- Add pre-commit hook script to misc/scripts

- Refer to it in CONTRIBUTING.md
- Add setup note in docs folder

CONTRIBUTING.md

@@ -38,6 +38,8 @@ If you have an idea for a query that you would like to share with other CodeQL u
 
     - The queries and libraries must be autoformatted, for example using the "Format Document" command in [CodeQL for Visual Studio Code](https://help.semmle.com/codeql/codeql-for-vscode/procedures/about-codeql-for-vscode.html).
 
+    If you prefer, you can use this [pre-commit hook](misc/scripts/pre-commit) that automatically checks whether your files are correctly formatted. See the [pre-commit hook installation guide](docs/install-pre-commit-hook.md) for instructions on how to install the hook.
+
 4. **Compilation**
 
     - Compilation of the query and any associated libraries and tests must be resilient to future development of the [supported](docs/supported-queries.md) libraries. This means that the functionality cannot use internal libraries, cannot depend on the output of `getAQlClass`, and cannot make use of regexp matching on `toString`.

docs/pre-commit-hook-setup.md

@@ -0,0 +1,13 @@
+# CodeQL pre-commit-hook setup
+
+As stated in [CONTRIBUTING](../CONTRIBUTING.md) all CodeQL files must be formatted according to our [CodeQL style guide](ql-style-guide.md). You can use our pre-commit hook to avoid committing incorrectly formatted code. To use it, simply copy the [pre-commit](../misc/scripts/pre-commit) script to `.git/modules/ql/hooks/pre-commit` and make sure that it is executable.
+
+The script will abort a commit that contains incorrectly formatted code in .ql or .qll files and print an error message like:
+
+```
+> git commit -m "My commit."
+code/ql/cpp/ql/src/Options.qll would change by autoformatting.
+code/ql/cpp/ql/src/printAst.ql would change by autoformatting.
+```
+
+If you prefer to have the script automatically format the code (and not abort the commit), you can replace the line `codeql query format --check-only` with `codeql query format --in-place` (and `exit $exitVal` with `exit 0`).

misc/scripts/pre-commit

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+exec 1>&2
+exitVal=0
+while read -r f
+do
+  filename="${f##*/}"
+  extension="${filename##*.}"
+  p="$PWD/$f";
+  if [[ -f "$p" ]] && { [ "$extension" == "ql" ] || [ "$extension" == "qll" ]; }
+  then
+    if ! codeql query format --check-only "$p"
+    then
+      exitVal=1
+    fi
+  fi
+done <<<"$(git diff --cached --relative --name-only)"
+exit $exitVal