Add a --write and --diff flag; require --write to be set before making any changes, for safety

go.mod

@@ -3,3 +3,8 @@ module cheese
 go 1.24.4
 
 require gopkg.in/yaml.v3 v3.0.1
+
+require (
+	github.com/hexops/gotextdiff v1.0.3 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+)

go.sum

@@ -1,3 +1,7 @@
+github.com/hexops/gotextdiff v1.0.3 h1:gitA9+qJrrTCsiCl7+kh75nPqQt1cx4ZkudSTLoUqJM=
+github.com/hexops/gotextdiff v1.0.3/go.mod h1:pSWU5MAI3yDq+fZBTazCSJysOMbxWL1BSow5/V2vxeg=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=

tesseract/genconf/env.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 )
 
-func generateEnv(yamlFile string) {
+func generateEnv(yamlFile string, wantDiff bool, allowWrite bool, useColor bool) {
 	config := loadConfig(yamlFile)
 
 	// Check that all local directories exist
@@ -24,7 +24,7 @@ func generateEnv(yamlFile string) {
 
 		// Create combined roots.pem file
 		rootsPemPath := filepath.Join(logEntry.LocalDirectory, "roots.pem")
-		err := createCombinedRootsPemWithStatus(config.Roots, logEntry.ExtraRoots, rootsPemPath)
+		err := createCombinedRootsPemWithStatus(config.Roots, logEntry.ExtraRoots, rootsPemPath, wantDiff, allowWrite, useColor)
 		if err != nil {
 			log.Fatalf("Failed to create %s: %v", rootsPemPath, err)
 		}
@@ -53,14 +53,14 @@ func generateEnv(yamlFile string) {
 		tesseractArgs := strings.Join(args, " ")
 		envContent := fmt.Sprintf("TESSERACT_ARGS=\"%s\"\nOTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318\n", tesseractArgs)
 
-		err = writeFileWithStatus(envPath, []byte(envContent))
+		err = writeFileWithStatus(envPath, []byte(envContent), wantDiff, allowWrite, useColor)
 		if err != nil {
 			log.Fatalf("Failed to write %s: %v", envPath, err)
 		}
 	}
 }
 
-func createCombinedRootsPemWithStatus(rootsFile, extraRootsFile, outputPath string) error {
+func createCombinedRootsPemWithStatus(rootsFile, extraRootsFile, outputPath string, wantDiff bool, allowWrite bool, useColor bool) error {
 	// Read main roots file
 	var combinedContent []byte
 	if rootsFile != "" {
@@ -80,5 +80,5 @@ func createCombinedRootsPemWithStatus(rootsFile, extraRootsFile, outputPath stri
 		combinedContent = append(combinedContent, extraRootsData...)
 	}
 
-	return writeFileWithStatus(outputPath, combinedContent)
+	return writeFileWithStatus(outputPath, combinedContent, wantDiff, allowWrite, useColor)
 }

tesseract/genconf/html.go

@@ -110,7 +110,7 @@ type TemporalInterval struct {
 	EndExclusive   string `json:"end_exclusive"`
 }
 
-func generateHTML(yamlFile string) {
+func generateHTML(yamlFile string, wantDiff bool, allowWrite bool, useColor bool) {
 	config := loadConfig(yamlFile)
 
 	// Check that all local directories exist
@@ -145,14 +145,14 @@ func generateHTML(yamlFile string) {
 		}
 
 		// Write file with status
-		err = writeFileWithStatus(indexPath, buf.Bytes())
+		err = writeFileWithStatus(indexPath, buf.Bytes(), wantDiff, allowWrite, useColor)
 		if err != nil {
 			log.Fatalf("Failed to write HTML to %s: %v", indexPath, err)
 		}
 
 		// Generate log.v3.json for this log
 		jsonPath := filepath.Join(logEntry.LocalDirectory, "log.v3.json")
-		err = generateLogJSONWithStatus(logEntry, jsonPath)
+		err = generateLogJSONWithStatus(logEntry, jsonPath, wantDiff, allowWrite, useColor)
 		if err != nil {
 			log.Fatalf("Failed to generate %s: %v", jsonPath, err)
 		}
@@ -209,7 +209,7 @@ func computeKeyInfo(logEntry *Log) error {
 	return nil
 }
 
-func generateLogJSONWithStatus(logEntry Log, outputPath string) error {
+func generateLogJSONWithStatus(logEntry Log, outputPath string, wantDiff bool, allowWrite bool, useColor bool) error {
 	logJSON := LogV3JSON{
 		Description:   fmt.Sprintf("%s.log.ct.ipng.ch", logEntry.ShortName),
 		SubmissionURL: fmt.Sprintf("%s/", logEntry.SubmissionPrefix),
@@ -228,5 +228,5 @@ func generateLogJSONWithStatus(logEntry Log, outputPath string) error {
 		return fmt.Errorf("failed to marshal JSON: %v", err)
 	}
 
-	return writeFileWithStatus(outputPath, jsonData)
+	return writeFileWithStatus(outputPath, jsonData, wantDiff, allowWrite, useColor)
 }

tesseract/genconf/key.go

@@ -12,7 +12,12 @@ import (
 	"path/filepath"
 )
 
-func generateKeys(yamlFile string) {
+func generateKeys(yamlFile string, wantDiff bool, allowWrite bool, useColor bool) {
+	if !allowWrite {
+		fmt.Printf("Key generation requires --write flag\n")
+		return
+	}
+
 	config := loadConfig(yamlFile)
 
 	// Generate keys for each log

tesseract/genconf/main.go

@@ -5,8 +5,12 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"strings"
 	"time"
 
+	"github.com/hexops/gotextdiff"
+	"github.com/hexops/gotextdiff/myers"
+	"github.com/hexops/gotextdiff/span"
 	"gopkg.in/yaml.v3"
 )
 
@@ -41,6 +45,9 @@ type Log struct {
 
 func main() {
 	configFile := flag.String("c", "./tesseract-staging.yaml", "Path to the YAML configuration file")
+	wantDiff := flag.Bool("diff", false, "Show unified diff of changes")
+	allowWrite := flag.Bool("write", false, "Allow writing files (required for actual file modifications)")
+	noColor := flag.Bool("no-color", false, "Disable colored diff output")
 	flag.Parse()
 
 	args := flag.Args()
@@ -51,15 +58,15 @@ func main() {
 
 	switch args[0] {
 	case "gen-html":
-		generateHTML(*configFile)
+		generateHTML(*configFile, *wantDiff, *allowWrite, !*noColor)
 	case "gen-env":
-		generateEnv(*configFile)
+		generateEnv(*configFile, *wantDiff, *allowWrite, !*noColor)
 	case "gen-key":
-		generateKeys(*configFile)
+		generateKeys(*configFile, *wantDiff, *allowWrite, !*noColor)
 	case "gen-nginx":
-		generateNginx(*configFile)
+		generateNginx(*configFile, *wantDiff, *allowWrite, !*noColor)
 	case "gen-roots":
-		generateRoots(args[1:])
+		generateRoots(args[1:], *wantDiff, *allowWrite, !*noColor)
 	default:
 		fmt.Fprintf(os.Stderr, "Unknown command: %s\n", args[0])
 		showHelp()
@@ -97,17 +104,94 @@ func loadConfig(yamlFile string) Config {
 	return config
 }
 
-func writeFileWithStatus(filename string, content []byte) error {
+// ANSI color codes
+const (
+	colorReset  = "\033[0m"
+	colorRed    = "\033[31m"
+	colorGreen  = "\033[32m"
+	colorYellow = "\033[33m"
+	colorCyan   = "\033[36m"
+)
+
+// colorizeUnifiedDiff adds ANSI color codes to unified diff output
+func colorizeUnifiedDiff(diff string) string {
+	lines := strings.Split(diff, "\n")
+	var colorizedLines []string
+
+	for _, line := range lines {
+		switch {
+		case strings.HasPrefix(line, "---"):
+			// File deletion header in cyan
+			colorizedLines = append(colorizedLines, colorCyan+line+colorReset)
+		case strings.HasPrefix(line, "+++"):
+			// File addition header in cyan
+			colorizedLines = append(colorizedLines, colorCyan+line+colorReset)
+		case strings.HasPrefix(line, "@@"):
+			// Hunk header in yellow
+			colorizedLines = append(colorizedLines, colorYellow+line+colorReset)
+		case strings.HasPrefix(line, "-"):
+			// Deleted lines in red
+			colorizedLines = append(colorizedLines, colorRed+line+colorReset)
+		case strings.HasPrefix(line, "+"):
+			// Added lines in green
+			colorizedLines = append(colorizedLines, colorGreen+line+colorReset)
+		default:
+			// Context lines unchanged
+			colorizedLines = append(colorizedLines, line)
+		}
+	}
+
+	return strings.Join(colorizedLines, "\n")
+}
+
+func writeFileWithStatus(filename string, content []byte, wantDiff bool, allowWrite bool, useColor bool) error {
 	existingContent, err := os.ReadFile(filename)
-	if os.IsNotExist(err) {
+	isNew := os.IsNotExist(err)
+	isUnchanged := false
+
+	if isNew {
+		if allowWrite {
 			fmt.Printf("Creating %s\n", filename)
+		} else {
+			fmt.Printf("Would create %s\n", filename)
+		}
 	} else if err != nil {
 		return fmt.Errorf("failed to read existing file %s: %v", filename, err)
 	} else if string(existingContent) == string(content) {
 		fmt.Printf("Unchanged %s\n", filename)
-		return nil
+		isUnchanged = true
 	} else {
+		if allowWrite {
 			fmt.Printf("Updating %s\n", filename)
+		} else {
+			fmt.Printf("Would update %s\n", filename)
+		}
+	}
+
+	if wantDiff && !isUnchanged {
+		if isNew {
+			// For new files, show the entire content as added
+			edits := myers.ComputeEdits(span.URIFromPath(filename), "", string(content))
+			diff := fmt.Sprint(gotextdiff.ToUnified("/dev/null", filename, "", edits))
+			if useColor {
+				fmt.Print(colorizeUnifiedDiff(diff))
+			} else {
+				fmt.Print(diff)
+			}
+		} else {
+			// For existing files, show the diff
+			edits := myers.ComputeEdits(span.URIFromPath(filename), string(existingContent), string(content))
+			diff := fmt.Sprint(gotextdiff.ToUnified(filename, filename+".new", string(existingContent), edits))
+			if useColor {
+				fmt.Print(colorizeUnifiedDiff(diff))
+			} else {
+				fmt.Print(diff)
+			}
+		}
+	}
+
+	if isUnchanged || !allowWrite {
+		return nil
 	}
 
 	err = os.WriteFile(filename, content, 0644)
@@ -119,8 +203,17 @@ func writeFileWithStatus(filename string, content []byte) error {
 
 func showHelp() {
 	fmt.Printf("Usage: %s [options] <command>\n\n", os.Args[0])
+	fmt.Printf("Note: Flags must come before the command name.\n\n")
 	fmt.Printf("Options:\n")
-	fmt.Printf("  -c <file>    Path to YAML configuration file (default: ./tesseract-staging.yaml)\n\n")
+	fmt.Printf("  -c <file>    Path to YAML configuration file (default: ./tesseract-staging.yaml)\n")
+	fmt.Printf("  --diff       Show unified diff of changes without writing files\n")
+	fmt.Printf("  --write      Allow writing files (required for actual file modifications)\n")
+	fmt.Printf("  --no-color   Disable colored diff output\n\n")
+	fmt.Printf("Examples:\n")
+	fmt.Printf("  %s --diff gen-html            # Show colored diffs without writing\n", os.Args[0])
+	fmt.Printf("  %s --diff --no-color gen-html # Show plain diffs without writing\n", os.Args[0])
+	fmt.Printf("  %s --write gen-html           # Write files\n", os.Args[0])
+	fmt.Printf("  %s --diff --write gen-html    # Show colored diffs and write files\n\n", os.Args[0])
 	fmt.Printf("Commands:\n")
 	fmt.Printf("  gen-html     Generate index.html and log.v3.json files in each log's localdirectory.\n")
 	fmt.Printf("               Creates HTML pages with log information and CT log metadata JSON.\n")

tesseract/genconf/nginx.go

@@ -76,7 +76,7 @@ type NginxTemplateData struct {
 	ListenPort     string
 }
 
-func generateNginx(yamlFile string) {
+func generateNginx(yamlFile string, wantDiff bool, allowWrite bool, useColor bool) {
 	config := loadConfig(yamlFile)
 
 	// Extract port from first listen address
@@ -123,7 +123,7 @@ func generateNginx(yamlFile string) {
 		}
 
 		// Write file with status
-		err = writeFileWithStatus(outputPath, buf.Bytes())
+		err = writeFileWithStatus(outputPath, buf.Bytes(), wantDiff, allowWrite, useColor)
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "Failed to write nginx config file %s: %v\n", outputPath, err)
 			continue

tesseract/genconf/roots.go

@@ -16,7 +16,7 @@ type CTLogRootsResponse struct {
 	Certificates []string `json:"certificates"`
 }
 
-func generateRoots(args []string) {
+func generateRoots(args []string, wantDiff bool, allowWrite bool, useColor bool) {
 	sourceURL := "https://rennet2027h2.log.ct.ipng.ch/"
 	outputFile := "roots.pem"
 
@@ -107,7 +107,7 @@ func generateRoots(args []string) {
 	}
 
 	// Write all certificates to file with status
-	err = writeFileWithStatus(outputFile, pemBuffer.Bytes())
+	err = writeFileWithStatus(outputFile, pemBuffer.Bytes(), wantDiff, allowWrite, useColor)
 	if err != nil {
 		log.Fatalf("Failed to write output file %s: %v", outputFile, err)
 	}