test: one run-directory writer for the packages that share it

internal/tracecorpus and cmd/internal-tools/exploration-reach declared the same
24-line writeRun, identical apart from whether the seed was a parameter. The
package's charter is on it: a helper goes here once it has callers in two
packages, and stays in a package's own tests until then.

cmd/internal-tools/exploration-reach/reach_test.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/priyanshujain/sanderling/internal/driver/ioscompanion"
 	"github.com/priyanshujain/sanderling/internal/hierarchy"
+	"github.com/priyanshujain/sanderling/internal/testsupport"
 	"github.com/priyanshujain/sanderling/internal/trace"
 	"github.com/priyanshujain/sanderling/internal/tracecorpus"
 )
@@ -136,27 +137,8 @@ func measureRun(t *testing.T, seed int64, dumps ...string) Reach {
 
 func writeRun(t *testing.T, seed int64, dumps ...string) string {
 	t.Helper()
-	directory := t.TempDir()
+	return testsupport.WriteRunFromDumps(
-	writer, err := trace.NewWriter(directory)
+		t, t.TempDir(), trace.Meta{Seed: seed, Platform: "web"}, dumps...)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if err := writer.WriteMeta(trace.Meta{Seed: seed, Platform: "web"}); err != nil {
-		t.Fatal(err)
-	}
-	for index, dump := range dumps {
-		tree, err := hierarchy.Parse(dump)
-		if err != nil {
-			t.Fatal(err)
-		}
-		if err := writer.WriteStep(trace.Step{Index: index + 1, Hierarchy: tree}); err != nil {
-			t.Fatal(err)
-		}
-	}
-	if err := writer.Close(); err != nil {
-		t.Fatal(err)
-	}
-	return directory
 }
 
 func appendFinalize(t *testing.T, directory string, index int) {

internal/testsupport/runs.go

@@ -0,0 +1,41 @@
+// Package testsupport holds test helpers that more than one package needs.
+// It exists because the suite's helper layer grew by copying: a helper written
+// inside one package's tests is invisible to the next package that needs it, so
+// the next package writes it again. Anything here has at least two callers in
+// different packages; a helper with one belongs in that package's own tests.
+package testsupport
+
+import (
+	"testing"
+
+	"github.com/priyanshujain/sanderling/internal/hierarchy"
+	"github.com/priyanshujain/sanderling/internal/trace"
+)
+
+// WriteRunFromDumps writes a complete run directory: meta.json, then one step
+// per hierarchy dump numbered from 1. It goes through trace.NewWriter rather
+// than composing the files by hand so a fixture cannot claim a trace_version
+// the writer would not have produced.
+func WriteRunFromDumps(t *testing.T, directory string, meta trace.Meta, dumps ...string) string {
+	t.Helper()
+	writer, err := trace.NewWriter(directory)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := writer.WriteMeta(meta); err != nil {
+		t.Fatal(err)
+	}
+	for index, dump := range dumps {
+		tree, err := hierarchy.Parse(dump)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := writer.WriteStep(trace.Step{Index: index + 1, Hierarchy: tree}); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := writer.Close(); err != nil {
+		t.Fatal(err)
+	}
+	return directory
+}

internal/tracecorpus/corpus_test.go

@@ -7,7 +7,7 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/priyanshujain/sanderling/internal/hierarchy"
+	"github.com/priyanshujain/sanderling/internal/testsupport"
 	"github.com/priyanshujain/sanderling/internal/trace"
 )
 
@@ -73,27 +73,8 @@ func TestLoadRefusesElementsWithNoStoredShape(t *testing.T) {
 
 func writeRun(t *testing.T, dumps ...string) string {
 	t.Helper()
-	directory := t.TempDir()
+	return testsupport.WriteRunFromDumps(
-	writer, err := trace.NewWriter(directory)
+		t, t.TempDir(), trace.Meta{Seed: 7, Platform: "web"}, dumps...)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if err := writer.WriteMeta(trace.Meta{Seed: 7, Platform: "web"}); err != nil {
-		t.Fatal(err)
-	}
-	for index, dump := range dumps {
-		tree, err := hierarchy.Parse(dump)
-		if err != nil {
-			t.Fatal(err)
-		}
-		if err := writer.WriteStep(trace.Step{Index: index + 1, Hierarchy: tree}); err != nil {
-			t.Fatal(err)
-		}
-	}
-	if err := writer.Close(); err != nil {
-		t.Fatal(err)
-	}
-	return directory
 }
 
 func downgrade(t *testing.T, path string) {