add peekCurrentJournalPosition() endpoint for sapling

Summary:
This is like getCurrentJournalPosition, except it does not count as "observing" the current journal position for journal subscription purposes (i.e. does not unblock sending new notifications).

Switch Sapling to use it for "sl debugwait" to ensure that Sapling is not increasing journal notification noise by constantly observing the journal position. If the server doesn't support it, we fall back to getCurrentJournalPosition.

Reviewed By: MichaelCuevas

Differential Revision: D95292274

fbshipit-source-id: 9b2685206416f1d33a693516f1c93b3572977d11

Author: muirdm

eden/fs/service/eden.thrift

@@ -2041,6 +2041,14 @@ struct ChangesSinceV2Result {
   2: list<ChangeNotification> changes;
 }
 
+struct PeekCurrentJournalPositionRequest {
+  1: MountId mountId;
+}
+
+struct PeekCurrentJournalPositionResponse {
+  1: JournalPosition position;
+}
+
 /**
  * Argument to changesSinceV2 API
  *
@@ -2448,6 +2456,14 @@ service EdenService extends fb303_core.BaseService {
     1: EdenError ex,
   );
 
+  /** Like getCurrentJournalPosition but does not mark the journal as observed.
+   * Use this when you only need the current position and don't want to
+   * affect subscriber notification coalescing.
+   */
+  PeekCurrentJournalPositionResponse peekCurrentJournalPosition(
+    1: PeekCurrentJournalPositionRequest params,
+  ) throws (1: EdenError ex);
+
   /** Returns the set of files (and dirs) that changed since a prior point.
    * If fromPosition.mountGeneration is mismatched with the current
    * mountGeneration, throws an EdenError with errorCode = ERANGE.