[#31029] YSQL: Avoid ExternalMiniCluster master/tserver startup race in initdb

ExternalMiniCluster exhibits the same master/tserver startup race as
MiniYBCluster (see dcd8d1d for the underlying bug). The C++ test
harness needs two pieces of wiring:

* In StartMaster(), set
  --TEST_master_min_live_tservers_before_initdb=num_tablet_servers next
  to --master_auto_run_initdb so the master waits for tservers before
  launching initdb. Mirrors the Java-side wiring in MiniYBCluster.

* In Start(), move the WaitForInitDb() call out of StartMasters() and
  to the point right after tservers have been launched. The previous
  ordering -- WaitForInitDb() synchronously inside StartMasters() --
  would deadlock with the new flag, because tservers are only launched
  after StartMasters() returns. ExternalMiniCluster::StartMasters() has
  no other callers in the codebase, so this deferral is safe.

This fix unmasks the ASAN LSan leak fix (45086aa) for the
PgObjectLocksTest family in src/yb/yql/pgwrapper/pg_object_locks-test.cc.
Without it, the LibPqTestBase-based tests in that family FAIL during
cluster bootstrap under ASAN, hiding the LSan fix's effect.

Test plan:
* asan-clang21 local, repeated, with --skip-cxx-build --skip-java-build -n N --tp 1:
  -> 5/5 PASSED for PgObjectLocksTest.VerifyLockTimeout (ticket #30090).
  -> 3/3 PASSED each for the four sibling tests Sergei flagged in #30090:
      PgObjectLocksTestMixModeDuringPromotion.TestMixModeDuringPromotion
      PgObjectLocksTestMixModeDuringPromotion.TestLocksTakenAfterPromotionWithExistingConnections
      PgObjectLocksTestRF1.TestShutdownWithWaiters
      PgObjectLocksTestRF1.TestDisableReuseAbortedPlainTxn
  Without this change, the three LibPqTestBase-based tests fail during
  cluster bootstrap. The two PgMiniTestBase-based RF1 tests pass with
  or without this change (in-process MiniCluster does not have the race).
* Production code path is gated behind the TEST flag defaulting to 0,
  so no observable change for any non-ExternalMiniCluster caller (and
  StartMasters() has no callers outside ExternalMiniCluster::Start()).

Author: hkimura

src/yb/integration-tests/external_mini_cluster.cc

@@ -406,6 +406,13 @@ Status ExternalMiniCluster::Start(rpc::Messenger* messenger) {
   } else {
     LOG(INFO) << "No need to start tablet servers";
   }
+  // WaitForInitDb is called here, after tservers have launched, rather than inside
+  // StartMasters(). The master gates initdb on the TEST_master_min_live_tservers_before_initdb
+  // flag set in StartMaster(), so initdb only completes after the tservers above register.
+  // StartMasters() is only called from this method, so the deferral is safe.
+  if (opts_.enable_ysql) {
+    RETURN_NOT_OK(WaitForInitDb());
+  }
   if (opts_.enable_ysql && opts_.wait_for_tservers_to_accept_ysql_connections) {
     RETURN_NOT_OK(WaitForTabletServersToAcceptYSQLConnection(
         MonoTime::Now() + kTabletServerRegistrationTimeout));
@@ -563,6 +570,16 @@ Result<ExternalMasterPtr> ExternalMiniCluster::StartMaster(
   if (opts_.enable_ysql) {
     flags.push_back("--enable_ysql=true");
     flags.push_back("--master_auto_run_initdb");
+    // Avoid GitHub #31029 startup race: initdb's postgres bootstrap calls CreateTable on the
+    // master (to create the global transaction status table) and fails with "Not enough live
+    // tablet servers ..." if no tservers have registered yet. Have the master wait for the
+    // tservers we are about to start before launching initdb. Mirrors the Java-side fix in
+    // MiniYBCluster.java. num_tablet_servers is the right ceiling because (a) we are about
+    // to start exactly that many tservers immediately after, and (b) the master's RF
+    // requirement is min(num_tablet_servers, FLAGS_replication_factor), so any test that
+    // survives the bootstrap eventually needs all of them anyway.
+    flags.push_back(Format(
+        "--TEST_master_min_live_tservers_before_initdb=$0", opts_.num_tablet_servers));
     if (opts_.enable_ysql_auth) {
       flags.push_back("--ysql_enable_auth=true");
     }
@@ -1281,9 +1298,10 @@ Status ExternalMiniCluster::StartMasters() {
     masters_.push_back(master);
   }
 
-  if (opts_.enable_ysql) {
-    RETURN_NOT_OK(WaitForInitDb());
-  }
+  // Note: WaitForInitDb() is intentionally not called here. It is called from Start() after
+  // tservers have launched, so that the master's wait on
+  // TEST_master_min_live_tservers_before_initdb does not deadlock against tservers that
+  // haven't been started yet. See the comment block in Start() for details.
 
   // Trigger an election to avoid an unnecessary 3s wait on every cluster startup.
   if (!masters_.empty()) {