fix tunable default update

Signed-off-by: Emelia Lei <wlei29@bloomberg.net>

Author: emelialei88

db/db_tunables.c

@@ -193,6 +193,7 @@ extern int gbl_all_prepare_abort;
 extern int gbl_all_prepare_leak;
 extern int gbl_flush_on_prepare;
 extern int gbl_debug_sleep_before_prepare;
+static char *gbl_debug_default_string_update = "debug_default_string_update_value";
 extern int gbl_wait_for_prepare_seqnum;
 extern int gbl_flush_replicant_on_prepare;
 extern int gbl_slow_rep_log_get_loop;
@@ -1407,6 +1408,14 @@ int register_tunable(comdb2_tunable *tunable)
     /* Keep tunable names in lower case (to be consistent). */
     tunable_tolower(t->name);
 
+    // Ensure the initial value for tunables without an update function
+    // is heap-allocated so the default update handler can safely free() it.
+    if ((tunable->type == TUNABLE_STRING || tunable->type == TUNABLE_RAW) && !tunable->update) {
+        if (tunable->var && *(char **)tunable->var) {
+            *((char **)tunable->var) = strdup(*(char **)tunable->var);
+        }
+    }
+
     t->descr = tunable->descr;
 
     if (tunable->type >= TUNABLE_INVALID) {

db/db_tunables.h

@@ -333,6 +333,8 @@ REGISTER_TUNABLE("sleep_before_dispatch", "Sleep before dispatching on master. (
                  &gbl_debug_sleep_before_dispatch, EXPERIMENTAL | INTERNAL, NULL, NULL, NULL, NULL);
 REGISTER_TUNABLE("debug_sleep_before_prepare", "Sleep for 5 seconds before preparing. (Default: off)", TUNABLE_BOOLEAN,
                  &gbl_debug_sleep_before_prepare, EXPERIMENTAL | INTERNAL, NULL, NULL, NULL, NULL);
+REGISTER_TUNABLE("debug_default_string_update", "Test default string update handler. (Default: debug_default_string_update_value)",
+                 TUNABLE_STRING, &gbl_debug_default_string_update, EXPERIMENTAL | INTERNAL, NULL, NULL, NULL, NULL);
 
 REGISTER_TUNABLE("disable_prefault_udp", "Disables 'enable_prefault_udp'", TUNABLE_BOOLEAN, &gbl_prefault_udp,
                  INVERSE_VALUE | NOARG, NULL, NULL, NULL, NULL);

tests/readearly_string_tunable.test/lrl.options

@@ -0,0 +1,2 @@
+iam_metrics_namespace comdb2-iam-test
+debug_default_string_update comdb2-string-tunable-test

tests/readearly_string_tunable.test/t00.expected

@@ -0,0 +1,2 @@
+(value='comdb2-iam-test')
+(value='comdb2-string-tunable-test')

tests/readearly_string_tunable.test/t00.sql

@@ -0,0 +1,4 @@
+# Verify that setting a string tunable via LRL does not crash.
+SELECT value FROM comdb2_tunables WHERE name = 'iam_metrics_namespace';
+PUT TUNABLE mask_internal_tunables 0
+SELECT value FROM comdb2_tunables WHERE name = 'debug_default_string_update';