storage: don't cache benchmark fixtures in metamorphic builds

In CI, we sometimes run benchmarks in metamorphic builds to ensure the
benchmarks pass. We don't want to cache the fixtures generated by these runs,
because we may end up caching fixtures that reflect metamorphic values. This
can lead to very non-deterministic behavior in the benchmarks depending on what
metamorphic settings were active when the fixture was generated.

Epic: none
Release note: none
Informs #135114.

pkg/storage/bench_data_test.go

@@ -23,6 +23,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/util/encoding"
 	"github.com/cockroachdb/cockroach/pkg/util/hlc"
 	"github.com/cockroachdb/cockroach/pkg/util/log"
+	"github.com/cockroachdb/cockroach/pkg/util/metamorphic"
 	"github.com/cockroachdb/cockroach/pkg/util/randutil"
 	"github.com/cockroachdb/pebble/vfs"
 	"github.com/stretchr/testify/require"
@@ -69,9 +70,23 @@ func getInitialStateEngine(
 	ctx context.Context, b *testing.B, initial initialState, inMemory bool,
 ) Engine {
 	name := strings.Join(initial.Key(), "-")
-	dir := testfixtures.ReuseOrGenerate(b, name, func(dir string) {
+
+	// In CI, we sometimes run benchmarks in metamorphic builds to ensure the
+	// benchmarks pass. We don't want to cache the fixtures generated by these
+	// runs, because we may end up caching fixtures that reflect metamorphic
+	// values. This can lead to very non-deterministic behavior in the
+	// benchmarks depending on what metamorphic settings were active when the
+	// fixture was generated.
+	var dir string
+	if metamorphic.IsMetamorphicBuild() {
+		b.Log("running in metamorphic build; ignoring existing fixtures and not caching new ones")
+		dir = b.TempDir()
 		buildInitialState(ctx, b, initial, dir)
-	})
+	} else {
+		dir = testfixtures.ReuseOrGenerate(b, name, func(dir string) {
+			buildInitialState(ctx, b, initial, dir)
+		})
+	}
 	dataDir := filepath.Join(dir, "data")
 
 	opts := append([]ConfigOption{