aix,ibmi,os390: make uv_exepath thread-safe

Fix thread-safety issue in uv_exepath() for AIX, IBM i, and z/OS platforms
by adding mutex protection around uv__search_path() calls.

The uv__search_path() function calls getenv("PATH") which is not thread-safe.
This fix adds proper mutex synchronization to prevent race conditions when
multiple threads call uv_exepath() concurrently.

Changes:
- aix-common.c: Add mutex protection around uv__search_path call
- os390.c: Add mutex protection and use cached original_exepath when available
- os390-proctitle.c: Make process_title_mutex and related functions non-static,
  add original_exepath variable
- ibmi.c: Add mutex protection around uv__search_path call in uv_setup_args

Fixes #4920

src/unix/aix-common.c

@@ -85,5 +85,8 @@ int uv_exepath(char* buffer, size_t* size) {
   if (res < 0)
     return UV_EINVAL;
 
-  return uv__search_path(args, buffer, size);
+  uv_mutex_lock(&process_title_mutex);
+  res = uv__search_path(args, buffer, size);
+  uv_mutex_unlock(&process_title_mutex);
+  return res;
 }

src/unix/ibmi.c

@@ -513,12 +513,12 @@ char** uv_setup_args(int argc, char** argv) {
   if (argc > 0) {
     /* Use argv[0] to determine value for uv_exepath(). */
     size = sizeof(exepath);
+    uv_once(&process_title_mutex_once, init_process_title_mutex_once);
+    uv_mutex_lock(&process_title_mutex);
     if (uv__search_path(argv[0], exepath, &size) == 0) {
-      uv_once(&process_title_mutex_once, init_process_title_mutex_once);
-      uv_mutex_lock(&process_title_mutex);
       original_exepath = uv__strdup(exepath);
-      uv_mutex_unlock(&process_title_mutex);
     }
+    uv_mutex_unlock(&process_title_mutex);
   }
 
   return argv;

src/unix/os390-proctitle.c

@@ -25,13 +25,14 @@
 #include <stdlib.h>
 #include <string.h>
 
-static uv_mutex_t process_title_mutex;
-static uv_once_t process_title_mutex_once = UV_ONCE_INIT;
+uv_mutex_t process_title_mutex;
+uv_once_t process_title_mutex_once = UV_ONCE_INIT;
+char* original_exepath = NULL;
 static char* process_title = NULL;
 static void* args_mem = NULL;
 
 
-static void init_process_title_mutex_once(void) {
+void init_process_title_mutex_once(void) {
   uv_mutex_init(&process_title_mutex);
 }
 

src/unix/os390.c

@@ -152,19 +152,41 @@ static int getexe(char* buf, size_t len) {
  * or through some libc APIs. The below approach is to parse the argv[0]'s pattern
  * and use it in conjunction with PATH environment variable to craft one.
  */
+extern char* original_exepath;
+extern uv_mutex_t process_title_mutex;
+extern uv_once_t process_title_mutex_once;
+extern void init_process_title_mutex_once(void);
+
 int uv_exepath(char* buffer, size_t* size) {
   int res;
   char args[PATH_MAX];
-  int pid;
+  size_t cached_len;
 
   if (buffer == NULL || size == NULL || *size == 0)
     return UV_EINVAL;
 
+  uv_once(&process_title_mutex_once, init_process_title_mutex_once);
+  uv_mutex_lock(&process_title_mutex);
+  if (original_exepath != NULL) {
+    cached_len = strlen(original_exepath);
+    *size -= 1;
+    if (*size > cached_len)
+      *size = cached_len;
+    memcpy(buffer, original_exepath, *size);
+    buffer[*size] = '\0';
+    uv_mutex_unlock(&process_title_mutex);
+    return 0;
+  }
+  uv_mutex_unlock(&process_title_mutex);
+
   res = getexe(args, sizeof(args));
   if (res < 0)
     return UV_EINVAL;
 
-  return uv__search_path(args, buffer, size);
+  uv_mutex_lock(&process_title_mutex);
+  res = uv__search_path(args, buffer, size);
+  uv_mutex_unlock(&process_title_mutex);
+  return res;
 }