prewarm wrapper should indicate it is active via the process environment

kitty_tests/prewarm.py

@@ -115,6 +115,14 @@ def socket_child_main(exit_code=0, initial_print=''):
                 pty.wait_till(lambda: f'Screen size changed: {cols + 3}' in pty.screen_contents())
                 os.close(pty.master_fd)
 
+        with self.subTest(msg='test env rewrite'):
+            pty = self.create_pty(
+                argv=[kitty_exe(), '+runpy', src + 'socket_child_main(initial_print="child ready:")'], cols=cols, env=env, cwd=cwd)
+            pty.wait_till(lambda: 'child ready:' in pty.screen_contents())
+            from kitty.child import environ_of_process
+            self.assertTrue(environ_of_process(pty.child_pid).get('KITTY_PWPR'))
+            os.close(pty.master_fd)
+
         with self.subTest(msg='test passing of data via cwd, env vars and stdin/stdout redirection'):
             stdin_r, stdin_w = os.pipe()
             os.set_inheritable(stdin_w, False)

launcher.c

@@ -268,11 +268,11 @@ read_exe_path(char *exe, size_t buf_sz) {
 }
 #endif // }}}
 
-extern void use_prewarmed_process(int argc, char *argv[]);
+extern void use_prewarmed_process(int argc, char *argv[], char *envp[]);
 
-int main(int argc, char *argv[]) {
+int main(int argc, char *argv[], char* envp[]) {
     if (argc < 1 || !argv) { fprintf(stderr, "Invalid argc/argv\n"); return 1; }
-    use_prewarmed_process(argc, argv);
+    use_prewarmed_process(argc, argv, envp);
     char exe[PATH_MAX+1] = {0};
     char exe_dir_buf[PATH_MAX+1] = {0};
     FREE_AFTER_FUNCTION const char *lc_ctype = NULL;

prewarm-launcher.c

@@ -673,7 +673,7 @@ check_socket_addr(char *addr) {
 }
 
 void
-use_prewarmed_process(int argc, char *argv[]) {
+use_prewarmed_process(int argc, char *argv[], char *envp[]) {
     char *env_addr = getenv("KITTY_PREWARM_SOCKET");
     if (!env_addr || !*env_addr || !is_prewarmable(argc, argv)) return;
     env_addr = check_socket_addr(env_addr);
@@ -698,6 +698,12 @@ use_prewarmed_process(int argc, char *argv[]) {
     to_child_tty.buf = from_child_tty.buf + IO_BUZ_SZ;
 #undef fail
 
+    while (*envp) {
+        if (strncmp(*envp, "KITTY_PREWARM_SOCKET=", 2) == 0) {
+            sprintf(*envp,  "KITTY_PWPR=%s", child_tty_name);
+        }
+        envp++;
+    }
     loop();
     flush_data();
     cleanup();