Clean Symbol Tables

Signed-off-by: SungJin1212 <tjdwls1201@gmail.com>

Author: SungJin1212

CHANGELOG.md

@@ -10,6 +10,7 @@
 * [BUGFIX] Fix nil when ingester_query_max_attempts > 1. #7369
 * [BUGFIX] Querier: Fix queryWithRetry and labelsWithRetry returning (nil, nil) on cancelled context by propagating ctx.Err(). #7370
 * [BUGFIX] Metrics Helper: Fix non-deterministic bucket order in merged histograms by sorting buckets after map iteration, matching Prometheus client library behavior. #7380
+* [BUGFIX] Fix memory leak in `ReuseWriteRequestV2` by explicitly clearing the `Symbols` backing array string pointers before returning the object to `sync.Pool`. #7373
 * [BUGFIX] Distributor: Return HTTP 401 Unauthorized when tenant ID resolution fails in the Prometheus Remote Write 2.0 path. #7389
 
 ## 1.21.0 in progress

pkg/cortexpb/timeseriesv2.go

@@ -77,6 +77,10 @@ func ReuseWriteRequestV2(req *PreallocWriteRequestV2) {
 		req.data = nil
 	}
 	req.Source = 0
+
+	for i := range req.Symbols {
+		req.Symbols[i] = ""
+	}
 	req.Symbols = req.Symbols[:0]
 	if req.Timeseries != nil {
 		ReuseSliceV2(req.Timeseries)

pkg/cortexpb/timeseriesv2_test.go

@@ -58,6 +58,38 @@ func TestTimeseriesV2FromPool(t *testing.T) {
 	})
 }
 
+func TestReuseWriteRequestV2(t *testing.T) {
+	req := PreallocWriteRequestV2FromPool()
+
+	// Populate req with some data.
+	req.Source = RULE
+	req.Symbols = append(req.Symbols, "", "__name__", "test")
+
+	tsSlice := PreallocTimeseriesV2SliceFromPool()
+	tsSlice = append(tsSlice, PreallocTimeseriesV2{TimeSeriesV2: TimeseriesV2FromPool()})
+	req.Timeseries = tsSlice
+
+	// Capture backing array before reuse
+	symbolsBackingArray := req.Symbols[:cap(req.Symbols)]
+	require.Equal(t, "__name__", symbolsBackingArray[1])
+	require.Equal(t, "test", symbolsBackingArray[2])
+
+	// Put the request back into the pool
+	ReuseWriteRequestV2(req)
+
+	// Verify clearing directly on the backing array
+	for i, s := range symbolsBackingArray[:3] {
+		assert.Equalf(t, "", s, "symbol at index %d not cleared", i)
+	}
+
+	// Source is reset to default
+	assert.Equal(t, API, req.Source)
+	// The symbol length is properly reset to 0.
+	assert.Len(t, req.Symbols, 0)
+	// Timeseries slice is nil
+	assert.Nil(t, req.Timeseries)
+}
+
 func BenchmarkMarshallWriteRequestV2(b *testing.B) {
 	ts := PreallocTimeseriesV2SliceFromPool()