test: wait until there is no pending tasks in compaction_manager_basi…

…c_test

before this change, after triggering the compaction,
compaction_manager_basic_test waits until the triggered compaction
completes. but since the regular compaction is run in a loop which
does not stop until either the daemon is stopping, or there is no
more sstables to be compacted, or the compaction is disabled.

but we only get the input sstables for compaction after swiching
to the "pending" state, and acquiring the read lock of the
compaction_state, and acquiring the read lock is implemented as
an coroutine, so there is chance that coroutine is suspended,
and the execution switches to the test. in this case, the test
will find that even after the triggered compaction completes,
there are still one or more pending compactions. hence the test
fails.

to address this problem, instead of just waiting for the compaction
to complete, we also wait until the number of pending compaction tasks
is 0. so that even if the test manages to sneak into the time window,
it won't proceed and starting check the compaction manager's stats.

Fixes #14865
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes #14889

test/boost/sstable_compaction_test.cc

@@ -173,8 +173,11 @@ SEASTAR_TEST_CASE(compaction_manager_basic_test) {
 
     BOOST_REQUIRE(cf->sstables_count() == idx.size());
     cf->trigger_compaction();
-    // wait for submitted job to finish.
-    do_until([&cm] { return cm.get_stats().completed_tasks > 0; }, [] {
+    // wait for submitted job to finish and there is no pending tasks
+    do_until([&cm] {
+        return (cm.get_stats().completed_tasks > 0 &&
+                cm.get_stats().pending_tasks == 0);
+    }, [] {
         return sleep(std::chrono::milliseconds(100));
     }).wait();
     // test no more running compactions