_posix_spawn_change(const environment_t &vars) {
     // If the variable is missing or empty, we default to true if allowed.
     if (auto var = vars.get(L"fish_use_posix_spawn")) {
@@ -354,7 +358,7 @@ static void run_inits(const environment_t &vars) {
     guess_emoji_width(vars);
     update_wait_on_escape_ms(vars);
     handle_read_limit_change(vars);
-    if (allow_use_posix_spawn) handle_fish_use_posix_spawn_change(vars);
+    if (allow_use_posix_spawn()) handle_fish_use_posix_spawn_change(vars);
     handle_fish_trace(vars);
 }
 

src/env_dispatch.h

@@ -15,4 +15,7 @@ void env_dispatch_init(const environment_t &vars);
 /// React to changes in variables like LANG which require running some code.
 void env_dispatch_var_change(const wcstring &key, env_stack_t &vars);
 
+/// Whether posix_spawn is configured and/or allowed to be in use globally.
+bool get_use_posix_spawn();
+
 #endif

src/exec.cpp

@@ -30,6 +30,7 @@
 #include "builtin.h"
 #include "common.h"
 #include "env.h"
+#include "env_dispatch.h"
 #include "exec.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "fds.h"