pre-receive: add prevent-foxtrot script

pre-receive.d/prevent-foxtrot-merge.sh

@@ -0,0 +1,44 @@
+#!/bin/bash
+
+# Copyright (c) 2016 G. Sylvie Davies. http://bit-booster.com/
+# Copyright (c) 2016 torek. http://stackoverflow.com/users/1256452/torek
+# Copyright (c) 2021 Alfred Melch. https://melch.pro
+# License: MIT license. https://opensource.org/licenses/MIT
+
+# From: https://blog.developer.atlassian.com/stop-foxtrots-now/
+
+NULL_REV="0000000000000000000000000000000000000000"
+
+# refname (e.g. refs/head/master) will be updated from oldrev (commit hash) to newrev (commit hash)
+while read oldrev newrev refname
+do
+  if [ $oldrev = $NULL_REV ]; then
+    # pushing a new branch
+    exit 0
+  fi
+  if [ $newrev = $NULL_REV ]; then
+    # deleting a branch 
+    exit 0
+  fi
+  if [ "$refname" = "refs/heads/master" ] || [ "$refname" = "refs/heads/main" ]; then
+    # Find the first parent of the commit where oldrev is a parent
+    # git log: list commits from newrev to oldrev
+    #   %H: commit hash
+    #   %P: parent hashes (list of parents)
+    # grep: filter for oldrev
+    #   Should appear once as a parent
+    # awk: print the second word 
+    #   The second word is the first parent. Third word would be second parent, ...
+    MATCH=`git log --first-parent --pretty='%H %P' $oldrev..$newrev |
+      grep $oldrev |
+      awk '{ print $2 }'`
+          # First parent should be the oldrev
+          if [ "$oldrev" = "$MATCH" ]; then
+            exit 0
+          else
+            echo >&2 "*** Push rejected! Foxtrot merge blocked! ***"
+            echo "See: https://blog.developer.atlassian.com/stop-foxtrots-now/"
+            exit 1
+          fi
+  fi
+done