[cmd/fetch_repo] make cache corruption failures more clear (#1782)

This failure was confusing to our users. We should make more clear error messages in cases when a `GOMODCACHE` might be corrupt.

Resolves #1774

Example failure:
```
ERROR: /home/user/go-code/WORKSPACE:69:26: fetching go_repository rule //external:com_github_bazelbuild_remote_apis: Traceback (most recent call last):
        File "/home/user/.cache/bazel/_bazel_tfrench/b97476d719d716accead0f2d5b93104f/external/bazel_gazelle/internal/go_repository.bzl", line 263, column 17, in _go_repository_impl
                fail("failed to fetch %s: %s" % (ctx.name, result.stderr))
Error in fail: failed to fetch com_github_bazelbuild_remote_apis: fetch_repo: resulting module with sum h1:xhO5v3Ac039HesaPqoVUwOy4WhRRJbmo9om245kp+AY=; expected sum h1:xhO5v3Ac039HesaPqoVUwOy4WhRRJbmo9om245kp+AY=, Please try clearing your module cache directory "/home/user/go-code/pkg/mod"
ERROR: Error computing the main repository mapping: no such package '@@com_github_bazelbuild_remote_apis//': failed to fetch com_github_bazelbuild_remote_apis: fetch_repo: resulting module with sum h1:xhO5v3Ac039HesaPqoVUwOy4WhRRJbmo9om245kp+AY=; expected sum h1:xhO5v3Ac039HesaPqoVUwOy4WhRRJbmo9om245kp+AY=, Please try clearing your module cache directory "/home/user/go-code/pkg/mod"
INFO: Build Event Protocol files produced successfully.
Computing main repo mapping: 
WARNING:Bazel failed with exit code 1
```

cmd/fetch_repo/copy_tree.go

@@ -37,23 +37,23 @@ func copyTree(destRoot, srcRoot string) error {
 
 		if info.IsDir() {
 			return os.Mkdir(dest, 0o777)
-		} else {
-			r, err := os.Open(src)
-			if err != nil {
-				return err
-			}
-			defer r.Close()
-			w, err := os.Create(dest)
-			if err != nil {
-				return err
-			}
-			defer func() {
-				if cerr := w.Close(); err == nil && cerr != nil {
-					err = cerr
-				}
-			}()
-			_, err = io.Copy(w, r)
+		}
+
+		r, err := os.Open(src)
+		if err != nil {
 			return err
 		}
+		defer r.Close()
+		w, err := os.Create(dest)
+		if err != nil {
+			return err
+		}
+		defer func() {
+			if cerr := w.Close(); err == nil && cerr != nil {
+				err = cerr
+			}
+		}()
+		_, err = io.Copy(w, r)
+		return err
 	})
 }

cmd/fetch_repo/go_mod_download.go

@@ -32,8 +32,8 @@ func findGoPath() string {
 }
 
 func runGoModDownload(dl *GoModDownloadResult, dest string, importpath string, version string) error {
-	buf := &bytes.Buffer{}
-	bufErr := &bytes.Buffer{}
+	buf := bytes.NewBuffer(nil)
+	bufErr := bytes.NewBuffer(nil)
 	cmd := exec.Command(findGoPath(), "mod", "download", "-json")
 	cmd.Dir = dest
 	cmd.Args = append(cmd.Args, "-modcacherw")
@@ -44,34 +44,28 @@ func runGoModDownload(dl *GoModDownloadResult, dest string, importpath string, v
 
 	cmd.Stdout = buf
 	cmd.Stderr = bufErr
-	fmt.Printf("Running: %s %s\n", cmd.Path, strings.Join(cmd.Args, " "))
 	dlErr := cmd.Run()
 	if dlErr != nil {
 		if _, ok := dlErr.(*exec.ExitError); !ok {
 			if bufErr.Len() > 0 {
-				return fmt.Errorf("!%s %s: %s", cmd.Path, strings.Join(cmd.Args, " "), bufErr.Bytes())
-			} else {
-				return fmt.Errorf("!!%s %s: %v", cmd.Path, strings.Join(cmd.Args, " "), dlErr)
+				return fmt.Errorf("go mod download exec error: %s %q: %s, %w", cmd.Path, strings.Join(cmd.Args, " "), bufErr.String(), dlErr)
 			}
+			return fmt.Errorf("go mod download exec error: %s %s: %v", cmd.Path, strings.Join(cmd.Args, " "), dlErr)
 		}
 	}
 
 	// Parse the JSON output.
 	if err := json.Unmarshal(buf.Bytes(), &dl); err != nil {
 		if bufErr.Len() > 0 {
-			return fmt.Errorf("3%s %s: %s", cmd.Path, strings.Join(cmd.Args, " "), bufErr.Bytes())
-		} else {
-			return fmt.Errorf("4%s %s: error parsing JSON: %v error: %v", cmd.Path, strings.Join(cmd.Args, " "), buf, err)
+			return fmt.Errorf("go mod download output format: `%s %s`: parsing JSON: %q stderr: %q error: %w", cmd.Path, strings.Join(cmd.Args, " "), buf.String(), bufErr.String(), err)
 		}
+		return fmt.Errorf("go mod download output format: `%s %s`: parsing JSON: %q error: %w", cmd.Path, strings.Join(cmd.Args, " "), buf.String(), err)
 	}
 	if dl.Error != "" {
-		return errors.New(dl.Error)
+		return errors.Join(errors.New(dl.Error), dlErr)
 	}
 	if dlErr != nil {
 		return dlErr
 	}
-
-	fmt.Printf("Downloaded: %s\n", dl.Dir)
-
 	return nil
 }

cmd/fetch_repo/module.go

@@ -49,24 +49,25 @@ func fetchModule(dest, importpath, version, sum string) error {
 	var dl = GoModDownloadResult{}
 	err = runGoModDownload(&dl, dest, importpath, version)
 	os.Remove("go.mod")
-
 	if err != nil {
 		return err
 	}
 
 	// Copy the module to the destination.
-	err = copyTree(dest, dl.Dir)
-	if err != nil {
+	if err := copyTree(dest, dl.Dir); err != nil {
 		return fmt.Errorf("failed copying repo: %w", err)
 	}
 
-	// Verify sum
+	// Verify sum of the directory itself against the go.sum.
 	repoSum, err := dirhash.HashDir(dest, importpath+"@"+version, dirhash.Hash1)
 	if err != nil {
 		return fmt.Errorf("failed computing sum: %w", err)
 	}
 
 	if repoSum != sum {
+		if goModCache := os.Getenv("GOMODCACHE"); goModCache != "" {
+			return fmt.Errorf("resulting module with sum %s; expected sum %s, Please try clearing your module cache directory %q", repoSum, sum, goModCache)
+		}
 		return fmt.Errorf("resulting module with sum %s; expected sum %s", repoSum, sum)
 	}