lib{mergset,storage}: prevent possible race condition with logging st… (#3900)

lib{mergset,storage}: prevent possible race condition with logging stats for merges

Previously partwrapper could be release by background process and reference for part may be invalid 
during logging stats. It will lead to panic at vmstorage
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3897

docs/CHANGELOG.md

@@ -17,6 +17,8 @@ The following tip changes can be tested by building VictoriaMetrics components f
 
 * FEATURE: [vmctl](https://docs.victoriametrics.com/vmctl.html): `vmctl` `vm-native` mode now splits the migration process on per-metric basis. This allows to migrate metrics one-by-one according to the specified filter. This change allows to retry export/import requests for a specific metric and provides a better understanding of the migration progress. See [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3600).
 
+* BUGFIX: prevent from possible panic during [background merge process](https://docs.victoriametrics.com/#storage). It may occur in rare case and was introduced at [v1.85.0](https://docs.victoriametrics.com/CHANGELOG.html#v1850) when implementing [this feature](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3337).
+
 ## [v1.88.1](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.88.1)
 
 Released at 2023-02-27

lib/mergeset/table.go

@@ -1164,14 +1164,6 @@ func (tb *Table) mergeParts(pws []*partWrapper, stopCh <-chan struct{}, isFinal
 	if err != nil {
 		return fmt.Errorf("cannot atomically register the created part: %w", err)
 	}
-	tb.swapSrcWithDstParts(pws, pwNew, dstPartType)
-
-	d := time.Since(startTime)
-	if d <= 30*time.Second {
-		return nil
-	}
-
-	// Log stats for long merges.
 	dstItemsCount := uint64(0)
 	dstBlocksCount := uint64(0)
 	dstSize := uint64(0)
@@ -1183,6 +1175,15 @@ func (tb *Table) mergeParts(pws []*partWrapper, stopCh <-chan struct{}, isFinal
 		dstSize = pDst.size
 		dstPartPath = pDst.path
 	}
+
+	tb.swapSrcWithDstParts(pws, pwNew, dstPartType)
+
+	d := time.Since(startTime)
+	if d <= 30*time.Second {
+		return nil
+	}
+
+	// Log stats for long merges.
 	durationSecs := d.Seconds()
 	itemsPerSec := int(float64(srcItemsCount) / durationSecs)
 	logger.Infof("merged (%d parts, %d items, %d blocks, %d bytes) into (1 part, %d items, %d blocks, %d bytes) in %.3f seconds at %d items/sec to %q",

lib/storage/partition.go

@@ -1376,14 +1376,7 @@ func (pt *partition) mergeParts(pws []*partWrapper, stopCh <-chan struct{}, isFi
 	if err != nil {
 		return fmt.Errorf("cannot atomically register the created part: %w", err)
 	}
-	pt.swapSrcWithDstParts(pws, pwNew, dstPartType)
 
-	d := time.Since(startTime)
-	if d <= 30*time.Second {
-		return nil
-	}
-
-	// Log stats for long merges.
 	dstRowsCount := uint64(0)
 	dstBlocksCount := uint64(0)
 	dstSize := uint64(0)
@@ -1395,6 +1388,15 @@ func (pt *partition) mergeParts(pws []*partWrapper, stopCh <-chan struct{}, isFi
 		dstSize = pDst.size
 		dstPartPath = pDst.String()
 	}
+
+	pt.swapSrcWithDstParts(pws, pwNew, dstPartType)
+
+	d := time.Since(startTime)
+	if d <= 30*time.Second {
+		return nil
+	}
+
+	// Log stats for long merges.
 	durationSecs := d.Seconds()
 	rowsPerSec := int(float64(srcRowsCount) / durationSecs)
 	logger.Infof("merged (%d parts, %d rows, %d blocks, %d bytes) into (1 part, %d rows, %d blocks, %d bytes) in %.3f seconds at %d rows/sec to %q",