roachtest/mixed-versions-compat: use corpus for master

Fixes: #91350

The CI build scripts take advantage of the branch name
to uplaod the corpus to GCS. Unfortunately, we have
no way of know if the current build is master inside
the roachtest. To address this, this patch supports
fetching the master corpus as a fallback.

Release note: None

pkg/cmd/roachtest/tests/mixed_version_decl_schemachange_compat.go

@@ -41,7 +41,11 @@ func registerDeclSchemaChangeCompatMixedVersions(r registry.Registry) {
 
 // fetchCorpusToTmp fetches corpus for a given version to tmp
 func fetchCorpusToTmpDir(
-	ctx context.Context, t test.Test, c cluster.Cluster, versionNumber string,
+	ctx context.Context,
+	t test.Test,
+	c cluster.Cluster,
+	versionNumber string,
+	alternateVersion string,
 ) (corpusFilePath string, cleanupFn func()) {
 	tmpDir, err := os.MkdirTemp("", "corpus")
 	if err != nil {
@@ -55,15 +59,29 @@ func fetchCorpusToTmpDir(
 			t.L().Printf("failed to clean up tmp directory %v", err)
 		}
 	}
-	err = c.RunE(ctx, c.Node(1),
-		fmt.Sprintf(" gsutil cp gs://cockroach-corpus/corpus-%s/corpus %s",
-			versionNumber,
-			corpusFilePath))
+	versionsToCheck := []string{versionNumber}
+	if len(alternateVersion) > 0 {
+		versionsToCheck = []string{versionNumber, alternateVersion}
+	}
+	for i, version := range versionsToCheck {
+		err = c.RunE(ctx, c.Node(1),
+			fmt.Sprintf(" gsutil cp gs://cockroach-corpus/corpus-%s/corpus %s",
+				version,
+				corpusFilePath))
+		if err != nil && i != len(versionsToCheck)-1 {
+			t.L().Printf("Failed to fetch corpus %s with error %v, trying the next one",
+				version,
+				err)
+		}
+		if err == nil {
+			t.L().Printf("Fetched validation corpus for %v", version)
+			break
+		}
+	}
 	if err != nil {
 		cleanupFn()
 		t.Fatalf("Missing validation corpus for %v (%v)", versionNumber, err)
 	}
-	t.L().Printf("Fetched validation corpus for %v", versionNumber)
 	return corpusFilePath, cleanupFn
 }
 
@@ -117,19 +135,20 @@ func runDeclSchemaChangeCompatMixedVersions(
 		t.Fatal(err)
 	}
 	// Test definitions which indicates which version of the corpus to fetch,
-	// and the bianry to validate against.
+	// and the binary to validate against.
 	compatTests := []struct {
-		testName      string
-		binaryVersion string
-		corpusVersion string
+		testName               string
+		binaryVersion          string
+		corpusVersion          string
+		alternateCorpusVersion string
 	}{
-		{"backwards compatibility", predecessorVersion, fmt.Sprintf("mixed-release-%d.%d", buildVersion.Major(), buildVersion.Minor())},
-		{"forwards compatibility", "", fmt.Sprintf("release-%s", versionRegex.FindStringSubmatch(predecessorVersion)[0])},
-		{"same version", "", fmt.Sprintf("release-%s", versionRegex.FindStringSubmatch(buildVersion.String())[0])},
+		{"backwards compatibility", predecessorVersion, fmt.Sprintf("mixed-release-%d.%d", buildVersion.Major(), buildVersion.Minor()), "mixed-master"},
+		{"forwards compatibility", "", fmt.Sprintf("release-%s", versionRegex.FindStringSubmatch(predecessorVersion)[0]), ""},
+		{"same version", "", fmt.Sprintf("release-%s", versionRegex.FindStringSubmatch(buildVersion.String())[0]), "master"},
 	}
-	for _, test := range compatTests {
-		binaryName := uploadVersion(ctx, t, c, c.All(), test.binaryVersion)
-		corpusPath, cleanupFn := fetchCorpusToTmpDir(ctx, t, c, test.corpusVersion)
+	for _, testInfo := range compatTests {
+		binaryName := uploadVersion(ctx, t, c, c.All(), testInfo.binaryVersion)
+		corpusPath, cleanupFn := fetchCorpusToTmpDir(ctx, t, c, testInfo.corpusVersion, testInfo.alternateCorpusVersion)
 		func() {
 			defer cleanupFn()
 			validateCorpusFile(ctx, t, c, binaryName, corpusPath)