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

@@ -15,6 +15,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 
 ## v1.87.x long-time support release (LTS)
 
+* 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).
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): do not register `vm_promscrape_config_*` metrics if `-promscrape.config` flag is not used. Previously those metrics were registered and never updated, which was confusing and could trigger false-positive alerts.
 * BUGFIX: [vmctl](https://docs.victoriametrics.com/vmctl.html): skip measurements with no fields when migrating data from influxdb. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3837).
 * BUGFIX: [vmauth](https://docs.victoriametrics.com/vmauth.html): fix `cannot serve http` panic when plain HTTP request is sent to `vmauth` configured to accept requests over [proxy protocol](https://www.haproxy.org/download/2.3/doc/proxy-protocol.txt)-encoded request (e.g. when `vmauth` runs with `-httpListenAddr.useProxyProtocol` command-line flag). The issue has been introduced at [v1.87.0](https://docs.victoriametrics.com/CHANGELOG.html#v1870) when implementing [this feature](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3335).

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",