Just nits. Sorry for the late review.

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @fqazi)

pkg/cmd/roachtest/tests/mixed_version_decl_schemachange_compat.go line 42 at r1 (raw file):

	ctx context.Context, logger *logger.Logger, c cluster.Cluster, versionNumber string,
) string {
	const tmpPath = "/tmp/"

this should use os.MkdirTemp

pkg/cmd/roachtest/tests/mixed_version_decl_schemachange_compat.go line 48 at r1 (raw file):

			tmpPath))
	if err != nil {
		logger.Printf("Missing validation corpus for %v", versionNumber)

delete the tempdir if there's an error? Or rather, should we propagate the error up and fail the test?

pkg/cmd/roachtest/tests/mixed_version_decl_schemachange_compat.go line 68 at r1 (raw file):

		t.Fatal(err)
	}
	failureRegex := regexp.MustCompile(`failed to validate.*`)

should we have the debug command exit with a non-zero exit code if the validation fails? Maybe it's a bit late for that.

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @ajwerner)

pkg/cmd/roachtest/tests/mixed_version_decl_schemachange_compat.go line 42 at r1 (raw file):

Done.

pkg/cmd/roachtest/tests/mixed_version_decl_schemachange_compat.go line 48 at r1 (raw file):

Done.

Moved it so that the test passed into these functions for a direct failure.

pkg/cmd/roachtest/tests/mixed_version_decl_schemachange_compat.go line 68 at r1 (raw file):

Done.

Modified the command to do this, and added support in the test detect both ways of failing. In the future we can drop the first method of checking.

Reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @fqazi)

pkg/cmd/roachtest/tests/mixed_version_decl_schemachange_compat.go line 114 at r2 (raw file):

		t.Fatal(err)
	}
	// Validate the mixed version of the corpus, against the previous

nit: this could be refactored to be more table-driven, the core of the logic seems duplicated

Code quote:

	// Validate the mixed version of the corpus, against the previous
	// version.
	binaryName := uploadVersion(ctx, t, c, c.All(), predecessorVersion)
	mixedVersionCorpus := fmt.Sprintf("%d.%d-mixed", buildVersion.Major(), buildVersion.Minor())
	corpusPath, cleanupFn := fetchCorpusToTmpDir(ctx, t, c, mixedVersionCorpus)
	if err != nil {
		t.Fatal("failed to locate backwards compatible corpus (%v)", err)
	}
	validateCorpusFile(ctx, t, c, binaryName, corpusPath)
	cleanupFn()
	// Forward compatibility testing of elements generated in the previous
	// version.
	binaryName = uploadVersion(ctx, t, c, c.All(), "")
	versionRegex := regexp.MustCompile(`(\d+\.\d+)`)
	versionNumber := versionRegex.FindStringSubmatch(predecessorVersion)[0]
	corpusPath, cleanupFn = fetchCorpusToTmpDir(ctx, t, c, versionNumber)
	if err != nil {
		t.Fatal("failed to locate backwards compatible corpus (%v)", err)
	}
	validateCorpusFile(ctx, t, c, binaryName, corpusPath)
	cleanupFn()
	// Validate the corpus generated on the same version as well.
	versionNumber = versionRegex.FindStringSubmatch(buildVersion.String())[0]
	corpusPath, cleanupFn = fetchCorpusToTmpDir(ctx, t, c, versionNumber)
	if err != nil {
		t.Fatal("failed to locate same version corpus (%v)", err)
	}
	validateCorpusFile(ctx, t, c, binaryName, corpusPath)
	cleanupFn()

@ajwerner TFTR!

Reviewable status: complete! 0 of 0 LGTMs obtained (and 1 stale) (waiting on @ajwerner)