build: Add a commit-msg hook that embeds Change-Id in commit messages.

Partially implements <https://issues.guix.gnu.org/66027>.

This will make it possible to track a merged commit back to its original
posting on the mailing list, and open the door to new opportunities such as
closing fully merged series automatically.

* Makefile.am (COMMIT_MSG_MAGIC): New variable.
(.git/hooks/commit-msg): New target.
* etc/git/commit-msg: New file.
* doc/contributing.texi (Configuring Git): Document Change-Id.

Series-changes: 3
 - Clarify documentation text, as suggested by Simon
Change-Id: Ia92fa958eae600fdd4e180bad494c85db8bb4dd6
Reviewed-by: Simon Tournier <zimon.toutoune@gmail.com>

Makefile.am

@@ -1170,7 +1170,17 @@ cuirass-jobs: $(GOBJECTS)
 	git config --add include.path ../etc/git/gitconfig; \
 	fi
 
-nodist_noinst_DATA = .git/hooks/pre-push .git/config
+COMMIT_MSG_MAGIC = VGhpcyBpcyB0aGUgY29tbWl0LW1zZyBob29rIG9mIEd1aXg=
+.git/hooks/commit-msg: etc/git/commit-msg
+	$(AM_V_at)if test -d .git; then \
+	if test -f $@  && ! grep -qF $(COMMIT_MSG_MAGIC) $@; then \
+	  mkdir -p $@.d && mv $@ $@.d && \
+	@ echo user commit-msg hook moved to $@.d/commit-msg; \
+	fi; \
+	cp etc/git/commit-msg $@; \
+	fi
+
+nodist_noinst_DATA = .git/hooks/pre-push .git/config .git/hooks/commit-msg
 
 # Downloading up-to-date PO files.
 

doc/contributing.texi

@@ -1575,8 +1575,16 @@ different name or email just for commits in this repository, you can
 use @command{git config --local}, or edit @file{.git/config} in the
 repository instead of @file{~/.gitconfig}.
 
+@cindex commit-msg hook
 Other important Git configuration will automatically be configured when
-building the project (@pxref{Building from Git}).
+building the project (@pxref{Building from Git}).  A
+@file{.git/hooks/commit-msg} hook will be installed that embeds
+@samp{Change-Id} Git @emph{trailers} in your commit messages for
+traceability purposes.  It is important to preserve these when editing
+your commit messages, particularly if a first version of your proposed
+changes was already submitted for review.  If you have a
+@file{commit-msg} hook of your own you would like to use with Guix, you
+can place it under the @file{.git/hooks/commit-msg.d/} directory.
 
 @node Sending a Patch Series
 @subsection Sending a Patch Series

etc/git/commit-msg

@@ -0,0 +1,94 @@
+#!/bin/sh
+# From Gerrit Code Review 3.6.1
+#
+# Part of Gerrit Code Review (https://www.gerritcodereview.com/)
+#
+# Copyright (C) 2009 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+### Guix modifications start
+COMMIT_MSG_MAGIC=VGhpcyBpcyB0aGUgY29tbWl0LW1zZyBob29rIG9mIEd1aXg=
+top=$(git rev-parse --show-toplevel)
+if test -d "$top/.git/hooks/commit-msg.d/"; then
+    for msg_hook in "$top/.git/hooks/commit-msg.d/"*; do
+        if ! sh "$msg_hook"; then
+            echo "error while running $msg_hook"
+            exit 1
+        fi
+    done
+fi
+### Guix modifications end
+
+set -u
+
+# avoid [[ which is not POSIX sh.
+if test "$#" != 1 ; then
+  echo "$0 requires an argument."
+  exit 1
+fi
+
+if test ! -f "$1" ; then
+  echo "file does not exist: $1"
+  exit 1
+fi
+
+# Do not create a change id if requested
+if test "false" = "$(git config --bool --get gerrit.createChangeId)" ; then
+  exit 0
+fi
+
+if git rev-parse --verify HEAD >/dev/null 2>&1; then
+  refhash="$(git rev-parse HEAD)"
+else
+  refhash="$(git hash-object -t tree /dev/null)"
+fi
+
+random=$({ git var GIT_COMMITTER_IDENT ; echo "$refhash" ; cat "$1"; } | git hash-object --stdin)
+dest="$1.tmp.${random}"
+
+trap 'rm -f "${dest}"' EXIT
+
+if ! git stripspace --strip-comments < "$1" > "${dest}" ; then
+   echo "cannot strip comments from $1"
+   exit 1
+fi
+
+if test ! -s "${dest}" ; then
+  echo "file is empty: $1"
+  exit 1
+fi
+
+reviewurl="$(git config --get gerrit.reviewUrl)"
+if test -n "${reviewurl}" ; then
+  if ! git interpret-trailers --parse < "$1" | grep -q '^Link:.*/id/I[0-9a-f]\{40\}$' ; then
+    if ! git interpret-trailers \
+          --trailer "Link: ${reviewurl%/}/id/I${random}" < "$1" > "${dest}" ; then
+      echo "cannot insert link footer in $1"
+      exit 1
+    fi
+  fi
+else
+  # Avoid the --in-place option which only appeared in Git 2.8
+  # Avoid the --if-exists option which only appeared in Git 2.15
+  if ! git -c trailer.ifexists=doNothing interpret-trailers \
+        --trailer "Change-Id: I${random}" < "$1" > "${dest}" ; then
+    echo "cannot insert change-id line in $1"
+    exit 1
+  fi
+fi
+
+if ! mv "${dest}" "$1" ; then
+  echo "cannot mv ${dest} to $1"
+  exit 1
+fi