bugfix: Setting OMPI_MPI_THREAD_LEVEL to a value different than

`requested` in `MPI_Init_thread` would invoke the error handler, even
though it is an useful override in some threaded library use cases.

Signed-off-by: Aurelien Bouteiller <abouteil@amd.com>

Author: abouteiller

ompi/mpi/c/init.c.in

@@ -15,6 +15,7 @@
  *                         and Technology (RIST). All rights reserved.
  * Copyright (c) 2024      Triad National Security, LLC. All rights
  *                         reserved.
+ * Copyright (c) 2025      Advanced Micro Devices, Inc. All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -46,7 +47,12 @@ PROTOTYPE INT init(INT_OUT argc, ARGV argv)
 
     if (NULL != (env = getenv("OMPI_MPI_THREAD_LEVEL"))) {
         required = atoi(env);
-        if (required < MPI_THREAD_SINGLE || required > MPI_THREAD_MULTIPLE) {
+        /* In the future we may have to contend with non-sequential (MPI ABI) values
+         * If you are implementing MPI ABI changes please refer to
+         * https://github.com/open-mpi/ompi/pull/13211#discussion_r2085086844
+         */
+        if (required != MPI_THREAD_SINGLE && required != MPI_THREAD_FUNNELED &&
+            required != MPI_THREAD_SERIALIZED && required != MPI_THREAD_MULTIPLE) {
             required = MPI_THREAD_MULTIPLE;
         }
     }

ompi/mpi/c/init_thread.c.in

@@ -18,6 +18,7 @@
  *                         reserved.
  * Copyright (c) 2024      Triad National Security, LLC. All rights
  *                         reserved.
+ * Copyright (c) 2025      Advanced Micro Devices, Inc. All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -40,21 +41,36 @@ PROTOTYPE ERROR_CLASS init_thread(INT_OUT argc, ARGV argv, INT required,
                                   INT_OUT provided)
 {
     int err, safe_required = MPI_THREAD_SERIALIZED;
+    bool err_arg_required = false;
     char *env;
 
     ompi_hook_base_mpi_init_thread_top(argc, argv, required, provided);
 
     /* Detect an incorrect thread support level, but dont report until we have the minimum
      * infrastructure setup.
      */
-    if( (MPI_THREAD_SINGLE == required) || (MPI_THREAD_SERIALIZED == required) ||
-        (MPI_THREAD_FUNNELED == required) || (MPI_THREAD_MULTIPLE == required) ) {
+    err_arg_required = (required != MPI_THREAD_SINGLE && required != MPI_THREAD_FUNNELED &&
+                        required != MPI_THREAD_SERIALIZED && required != MPI_THREAD_MULTIPLE);
+    if (!err_arg_required) {
+        safe_required = required;
+    }
 
-        if (NULL != (env = getenv("OMPI_MPI_THREAD_LEVEL")))  {
-            safe_required = atoi(env);
-        }
-        else {
-            safe_required = required;
+    /* check for environment overrides for required thread level. If
+     * there is, check to see that it is a valid/supported thread level.
+     * If valid, the environment variable always override the provided thread
+     * level (even if lower than argument `required`). A user program can
+     * check `provided != required` to check if `required` has been overruled.
+     */
+    if (NULL != (env = getenv("OMPI_MPI_THREAD_LEVEL"))) {
+        int env_required = atoi(env);
+        /* In the future we may have to contend with non-sequential (MPI ABI) values
+         * If you are implementing MPI ABI changes please refer to
+         * https://github.com/open-mpi/ompi/pull/13211#discussion_r2085086844
+         */
+        err_arg_required |= (env_required != MPI_THREAD_SINGLE && env_required != MPI_THREAD_FUNNELED &&
+                             env_required != MPI_THREAD_SERIALIZED && env_required != MPI_THREAD_MULTIPLE);
+        if (!err_arg_required) {
+            safe_required = env_required;
         }
     }
 
@@ -70,7 +86,7 @@ PROTOTYPE ERROR_CLASS init_thread(INT_OUT argc, ARGV argv, INT required,
         err = ompi_mpi_init(0, NULL, safe_required, provided, false);
     }
 
-    if( safe_required != required ) {
+    if (err_arg_required) {
         /* Trigger the error handler for the incorrect argument. Keep it separate from the
          * check on the ompi_mpi_init return and report a nice, meaningful error message to
          * the user. */