Automatically run bazel mod tidy in @rules_go//go

.gitignore

@@ -5,3 +5,5 @@
 /examples/*/bazel-*
 /.ijwb/
 /tests/bcr/.ijwb/
+
+MODULE.bazel.lock

docs/go/core/bzlmod.md

@@ -147,8 +147,8 @@ use_repo(
 )
 ```
 
-Bazel emits a warning if the `use_repo` statement is out of date or missing entirely (requires Bazel 6.2.0 or higher).
-The warning contains a `buildozer` command to automatically fix the `MODULE.bazel` file (requires buildozer 6.1.1 or higher).
+When using Bazel 7.1.1 or higher, the [`@rules_go//go` target](#using-a-go-sdk) automatically updates the `use_repo` call whenever the `go.mod` file changes, using `bazel mod tidy`.
+With older versions of Bazel, a warning with a fixup command will be emitted during a build if the `use_repo` call is out of date or missing.
 
 Alternatively, you can specify a module extension tag to add an individual dependency:
 

go/private/polyfill_bazel_features.bzl

@@ -7,7 +7,11 @@
 _POLYFILL_BAZEL_FEATURES = """bazel_features = struct(
   cc = struct(
     find_cpp_toolchain_has_mandatory_param = {find_cpp_toolchain_has_mandatory_param},
-  )
+  ),
+  external_deps = struct(
+    # WORKSPACE users have no use for bazel mod tidy.
+    bazel_mod_tidy = False,
+  ),
 )
 """
 

go/tools/go_bin_runner/BUILD.bazel

@@ -1,5 +1,6 @@
 # gazelle:exclude
 
+load("@io_bazel_rules_go_bazel_features//:features.bzl", "bazel_features")
 load("//go:def.bzl", "go_binary", "go_library")
 load("//go/private:common.bzl", "RULES_GO_IS_BZLMOD_REPO")
 load("//go/private/rules:go_bin_for_host.bzl", "go_bin_for_host")
@@ -36,6 +37,7 @@ go_binary(
     x_defs = {
         "GoBinRlocationPath": "$(rlocationpath :go_bin_for_host)",
         "ConfigRlocationPath": "$(rlocationpath @bazel_gazelle_go_repository_config//:config.json)" if RULES_GO_IS_BZLMOD_REPO else "WORKSPACE",
+        "HasBazelModTidy": str(bazel_features.external_deps.bazel_mod_tidy),
     },
 )
 

go/tools/go_bin_runner/main.go

@@ -1,21 +1,29 @@
 package main
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
+	"fmt"
+	"io"
 	"log"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"sort"
+	"strings"
 
 	"github.com/bazelbuild/rules_go/go/runfiles"
 )
 
 var GoBinRlocationPath = "not set"
 var ConfigRlocationPath = "not set"
+var HasBazelModTidy = "not set"
 
 // Produced by gazelle's go_deps extension.
 type Config struct {
-	GoEnv map[string]string `json:"go_env"`
+	GoEnv     map[string]string `json:"go_env"`
+	DepsFiles []string          `json:"dep_files"`
 }
 
 func main() {
@@ -34,8 +42,39 @@ func main() {
 		log.Fatal(err)
 	}
 
+	hashesBefore, err := hashWorkspaceRelativeFiles(cfg.DepsFiles)
+	if err != nil {
+		log.Fatal(err)
+	}
+
 	args := append([]string{goBin}, os.Args[1:]...)
-	log.Fatal(runProcess(args, env, os.Getenv("BUILD_WORKING_DIRECTORY")))
+	cwd := os.Getenv("BUILD_WORKING_DIRECTORY")
+	err = runProcess(args, env, cwd)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	hashesAfter, err := hashWorkspaceRelativeFiles(cfg.DepsFiles)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	diff := diffMaps(hashesBefore, hashesAfter)
+	if len(diff) > 0 {
+		if HasBazelModTidy == "True" {
+			bazel := os.Getenv("BAZEL")
+			if bazel == "" {
+				bazel = "bazel"
+			}
+			_, _ = fmt.Fprintf(os.Stderr, "\nrules_go: Running '%s mod tidy' since %s changed...\n", bazel, strings.Join(diff, ", "))
+			err = runProcess([]string{bazel, "mod", "tidy"}, os.Environ(), cwd)
+			if err != nil {
+				log.Fatal(err)
+			}
+		} else {
+			_, _ = fmt.Fprintf(os.Stderr, "\nrules_go: %s changed, please apply any buildozer fixes suggested by Bazel\n", strings.Join(diff, ", "))
+		}
+	}
 }
 
 func parseConfig() (Config, error) {
@@ -76,6 +115,45 @@ func getGoEnv(goBin string, cfg Config) ([]string, error) {
 	return append(env, "GOROOT="+goRoot), nil
 }
 
+func hashWorkspaceRelativeFiles(relativePaths []string) (map[string]string, error) {
+	workspace := os.Getenv("BUILD_WORKSPACE_DIRECTORY")
+
+	hashes := make(map[string]string)
+	for _, p := range relativePaths {
+		h, err := hashFile(filepath.Join(workspace, p))
+		if err != nil {
+			return nil, err
+		}
+		hashes[p] = h
+	}
+	return hashes, nil
+}
+
+// diffMaps returns the keys that have different values in a and b.
+func diffMaps(a, b map[string]string) []string {
+	var diff []string
+	for k, v := range a {
+		if b[k] != v {
+			diff = append(diff, k)
+		}
+	}
+	sort.Strings(diff)
+	return diff
+}
+
+func hashFile(path string) (string, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+	h := sha256.New()
+	if _, err := io.Copy(h, f); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}
+
 func runProcess(args, env []string, dir string) error {
 	cmd := exec.Command(args[0], args[1:]...)
 	cmd.Dir = dir
@@ -85,8 +163,6 @@ func runProcess(args, env []string, dir string) error {
 	err := cmd.Run()
 	if exitErr, ok := err.(*exec.ExitError); ok {
 		os.Exit(exitErr.ExitCode())
-	} else if err == nil {
-		os.Exit(0)
 	}
 	return err
 }