asim: move plots to testdata/generated

I have work in progress that automatically generates all possible plots for each test.
It's better to keep them apart at that point since there's a fair number of files.
Additionally, I'd like to make "reports" (like an HTML file) comparing different
allocator configurations, and so we're going to have even more structure that
should be outside the realm of the datadriven file walker.

Epic: CRDB-25222

Author: tbg

pkg/kv/kvserver/asim/tests/BUILD.bazel

@@ -33,6 +33,7 @@ go_test(
     srcs = [
         "datadriven_simulation_test.go",
         "helpers_test.go",
+        "plotting_test.go",
         "rand_test.go",
     ],
     data = glob(["testdata/**"]),

pkg/kv/kvserver/asim/tests/datadriven_simulation_test.go

@@ -9,7 +9,6 @@ import (
 	"context"
 	"fmt"
 	"hash/fnv"
-	"io"
 	"math/rand"
 	"os"
 	"path/filepath"
@@ -37,10 +36,6 @@ import (
 	"github.com/cockroachdb/datadriven"
 	"github.com/cockroachdb/logtags"
 	"github.com/stretchr/testify/require"
-	"gonum.org/v1/plot"
-	"gonum.org/v1/plot/plotter"
-	"gonum.org/v1/plot/plotutil"
-	"gonum.org/v1/plot/vg"
 )
 
 var runAsimTests = envutil.EnvOrDefaultBool("COCKROACH_RUN_ASIM_TESTS", false)
@@ -197,9 +192,6 @@ func TestDataDriven(t *testing.T) {
 		leakTestAfter()
 	})
 	datadriven.Walk(t, dir, func(t *testing.T, path string) {
-		if filepath.Ext(path) != ".txt" {
-			return
-		}
 		ctx := logtags.AddTag(context.Background(), "name", filepath.Base(path))
 		// The inline comment below is required for TestLint/TestTParallel.
 		// We use t.Cleanup to work around the issue this lint is trying to prevent.
@@ -213,7 +205,6 @@ func TestDataDriven(t *testing.T) {
 		assertions := []assertion.SimulationAssertion{}
 		var stateStrAcrossSamples []string
 		var runs []history.History
-		plotNum := 0
 		datadriven.RunTest(t, path, func(t *testing.T, d *datadriven.TestData) string {
 			defer func() {
 				require.Empty(t, d.CmdArgs, "leftover arguments for %s", d.Cmd)
@@ -411,7 +402,7 @@ func TestDataDriven(t *testing.T) {
 
 				return ""
 			case "eval":
-				t.Logf("running eval for %s", path)
+				t.Logf("running eval for %s", filepath.Base(path))
 				samples := 1
 				seed := rand.Int63()
 				duration := 30 * time.Minute
@@ -592,52 +583,27 @@ func TestDataDriven(t *testing.T) {
 				buf.WriteString(s)
 				buf.WriteString("\n")
 
-				// The plot's filename is determined by the name of the test file and
-				// the plot counter within the file.
-				plotNum++
-				plotDir := filepath.Dir(path)
-				fileName := fmt.Sprintf(
-					"%s_%d_%s.png",
-					strings.TrimSuffix(filepath.Base(path), filepath.Ext(path)),
-					plotNum,
-					stat,
-				)
-				filePath := filepath.Join(plotDir, fileName)
+				testFileName := strings.TrimSuffix(filepath.Base(path), filepath.Ext(path))
+				plotDir := datapathutils.TestDataPath(t, "generated", testFileName)
+
+				// To ensure the plots are up to date, the datapoints are hashed and
+				// printed in the output.
+				hasher := fnv.New64a()
+				_, err := hasher.Write([]byte(fmt.Sprint(ts[stat])))
+				require.NoError(t, err)
+
+				plotFileName := fmt.Sprintf("%s_%d_%s.png", testFileName, sample, stat)
 
 				var rewrite bool
 				require.NoError(t, sniffarg.DoEnv("rewrite", &rewrite))
 				if rewrite {
-					p := plot.New()
-					p.Title.Text = stat
-					p.X.Label.Text = "Ticks"
-					p.Y.Label.Text = stat
-
-					var plotArgs []interface{}
-					for storeIdx, storeTS := range ts[stat] {
-						storeID := storeIdx + 1
-						pts := make(plotter.XYs, len(storeTS))
-						for i, val := range storeTS {
-							pts[i].X = float64(i) // tick
-							pts[i].Y = val
-						}
-						plotArgs = append(plotArgs, fmt.Sprintf("store-%d", storeID), pts)
-					}
-
-					err := plotutil.AddLinePoints(p, plotArgs...)
-					require.NoError(t, err)
-					require.NoError(t, p.Save(8*vg.Inch, 6*vg.Inch, filePath))
+					_ = os.MkdirAll(plotDir, 0755)
+					plotPath := filepath.Join(plotDir, plotFileName)
+					b := generatePlot(t, stat, ts[stat])
+					require.NoError(t, os.WriteFile(plotPath, b, 0644))
 				}
 
-				f, err := os.Open(filePath)
-				require.NoError(t, err, "cannot open plot %s; use -rewrite", fileName)
-				defer f.Close()
-
-				// To ensure the plots are up to date, the file contents are hashed and
-				// printed in the output.
-				hasher := fnv.New64a()
-				_, err = io.Copy(hasher, f)
-				require.NoError(t, err)
-				buf.WriteString(fmt.Sprintf("%s (%x)", fileName, hasher.Sum(nil)))
+				buf.WriteString(fmt.Sprintf("%s (%x)", plotFileName, hasher.Sum(nil)))
 				return buf.String()
 			default:
 				return fmt.Sprintf("unknown command: %s", d.Cmd)

pkg/kv/kvserver/asim/tests/plotting_test.go

@@ -0,0 +1,50 @@
+// Copyright 2025 The Cockroach Authors.
+//
+// Use of this software is governed by the CockroachDB Software License
+// included in the /LICENSE file.
+
+package tests
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"gonum.org/v1/plot"
+	"gonum.org/v1/plot/plotter"
+	"gonum.org/v1/plot/plotutil"
+	"gonum.org/v1/plot/vg"
+)
+
+// generatePlot creates a single plot for the given stat from simulation history.
+// Hashes the plot file content directly to the provided hasher.
+// If rewrite is false, the plot is generated but not saved to disk.
+// Returns the filename or an error if plot generation fails.
+func generatePlot(t *testing.T, stat string, sl [][]float64) []byte {
+	p := plot.New()
+	p.Title.Text = stat
+	// TODO(tbg): convert to time.
+	p.X.Label.Text = "Ticks"
+	p.Y.Label.Text = stat
+
+	var plotArgs []interface{}
+	for storeIdx, storeTS := range sl {
+		storeID := storeIdx + 1
+		pts := make(plotter.XYs, len(storeTS))
+		for i, val := range storeTS {
+			pts[i].X = float64(i) // tick
+			pts[i].Y = val
+		}
+		plotArgs = append(plotArgs, fmt.Sprintf("s%d", storeID), pts)
+	}
+
+	err := plotutil.AddLinePoints(p, plotArgs...)
+	require.NoError(t, err)
+	wt, err := p.WriterTo(8*vg.Inch, 6*vg.Inch, "png")
+	require.NoError(t, err)
+	var buf bytes.Buffer
+	_, err = wt.WriteTo(&buf)
+	require.NoError(t, err)
+	return buf.Bytes()
+}