Add git-auto-merge script.

git-auto-merge

@@ -0,0 +1,69 @@
+#!/bin/bash
+# Version: 0.1.0
+# Copyright (c) 2023:
+#   Darren 'Tadgy' Austin <darren (at) afterdark.org.uk>
+# Licensed under the terms of the GNU General Public License version 3.
+#
+# Automatically fetch and merge (if possible) upstream changes into a local git repository.
+
+# What platform we're running on.
+PLATFORM="$(uname -s)"
+
+# Only operate on repositories that are set for maintenance work in ~/.gitconfig.
+while IFS= read -r REPO; do
+  # Get into the working directory.
+  pushd "$REPO" >/dev/null 2>&1 || {
+    printf "%s: %s\\n" "${0##*/}" "failed to change to working directory '$REPO'" >&2
+    RET=1
+    continue
+  }
+
+  # Skip processing if auto-merging is disabled in this repository.
+  [[ "$(git config --local --get --type bool script.AutoMergeDisabled 2>/dev/null)" != "true" ]] && {
+    # Fetch the latest updates.
+    if git fetch --all --atomic --tags >/dev/null 2>&1; then
+      # Try to merge using fast-forward only, so no clobbering is done.
+      if MERGE="$(git merge --ff-only 2>/dev/null)"; then
+        [[ "$(printf "%s" "$MERGE" | awk '{ print $1; exit }')" == "Updating" ]] && {
+          if [[ "$PLATFORM" == "Linux" ]]; then
+            # Linux, how I love thee.
+            NOW="$(date +'%s%3N')"
+          elif [[ "$PLATFORM" == "Darwin" ]]; then
+            # This sucks... but so does Darwin.
+            NOW="$(perl -e 'use Time::HiRes; printf "%.3f", Time::HiRes::time();')"
+            NOW="${NOW/.}"
+          else
+            printf "%s: %s\\n" "${0##*/}" "unsupported platform: $PLATFORM" >&2
+            RET=2
+            break
+          fi
+          git config --local --replace-all --type int script.AutoMergeLast "$NOW" >/dev/null 2>&1 || {
+            printf "%s: %s\\n" "${0##*/}" "updating AutoMergeLast timestamp failed for '$REPO'" >&2
+            RET=1
+          }
+          git config --local --replace-all --type bool script.AutoMergeSuccess true >/dev/null 2>&1 || {
+            printf "%s: %s\\n" "${0##*/}" "updating AutoMergeSuccess marker failed for '$REPO'" >&2
+            RET=1
+          }
+        }
+      else
+        git config --local --replace-all --type bool script.AutoMergeSuccess false >/dev/null 2>&1 || {
+          printf "%s: %s\\n" "${0##*/}" "updating AutoMergeSuccess marker failed for '$REPO'" >&2
+          RET=1
+        }
+      fi
+    else
+      # Mark that the auto-merge failed.
+      printf "%s: %s\\n" "${0##*/}" "git fetch failed for '$REPO'" >&2
+      git config --local --replace-all --type bool script.AutoMergeSuccess false >/dev/null 2>&1 || {
+        printf "%s: %s\\n" "${0##*/}" "updating AutoMergeSuccess marker failed for '$REPO'" >&2
+        RET=1
+      }
+    fi
+  }
+
+  # Back to where we started...
+  popd >/dev/null 2>&1 || continue
+done < <(git config --global --get-all maintenance.repo 2>/dev/null)
+
+exit "${RET:-0}"