Avoid updating target file mtime in 'generate'

If the target file exists on disk already, compare its contents
to the generated source in memory: only update the target file
on disk if the contents differ or if the target file is missing.

This should allow build tools downstream of sqlc to make conditional
decisions based on whether the generated code files have been updated
(e.g. mockgen).

Author: n-oden

internal/cmd/cmd.go

@@ -205,10 +205,28 @@ var genCmd = &cobra.Command{
 		defer trace.StartRegion(cmd.Context(), "writefiles").End()
 		for filename, source := range output {
 			os.MkdirAll(filepath.Dir(filename), 0755)
-			if err := os.WriteFile(filename, []byte(source), 0644); err != nil {
+			// if the target file does not exist, we can just write to it
+			if _, err := os.Stat(filename); os.IsNotExist(err) {
+				if err := os.WriteFile(filename, []byte(source), 0644); err != nil {
+					fmt.Fprintf(stderr, "%s: %s\n", filename, err)
+					return err
+				}
+				return nil
+			}
+			// otherwise, if the target file exists, read its contents and compare it to the source
+			targetFile, err := os.ReadFile(filename)
+			if err != nil {
 				fmt.Fprintf(stderr, "%s: %s\n", filename, err)
 				return err
 			}
+			// if the target file is the same as the source, we can just remove the temp file
+			if bytes.Equal(targetFile, []byte(source)) {
+				if err := os.WriteFile(filename, []byte(source), 0644); err != nil {
+					fmt.Fprintf(stderr, "%s: %s\n", filename, err)
+					return err
+				}
+				continue
+			}
 		}
 		return nil
 	},