fs: avoid hanging os.Stat() in NodeGetVolumeStats after restart

If a node-plugin pod restarts while the backend (e.g. MDS) is down,
NodeGetVolumeStats used to call os.Stat() on the target path
immediately after (re)starting the health checker, before the
checker had a chance to detect the outage. Since os.Stat() blocks
in the kernel on an unresponsive mount, this held the VolumeLock
forever and made every later call for that path fail with Aborted.

Return early with an "not yet available" condition instead of
calling os.Stat() when the checker was just (re)started, for
CephFS.

Signed-off-by: SanjalKatiyar <sanjaldhir@gmail.com>

Author: SanjalKatiyar

internal/cephfs/nodeserver.go

@@ -950,10 +950,27 @@ func (ns *cephfsNodeServer) NodeGetVolumeStats(
 		// FileChecker is started with the stagingTargetPath, but we can't
 		// get the stagingPath from the request easily.
 		// TODO: resolve the stagingPath like rbd.getStagingPath() does
+		// NOTE: rbd.getStagingPath() uses os.Stat() internally which
+		// if called synchronously, could block indefinitely.
+
+		// Start the background checker but return
+		// immediately instead of calling os.Stat() on this goroutine.
+		// If the mount is unresponsive, os.Stat() would block,
+		// holding the VolumeLock (acquired above) and preventing all future
+		// calls for this path from reaching isHealthy().
+		// The background checker will do the stat(), the next periodic
+		// call will pick up the result (or detect the 75s timeout).
 		err = ns.healthChecker.StartChecker(req.GetVolumeId(), targetPath, hc.StatCheckerType)
 		if err != nil {
 			log.WarningLog(ctx, "failed to start healthchecker: %v", err)
 		}
+
+		return &csi.NodeGetVolumeStatsResponse{
+			VolumeCondition: &csi.VolumeCondition{
+				Abnormal: false,
+				Message:  "health checker started, status not yet available",
+			},
+		}, nil
 	}
 
 	// !healthy indicates a problem with the volume
@@ -966,7 +983,7 @@ func (ns *cephfsNodeServer) NodeGetVolumeStats(
 		}, nil
 	}
 
-	// warning: stat() may hang on an unhealthy volume
+	// no indefite block, if flow reaches here
 	stat, err := os.Stat(targetPath)
 	if err != nil {
 		if util.IsCorruptedMountError(err) {