reduce: show original test case output

The `reduce` tool now shows the original test case output in verbose
mode if it is not found to be interesting. This makes it easier for a
user to determine why a reproduction is not producing output that
matches the `--contains` regex.

Release note: None

pkg/cmd/reduce/main.go

@@ -52,7 +52,7 @@ var (
 	flags           = flag.NewFlagSet(os.Args[0], flag.ExitOnError)
 	binary          = flags.String("binary", "./cockroach", "path to cockroach binary")
 	file            = flags.String("file", "", "the path to a file containing a SQL query to reduce")
-	verbose         = flags.Bool("v", false, "print progress to standard output")
+	verbose         = flags.Bool("v", false, "print progress to standard output and the original test case output if it is not interesting")
 	contains        = flags.String("contains", "", "error regex to search for")
 	unknown         = flags.Bool("unknown", false, "print unknown types during walk")
 	workers         = flags.Int("goroutines", goroutines, "number of worker goroutines (defaults to NumCPU/3")
@@ -141,7 +141,7 @@ func reduceSQL(
 		chunkReducer = reducesql.NewSQLChunkReducer(chunkReductions)
 	}
 
-	interesting := func(ctx context.Context, sql string) bool {
+	isInteresting := func(ctx context.Context, sql string) (interesting bool, logOriginalHint func()) {
 		// Disable telemetry and license generation.
 		cmd := exec.CommandContext(ctx, binary,
 			"demo",
@@ -162,13 +162,18 @@ func reduceSQL(
 		case errors.HasType(err, (*os.PathError)(nil)):
 			log.Fatal(err)
 		}
-		return containsRE.Match(out)
+		if verbose {
+			logOriginalHint = func() {
+				fmt.Fprintf(logger, "output did not match regex %s:\n\n%s", contains, string(out))
+			}
+		}
+		return containsRE.Match(out), logOriginalHint
 	}
 
 	out, err := reduce.Reduce(
 		logger,
 		inputSQL,
-		interesting,
+		isInteresting,
 		workers,
 		reduce.ModeInteresting,
 		chunkReducer,

pkg/testutils/reduce/reduce.go

@@ -56,8 +56,11 @@ const (
 type State interface{}
 
 // InterestingFn returns true if the string triggers the target test failure. It
-// should be context-aware and stop work if the context is canceled.
-type InterestingFn func(context.Context, string) bool
+// should be context-aware and stop work if the context is canceled. It can
+// return a function which will be called if the original test case is not
+// interesting. The function should log a hint that will help the user
+// understand why the original test case is not interesting.
+type InterestingFn func(context.Context, string) (_ bool, logOriginalHint func())
 
 // Mode is an enum specifying how to determine if forward progress was made.
 type Mode int
@@ -89,7 +92,10 @@ func Reduce(
 	}
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
-	if !isInteresting(ctx, originalTestCase) {
+	if interesting, logHint := isInteresting(ctx, originalTestCase); !interesting {
+		if logHint != nil {
+			logHint()
+		}
 		return "", errors.New("original test case not interesting")
 	}
 
@@ -162,7 +168,7 @@ func Reduce(
 		for i := 0; i < numGoroutines; i++ {
 			g.GoCtx(func(ctx context.Context) error {
 				for vs := range variants {
-					if isInteresting(ctx, vs.file) {
+					if interesting, _ := isInteresting(ctx, vs.file); interesting {
 						// Wait for the previous test to finish.
 						select {
 						case <-ctx.Done():
@@ -327,7 +333,7 @@ func attemptChunkReduction(
 		end := rand.Intn(chunkReducer.NumSegments()-start) + start + 1
 
 		localReduced := chunkReducer.DeleteSegments(start, end)
-		if isInteresting(ctx, localReduced) {
+		if interesting, _ := isInteresting(ctx, localReduced); interesting {
 			reduced = localReduced
 			log(logger, "\tchunk reduction: %d bytes\n", len(reduced))
 			failedAttempts = 0

pkg/testutils/reduce/reduce_test.go

@@ -52,11 +52,11 @@ var (
 )
 
 func isInterestingGo(contains string) reduce.InterestingFn {
-	return func(ctx context.Context, f string) bool {
+	return func(ctx context.Context, f string) (bool, func()) {
 		_, err := parser.ParseExpr(f)
 		if err == nil {
-			return false
+			return false, nil
 		}
-		return strings.Contains(err.Error(), contains)
+		return strings.Contains(err.Error(), contains), nil
 	}
 }

pkg/testutils/reduce/reducesql/reducesql_test.go

@@ -38,7 +38,7 @@ func TestReduceSQL(t *testing.T) {
 }
 
 func isInterestingSQL(contains string) reduce.InterestingFn {
-	return func(ctx context.Context, f string) bool {
+	return func(ctx context.Context, f string) (bool, func()) {
 		args := base.TestServerArgs{
 			Insecure: true,
 		}
@@ -67,8 +67,8 @@ func isInterestingSQL(contains string) reduce.InterestingFn {
 		}
 		_, err = db.Exec(ctx, f)
 		if err == nil {
-			return false
+			return false, nil
 		}
-		return strings.Contains(err.Error(), contains)
+		return strings.Contains(err.Error(), contains), nil
 	}
 }