Let pre-commit hook run go mod tidy

amend

Author: flowrean

DEV.md

@@ -2,7 +2,7 @@
 
 ## Development
 
-After cloning this repository, set up the pre-commit hook to ensure proper formatting of the Go code:
+After cloning this repository, set up the pre-commit hook to ensure proper formatting of the Go code and the `go.mod` and `go.sum` files:
 ```shell
 ln -s ../../git-hooks/pre-commit .git/hooks/pre-commit
 ```

git-hooks/pre-commit

@@ -3,24 +3,31 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-# git gofmt pre-commit hook
+# git gofmt & go mod tidy pre-commit hook
 #
 # To use, store as .git/hooks/pre-commit inside your repository and make sure
 # it has execute permissions.
 #
 # This script does not handle file names that contain spaces.
 
+function failWithMessage {
+	echo >&2 $1
+	exit 1
+}
+
+# Adapted from dnephin/pre-commit-golang
+go mod tidy -v 2>&1 | grep -q "updates to go.mod needed" && failWithMessage "go mod tidy: updates to go.mod needed"
+git diff --exit-code go.* &> /dev/null || failWithMessage "go.mod or go.sum differs, please re-add it to your commit"
+
 gofiles=$(git diff --cached --name-only --diff-filter=ACM | grep '\.go$')
 [ -z "$gofiles" ] && exit 0
 
 unformatted=$(gofmt -l $gofiles)
 [ -z "$unformatted" ] && exit 0
 
 # Some files are not gofmt'd. Print message and fail.
-
-echo >&2 "Go files must be formatted with gofmt. Please run:"
+msg="Go files must be formatted with gofmt. Please run:"
 for fn in $unformatted; do
-	echo >&2 "  gofmt -w $PWD/$fn"
+	msg+="\n  gofmt -w $PWD/$fn"
 done
-
-exit 1
+failWithMessage msg