Skip rewriting output files if unchanged

When running mockgen with the output option this checks if the existing file content already exists and skips writing if there is nothing to change.

This will help reduce i/o when changing lots of files, but also reduce the re-indexing triggering in IDEs.

Solves golang#604

.gitignore

@@ -17,3 +17,6 @@ mockgen/mockgen
 # Editors
 .vscode
 .idea
+
+# vendor directory used for IDEs
+/vendor

mockgen/mockgen.go

@@ -21,6 +21,7 @@ package main
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"go/token"
@@ -107,19 +108,6 @@ func main() {
 		return
 	}
 
-	dst := os.Stdout
-	if len(*destination) > 0 {
-		if err := os.MkdirAll(filepath.Dir(*destination), os.ModePerm); err != nil {
-			log.Fatalf("Unable to create directory: %v", err)
-		}
-		f, err := os.Create(*destination)
-		if err != nil {
-			log.Fatalf("Failed opening destination file: %v", err)
-		}
-		defer f.Close()
-		dst = f
-	}
-
 	outputPackageName := *packageOut
 	if outputPackageName == "" {
 		// pkg.Name in reflect mode is the base name of the import path,
@@ -171,7 +159,28 @@ func main() {
 	if err := g.Generate(pkg, outputPackageName, outputPackagePath); err != nil {
 		log.Fatalf("Failed generating mock: %v", err)
 	}
-	if _, err := dst.Write(g.Output()); err != nil {
+	output := g.Output()
+	dst := os.Stdout
+	if len(*destination) > 0 {
+		if err := os.MkdirAll(filepath.Dir(*destination), os.ModePerm); err != nil {
+			log.Fatalf("Unable to create directory: %v", err)
+		}
+		existing, err := os.ReadFile(*destination)
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
+			log.Fatalf("Failed reading pre-exiting destination file: %v", err)
+		}
+		if len(existing) == len(output) && bytes.Compare(existing, output) == 0 {
+			log.Println("Pre-existing file is already up to date.")
+			return
+		}
+		f, err := os.Create(*destination)
+		if err != nil {
+			log.Fatalf("Failed opening destination file: %v", err)
+		}
+		defer f.Close()
+		dst = f
+	}
+	if _, err := dst.Write(output); err != nil {
 		log.Fatalf("Failed writing to destination: %v", err)
 	}
 }