Add Change-Id trailer to commit message hook

Append a deterministic Gerrit-style Change-Id to every commit message:
hash of tree, parent, author/committer identity, and cleaned message
body. The awk block inserts the trailer in the footer, respecting
existing trailer ordering.

Handles core.commentChar (including 'auto'), validates the generated
hash, and fails loudly on write errors instead of silently proceeding.

Change-Id: Id606a2c2b9deae4f6bd10093cfe506fe9203da1a

Author: jserv

scripts/commit-msg.hook

@@ -364,7 +364,7 @@ while true; do
     validate_commit_message
 
     # if there are no WARNINGS are empty then we're good to break out of here
-    test ${#WARNINGS[@]} -eq 0 && exit 0
+    test ${#WARNINGS[@]} -eq 0 && break
 
     display_warnings
 
@@ -389,3 +389,100 @@ while true; do
     esac
 
 done
+
+#
+# Append a deterministic Change-Id (Gerrit algorithm) when not already present.
+#
+
+if grep -q '^Change-Id: I[0-9a-f]\{40\}' "$COMMIT_MSG_FILE"; then
+    exit 0
+fi
+
+clean_message=$(sed -n -e '/^#/d' -e '/^diff --git /q' \
+    -e '/^Signed-off-by:/d' -e p "$COMMIT_MSG_FILE" | git stripspace)
+
+# Nothing to sign: skip.
+[ -z "$clean_message" ] && exit 0
+
+_gen_changeid_input()
+{
+    local tree parent author committer
+
+    tree=$(git write-tree) || return 1
+    author=$(git var GIT_AUTHOR_IDENT) || return 1
+    committer=$(git var GIT_COMMITTER_IDENT) || return 1
+
+    echo "tree $tree"
+    if parent=$(git rev-parse "HEAD^0" 2> /dev/null); then
+        echo "parent $parent"
+    fi
+    echo "author $author"
+    echo "committer $committer"
+    echo
+    printf '%s' "$clean_message"
+}
+
+if ! change_id_input=$(_gen_changeid_input); then
+    echo "Failed to build Change-Id input" >&2
+    exit 1
+fi
+
+if ! change_id=$(printf '%s' "$change_id_input" | git hash-object -t commit --stdin); then
+    echo "Failed to compute Change-Id" >&2
+    exit 1
+fi
+
+if [[ ! "$change_id" =~ ^[0-9a-f]{40}$ ]]; then
+    echo "Failed to compute valid Change-Id" >&2
+    exit 1
+fi
+
+commentChar=$(git config --get core.commentChar 2> /dev/null || echo '#')
+# 'auto' means Git picks dynamically; treat as '#' for our purposes.
+[[ "$commentChar" == "auto" || -z "$commentChar" ]] && commentChar='#'
+
+awk -v id="$change_id" -v cc="$commentChar" '
+BEGIN {
+    isFooter = 0; footerComment = 0; blankLines = 0
+}
+index($0, cc) == 1 { next }
+/^diff --git / { blankLines = 0; while (getline) {}; next }
+/^$/ && (footerComment == 0) { blankLines++; next }
+/^\[[a-zA-Z0-9-]+:/ && (isFooter == 1) { footerComment = 1 }
+/]$/ && (footerComment == 1) { footerComment = 2 }
+(blankLines > 0) {
+    print lines
+    for (i = 0; i < blankLines; i++) print ""
+    lines = ""; blankLines = 0; isFooter = 1; footerComment = 0
+}
+(footerComment == 0) && (!/^\[?[a-zA-Z0-9-]+:/ || /^[a-zA-Z0-9-]+:\/\//) {
+    isFooter = 0
+}
+{
+    if (footerComment == 2) footerComment = 0
+    if (lines != "") lines = lines "\n"
+    lines = lines $0
+}
+END {
+    unprinted = 1
+    if (isFooter == 0) { print lines "\n"; lines = "" }
+    numlines = split(lines, footer, "\n")
+    trailers = ""; other = ""
+    for (i = 1; i <= numlines; i++) {
+        low = tolower(footer[i])
+        if (low ~ /^(signed-off|reviewed|co-authored|acked|suggested|tested|reported)-by:/) {
+            trailers = trailers footer[i] "\n"
+        } else {
+            other = other footer[i] "\n"
+        }
+    }
+    if (other != "") printf "%s", other
+    if (trailers != "") printf "%s", trailers
+    printf "Change-Id: I%s\n", id
+}' "$COMMIT_MSG_FILE" > "${COMMIT_MSG_FILE}.tmp"
+
+if ! mv "${COMMIT_MSG_FILE}.tmp" "$COMMIT_MSG_FILE"; then
+    rm -f "${COMMIT_MSG_FILE}.tmp"
+    echo "Failed to write Change-Id to commit message" >&2
+    exit 1
+fi