skip rewriting output files if unchanged (#626)

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 #604

Author: sodul

.gitignore

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

mockgen/internal/tests/mock_in_test_package/mock_test.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 +159,27 @@ 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 := ioutil.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 {
+			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)
 	}
 }