build: add gitlint commit-msg hook

gitlint provides an excellent standard ruleset for conventional
commits

the commit-msg shell script is default produced by gitlint

Consideration was given to using a tool such as pre-commit-hooks to
produce git hooks but ultimately using shell scripts reduces
dependancies and simplifies producing a determinaistic development
enviornment with nix.

flake.nix

@@ -8,19 +8,21 @@
     };
   };
 
-  outputs = {
-    self,
-    nixpkgs,
-    gitignore,
-    flake-utils,
-    ...
-  }:
+  outputs =
+    { self
+    , nixpkgs
+    , gitignore
+    , flake-utils
+    , ...
+    }:
     flake-utils.lib.eachDefaultSystem (
-      system: let
+      system:
+      let
         pkgs = nixpkgs.legacyPackages.${system};
         packageJSON = pkgs.lib.importJSON ./package.json;
         gitignoreSource = gitignore.lib.gitignoreSource;
-      in {
+      in
+      {
         packages = rec {
           site-src = pkgs.mkYarnPackage rec {
             name = "${packageJSON.name}-site-${version}";
@@ -36,7 +38,7 @@
 
           default = pkgs.writeShellApplication {
             name = packageJSON.name;
-            runtimeInputs = [site-src pkgs.nodejs];
+            runtimeInputs = [ site-src pkgs.nodejs ];
             text = ''
               node ${site-src}/libexec/${packageJSON.name}/deps/${packageJSON.name}/build
             '';
@@ -44,7 +46,17 @@
         };
 
         devShell = pkgs.mkShell {
-          buildInputs = [pkgs.yarn pkgs.nodejs];
+          buildInputs = [
+            pkgs.gitlint
+            pkgs.nodejs
+            pkgs.yarn
+          ];
+          shellHook = ''
+            # auto-install git hooks
+            dot_git="$(git rev-parse --git-common-dir)"
+            if [[ ! -d "$dot_git/hooks" ]]; then mkdir "$dot_git/hooks"; fi
+            for hook in git_hooks/* ; do ln -sf "$(pwd)/$hook" "$dot_git/hooks/" ; done
+          '';
         };
       }
     );

git_hooks/commit-msg

@@ -0,0 +1,35 @@
+#!/bin/sh
+### gitlint commit-msg hook start ###
+
+# Determine whether we have a tty available by trying to access it.
+# This allows us to deal with UI based gitclient's like Atlassian SourceTree.
+# NOTE: "exec < /dev/tty" sets stdin to the keyboard
+stdin_available=1
+(exec < /dev/tty) 2> /dev/null || stdin_available=0
+
+if [ $stdin_available -eq 1 ]; then
+    # Now that we know we have a functional tty, set stdin to it so we can ask the user questions :-)
+    exec < /dev/tty
+
+    # On Windows, we need to explicitly set our stdout to the tty to make terminal editing work (e.g. vim)
+    # See SO for windows detection in bash (slight modified to work on plain shell (not bash)): 
+    # https://stackoverflow.com/questions/394230/how-to-detect-the-os-from-a-bash-script
+    if [ "$OSTYPE" = "cygwin" ] || [ "$OSTYPE" = "msys" ] || [ "$OSTYPE" = "win32" ]; then
+        exec > /dev/tty
+    fi
+fi
+
+gitlint --staged --msg-filename "$1" run-hook
+exit_code=$?
+
+# If we fail to find the gitlint binary (command not found), let's retry by executing as a python module.
+# This is the case for Atlassian SourceTree, where $PATH deviates from the user's shell $PATH.
+if [ $exit_code -eq 127 ]; then
+    echo "Fallback to python module execution"
+    python -m gitlint.cli --staged --msg-filename "$1" run-hook
+    exit_code=$?
+fi
+
+exit $exit_code
+
+### gitlint commit-msg hook end ###