pageserver: add wait_until_flushed parameter for timeline checkpoint (#10013)

## Problem

I'm writing an ingest benchmark in #9812. To time S3 uploads, I need to
schedule a flush of the Pageserver's in-memory layer, but don't actually
want to wait around for it to complete (which will take a minute).

## Summary of changes

Add a parameter `wait_until_flush` (default `true`) for
`timeline/checkpoint` to control whether to wait for the flush to
complete.

pageserver/src/http/routes.rs

@@ -2148,16 +2148,20 @@ async fn timeline_checkpoint_handler(
     // By default, checkpoints come with a compaction, but this may be optionally disabled by tests that just want to flush + upload.
     let compact = parse_query_param::<_, bool>(&request, "compact")?.unwrap_or(true);
 
+    let wait_until_flushed: bool =
+        parse_query_param(&request, "wait_until_flushed")?.unwrap_or(true);
+
     let wait_until_uploaded =
         parse_query_param::<_, bool>(&request, "wait_until_uploaded")?.unwrap_or(false);
 
     async {
         let ctx = RequestContext::new(TaskKind::MgmtRequest, DownloadBehavior::Download);
         let timeline = active_timeline_of_active_tenant(&state.tenant_manager, tenant_shard_id, timeline_id).await?;
-        timeline
-            .freeze_and_flush()
-            .await
-            .map_err(|e| {
+        if wait_until_flushed {
+            timeline.freeze_and_flush().await
+        } else {
+            timeline.freeze().await.and(Ok(()))
+        }.map_err(|e| {
                 match e {
                     tenant::timeline::FlushLayerError::Cancelled => ApiError::ShuttingDown,
                     other => ApiError::InternalServerError(other.into()),

pageserver/src/tenant/timeline.rs

@@ -1457,23 +1457,31 @@ impl Timeline {
         Ok(lease)
     }
 
-    /// Flush to disk all data that was written with the put_* functions
+    /// Freeze the current open in-memory layer. It will be written to disk on next iteration.
+    /// Returns the flush request ID which can be awaited with wait_flush_completion().
+    #[instrument(skip(self), fields(tenant_id=%self.tenant_shard_id.tenant_id, shard_id=%self.tenant_shard_id.shard_slug(), timeline_id=%self.timeline_id))]
+    pub(crate) async fn freeze(&self) -> Result<u64, FlushLayerError> {
+        self.freeze0().await
+    }
+
+    /// Freeze and flush the open in-memory layer, waiting for it to be written to disk.
     #[instrument(skip(self), fields(tenant_id=%self.tenant_shard_id.tenant_id, shard_id=%self.tenant_shard_id.shard_slug(), timeline_id=%self.timeline_id))]
     pub(crate) async fn freeze_and_flush(&self) -> Result<(), FlushLayerError> {
         self.freeze_and_flush0().await
     }
 
+    /// Freeze the current open in-memory layer. It will be written to disk on next iteration.
+    /// Returns the flush request ID which can be awaited with wait_flush_completion().
+    pub(crate) async fn freeze0(&self) -> Result<u64, FlushLayerError> {
+        let mut g = self.write_lock.lock().await;
+        let to_lsn = self.get_last_record_lsn();
+        self.freeze_inmem_layer_at(to_lsn, &mut g).await
+    }
+
     // This exists to provide a non-span creating version of `freeze_and_flush` we can call without
     // polluting the span hierarchy.
     pub(crate) async fn freeze_and_flush0(&self) -> Result<(), FlushLayerError> {
-        let token = {
-            // Freeze the current open in-memory layer. It will be written to disk on next
-            // iteration.
-            let mut g = self.write_lock.lock().await;
-
-            let to_lsn = self.get_last_record_lsn();
-            self.freeze_inmem_layer_at(to_lsn, &mut g).await?
-        };
+        let token = self.freeze0().await?;
         self.wait_flush_completion(token).await
     }