Add Update GHC workflow

This workflow is scheduled weekly and fetches the latest GHC release for each of the current
stable versions.

It adds the version to the gen_ghc_bindist.py file and runs this script to update the
ghc_bindist_generated.bzl file.

.github/update-ghc.py

@@ -0,0 +1,110 @@
+#!/usr/bin/env python
+
+import os, sys, re
+import importlib
+import importlib.util
+import importlib.machinery
+import http.client
+import json
+from pathlib import PurePosixPath, Path
+from pprint import pprint
+from urllib.parse import quote
+from subprocess import Popen, PIPE, STDOUT, DEVNULL, check_output, run
+
+def import_path(path):
+    module_name = os.path.basename(path).replace('-', '_')
+    spec = importlib.util.spec_from_loader(
+        module_name,
+        importlib.machinery.SourceFileLoader(module_name, path)
+    )
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    sys.modules[module_name] = module
+    return module
+
+
+GHC_GITLAB_SERVER="gitlab.haskell.org"
+
+def get_gitlab_tags_with_prefix(project_id, prefix):
+    connection = http.client.HTTPSConnection(GHC_GITLAB_SERVER)
+    encoded_project_id = encoded_project_id = quote(project_id, safe='')
+    endpoint = f"/api/v4/projects/{encoded_project_id}/repository/tags?search=^{prefix}&order-by=version"
+
+    connection.request("GET", endpoint)
+    response = connection.getresponse()
+
+    if response.status == 200:
+        tags_data = response.read().decode("utf-8")
+        tags = json.loads(tags_data)
+        return tags
+    else:
+        print(f"Error: {response.status} - {response.reason}")
+        return None
+
+project_id = "ghc/ghc"
+
+# matches GHC release versions
+version_re = re.compile("ghc-(?P<version>.*)-release")
+
+def parse_version(s):
+    return tuple(map(int, s.split(".")))
+
+def main():
+    SCRIPT_PATH = Path(__file__)
+
+    if len(sys.argv) != 2:
+        sys.exit(f"usage: {SCRIPT_PATH.name} GHC_MAJOR_MINOR")
+
+    bindist = import_path(str(SCRIPT_PATH.parent.parent.joinpath('haskell/private/ghc_bindist_generated.bzl')))
+
+    GHC_MAJOR_MINOR = sys.argv[1]
+
+    versions = [ version for version in bindist.GHC_BINDIST if version.startswith(GHC_MAJOR_MINOR) ]
+
+    prefix = f"ghc-{GHC_MAJOR_MINOR}."
+
+    tags = get_gitlab_tags_with_prefix(project_id, prefix)
+
+    if tags:
+        releases = [ m.group("version") for tag in tags if (m := version_re.match(tag["name"])) ]
+
+        latest_release = releases[0]
+
+        if latest_release in versions:
+            print(f"GHC {GHC_MAJOR_MINOR} is up-to-date", file=sys.stderr)
+        else:
+            print("found update:", latest_release, file=sys.stderr)
+
+            replace = re.compile(r'^(?P<indent>\s+)\{\s*"version"\s*:\s*"' + re.escape(GHC_MAJOR_MINOR + '.'), re.MULTILINE)
+
+            gen_script_path = SCRIPT_PATH.parent.parent.joinpath('haskell/gen_ghc_bindist.py')
+            with open(gen_script_path, 'r+') as gen:
+                gen_script = gen.read()
+
+                print(" 1. modify haskell/gen_ghc_bindist.py", file=sys.stderr)
+
+                added_version = replace.sub(fr'''\g<indent>{{ "version": { repr(latest_release) },
+\g<indent>  "ignore_suffixes": [".bz2", ".lz", ".zip"] }},
+\g<0>''', gen_script, count=1)
+
+                if added_version is gen_script:
+                    sys.exit(f"could not add new version {latest_release} using regex {replace}")
+                else:
+                    gen.truncate(0)
+                    gen.seek(0)
+                    gen.write(added_version)
+
+            print(" 2. call haskell/gen_ghc_bindist.py", file=sys.stderr)
+
+            bzl = check_output([sys.executable, 'haskell/gen_ghc_bindist.py'])
+
+            print(" 3. format bzl code, write to haskell/private/ghc_bindist_generated.bzl", file=sys.stderr)
+            with open('haskell/private/ghc_bindist_generated.bzl', 'r+') as generated:
+                run(['buildifier'], check=True, input=bzl, stdout=generated)
+                generated.truncate()
+    else:
+        print("no tags found for prefix", prefix, file=sys.stderr)
+
+
+if __name__ == '__main__':
+    main()

.github/workflows/update-ghc.yaml

@@ -0,0 +1,27 @@
+name: Update GHC
+on:
+  schedule:
+    # run weekly on Thursday
+    - cron:  '0 15 * * THU'
+  workflow_dispatch: # allows manual triggering
+
+jobs:
+  update_ghc:
+    name: GHC ${{ matrix.ghc }} Update
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ghc:
+          - '9.2'
+          - '9.4'
+          - '9.6'
+    steps:
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v22
+        with:
+          nix_path: nixpkgs=nixpkgs/default.nix
+      - name: Fetch updates
+        run: nix shell 'nixpkgs#bazel-buildtools' 'nixpkgs#python3' --command python .github/update-ghc.py ${{ matrix.ghc }}
+      - run: git diff
+