app/vmselect: remove data race on updating EvalConfig.IsPartialResponse from concurrently running goroutines

This properly returns `is_partial: true` for partial responses.

app/vmselect/prometheus/prometheus.go

@@ -884,7 +884,7 @@ func QueryHandler(qt *querytracer.Tracer, startTime time.Time, at *auth.Token, w
 	qtDone := func() {
 		qt.Donef("query=%s, time=%d: series=%d", query, start, len(result))
 	}
-	WriteQueryResponse(bw, ec.IsPartialResponse, result, qt, qtDone)
+	WriteQueryResponse(bw, ec.IsPartialResponse.Load(), result, qt, qtDone)
 	if err := bw.Flush(); err != nil {
 		return fmt.Errorf("cannot flush query response to remote client: %w", err)
 	}
@@ -992,7 +992,7 @@ func queryRangeHandler(qt *querytracer.Tracer, startTime time.Time, at *auth.Tok
 	qtDone := func() {
 		qt.Donef("start=%d, end=%d, step=%d, query=%q: series=%d", start, end, step, query, len(result))
 	}
-	WriteQueryRangeResponse(bw, ec.IsPartialResponse, result, qt, qtDone)
+	WriteQueryRangeResponse(bw, ec.IsPartialResponse.Load(), result, qt, qtDone)
 	if err := bw.Flush(); err != nil {
 		return fmt.Errorf("cannot send query range response to remote client: %w", err)
 	}

app/vmselect/promql/eval.go

@@ -137,7 +137,7 @@ type EvalConfig struct {
 	DenyPartialResponse bool
 
 	// IsPartialResponse is set during query execution and can be used by Exec caller after query execution.
-	IsPartialResponse bool
+	IsPartialResponse atomic.Bool
 
 	timestamps     []int64
 	timestampsOnce sync.Once
@@ -159,16 +159,14 @@ func copyEvalConfig(src *EvalConfig) *EvalConfig {
 	ec.EnforcedTagFilterss = src.EnforcedTagFilterss
 	ec.GetRequestURI = src.GetRequestURI
 	ec.DenyPartialResponse = src.DenyPartialResponse
-	ec.IsPartialResponse = src.IsPartialResponse
+	ec.IsPartialResponse.Store(src.IsPartialResponse.Load())
 
 	// do not copy src.timestamps - they must be generated again.
 	return &ec
 }
 
 func (ec *EvalConfig) updateIsPartialResponse(isPartialResponse bool) {
-	if !ec.IsPartialResponse {
-		ec.IsPartialResponse = isPartialResponse
-	}
+	ec.IsPartialResponse.CompareAndSwap(false, isPartialResponse)
 }
 
 func (ec *EvalConfig) validate() {
@@ -854,7 +852,7 @@ func evalRollupFuncWithoutAt(qt *querytracer.Tracer, ec *EvalConfig, funcName st
 	if funcName == "absent_over_time" {
 		rvs = aggregateAbsentOverTime(ec, re.Expr, rvs)
 	}
-	ec.updateIsPartialResponse(ecNew.IsPartialResponse)
+	ec.updateIsPartialResponse(ecNew.IsPartialResponse.Load())
 	if offset != 0 && len(rvs) > 0 {
 		// Make a copy of timestamps, since they may be used in other values.
 		srcTimestamps := rvs[0].Timestamps
@@ -914,7 +912,7 @@ func evalRollupFuncWithSubquery(qt *querytracer.Tracer, ec *EvalConfig, funcName
 	if err != nil {
 		return nil, err
 	}
-	ec.updateIsPartialResponse(ecSQ.IsPartialResponse)
+	ec.updateIsPartialResponse(ecSQ.IsPartialResponse.Load())
 	if len(tssSQ) == 0 {
 		return nil, nil
 	}

docs/CHANGELOG.md

@@ -29,6 +29,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 * BUGFIX: vmselect: reduce memory usage and CPU usage when performing heavy queries. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3692).
 * BUGFIX: prevent from possible `invalid memory address or nil pointer dereference` panic during [background merge](https://docs.victoriametrics.com/#storage). The issue has been introduced at [v1.85.0](https://docs.victoriametrics.com/CHANGELOG.html#v1850). See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3897).
 * BUGFIX: prevent from possible `SIGBUS` crash on ARM architectures (Raspberry Pi), which deny unaligned access to 8-byte words. Thanks to @oliverpool for narrowing down the issue and for [the initial attempt to fix it](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3927).
+* BUGFIX: [VictoriaMetrics cluster](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html): always return `is_partial: true` in partial responses. Previously partial responses could be returned as non-partial in some cases.
 * BUGFIX: [VictoriaMetrics cluster](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html): properly take into account `-rpc.disableCompression` command-line flag at `vmstorage`. It was ignored since [v1.78.0](https://docs.victoriametrics.com/CHANGELOG.html#v1780). See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3932).
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): fix panic when [writing data to Kafka](https://docs.victoriametrics.com/vmagent.html#writing-metrics-to-kafka). The panic has been introduced in [v1.88.0](https://docs.victoriametrics.com/CHANGELOG.html#v1880).
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): stop showing `Please enter a valid Query and execute it` error message on the first load of vmui.