feat(pageserver): add compact queue http endpoint (#10173)

## Problem

We cannot get the size of the compaction queue and access the info.

Part of #9114 

## Summary of changes

* Add an API endpoint to get the compaction queue.
* gc_compaction test case now waits until the compaction finishes.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

test_runner/fixtures/pageserver/http.py

@@ -738,6 +738,18 @@ class PageserverHttpClient(requests.Session, MetricsGetter):
         res_json = res.json()
         assert res_json is None
 
+    def timeline_compact_info(
+        self,
+        tenant_id: TenantId | TenantShardId,
+        timeline_id: TimelineId,
+    ) -> Any:
+        res = self.get(
+            f"http://localhost:{self.port}/v1/tenant/{tenant_id}/timeline/{timeline_id}/compact",
+        )
+        self.verbose_error(res)
+        res_json = res.json()
+        return res_json
+
     def timeline_compact(
         self,
         tenant_id: TenantId | TenantShardId,
@@ -749,7 +761,6 @@ class PageserverHttpClient(requests.Session, MetricsGetter):
         enhanced_gc_bottom_most_compaction=False,
         body: dict[str, Any] | None = None,
     ):
-        self.is_testing_enabled_or_skip()
         query = {}
         if force_repartition:
             query["force_repartition"] = "true"

test_runner/regress/test_compaction.py

@@ -176,6 +176,12 @@ def test_pageserver_gc_compaction_smoke(neon_env_builder: NeonEnvBuilder):
 
         workload.churn_rows(row_count, env.pageserver.id)
 
+    def compaction_finished():
+        queue_depth = len(ps_http.timeline_compact_info(tenant_id, timeline_id))
+        assert queue_depth == 0
+
+    wait_until(compaction_finished, timeout=60)
+
     # ensure gc_compaction is scheduled and it's actually running (instead of skipping due to no layers picked)
     env.pageserver.assert_log_contains(
         "scheduled_compact_timeline.*picked .* layers for compaction"