replace a panic with an error in merge-logs

Release justification: bug fix

Release note (bug fix): Users would receive a panic message when the log parser
fails to extract log file formats. This has been replaced with a helpful
error message.

pkg/cli/debug_merge_logs.go

@@ -499,9 +499,8 @@ func (s *fileLogStream) open() bool {
 	if s.err = seekToFirstAfterFrom(s.f, s.from, s.editMode, s.format); s.err != nil {
 		return false
 	}
-	var err error
-	if s.d, err = log.NewEntryDecoderWithFormat(bufio.NewReaderSize(s.f, readBufSize), s.editMode, s.format); err != nil {
-		panic(err)
+	if s.d, s.err = log.NewEntryDecoderWithFormat(bufio.NewReaderSize(s.f, readBufSize), s.editMode, s.format); s.err != nil {
+		return false
 	}
 	return true
 }

pkg/cli/debug_merge_logs_test.go

@@ -12,6 +12,7 @@ package cli
 
 import (
 	"bytes"
+	"fmt"
 	"io/ioutil"
 	"path/filepath"
 	"testing"
@@ -184,14 +185,22 @@ func getCases(format string) []testCase {
 	}
 }
 
-func (c testCase) run(t *testing.T) {
-	outBuf := bytes.Buffer{}
+func resetDebugMergeLogFlags(errorFn func(s string)) {
 	debugMergeLogsCmd.Flags().Visit(func(f *pflag.Flag) {
 		if err := f.Value.Set(f.DefValue); err != nil {
-			t.Fatalf("Failed to set flag to default: %v", err)
+			errorFn(fmt.Sprintf("Failed to set flag to default: %v", err))
 		}
 	})
+}
+
+func (c testCase) run(t *testing.T) {
+	resetDebugMergeLogFlags(func(s string) { t.Fatal(s) })
+	outBuf := bytes.Buffer{}
 	debugMergeLogsCmd.SetOut(&outBuf)
+	// Ensure that the original writer is restored when the test
+	// completes. Otherwise, subsequent tests may not see their output.
+	defer debugMergeLogsCmd.SetOut(nil)
+
 	if err := debugMergeLogsCmd.ParseFlags(c.flags); err != nil {
 		t.Fatalf("Failed to set flags: %v", err)
 	}
@@ -235,3 +244,16 @@ func TestCrdbV1DebugMergeLogs(t *testing.T) {
 		t.Run(c.name, c.run)
 	}
 }
+
+func Example_format_error() {
+	c := NewCLITest(TestCLIParams{NoServer: true})
+	defer c.Cleanup()
+
+	resetDebugMergeLogFlags(func(s string) { fmt.Fprintf(stderr, "ERROR: %v", s) })
+
+	c.RunWithArgs([]string{"debug", "merge-logs", "testdata/merge_logs_v1/missing_format/*"})
+
+	// Output:
+	// debug merge-logs testdata/merge_logs_v1/missing_format/*
+	// ERROR: decoding format: failed to extract log file format from the log
+}

pkg/cli/testdata/merge_logs_v1/missing_format/cockroach.test-0001.ubuntu.2018-11-30T22_06_47Z.004130.log

@@ -0,0 +1,3 @@
+I210801 21:05:59.364923 1 util/log/sync_buffer.go:70  [config] binary: CockroachDB CCL v20.1.17 (x86_64-apple-darwin14, built 2021/05/17 16:30:22,
+I210801 21:05:59.364923 1 util/log/sync_buffer.go:70  [config] arguments: [./cockroach start]
+I210801 21:05:59.364923 1 util/log/sync_buffer.go:70  line format: [IWEF]yymmdd hh:mm:ss.uuuuuu goid file:line msg utf8=✓