Preserve ignored error snapshots

tests/err_threads_pool.c

@@ -90,6 +90,9 @@ static void one_checkout(akerr_ThreadArg *arg)
 static void *pool_body(void *raw)
 {
     akerr_ThreadArg *arg = raw;
+    char expected[64];
+
+    snprintf(expected, sizeof(expected), "ignored by thread %d", arg->id);
     pthread_barrier_wait(arg->barrier);
 
     for ( int i = 0; i < ITERATIONS; i++ ) {
@@ -97,10 +100,21 @@ static void *pool_body(void *raw)
 	one_checkout(arg);
     }
 
-    /* IGNORE's scratch pointer is thread-local while logging and cleared after
+    /* IGNORE's snapshot is thread-local while logging and remains valid after
      * release. Concurrent ignored errors must all return their pool slots. */
     IGNORE(ignorable(arg));
-    AKERR_TCHECK(arg, __akerr_last_ignored == NULL);
+    AKERR_TCHECK(arg, __akerr_last_ignored.status == AKERR_IO);
+    AKERR_TCHECK(arg, strcmp(__akerr_last_ignored.message, expected) == 0);
+
+    /* Reuse a slot after IGNORE and prove that the copied snapshot did not
+     * become an alias for the newly acquired context. */
+    akerr_ErrorContext *reused = akerr_next_error();
+    AKERR_TCHECK(arg, reused != NULL);
+    if ( reused != NULL ) {
+	RELEASE_ERROR(reused);
+    }
+    AKERR_TCHECK(arg, __akerr_last_ignored.status == AKERR_IO);
+    AKERR_TCHECK(arg, strcmp(__akerr_last_ignored.message, expected) == 0);
 
     return NULL;
 }