From c9b37f2477677e30c8115b944d11d8a6d55776e7 Mon Sep 17 00:00:00 2001
From: SBPro <slim.b.pro@gmail.com>
Date: Tue, 17 Mar 2026 14:41:04 +0300
Subject: [PATCH] Fix linearize fallback applying wrong diff (full tree instead
 of parent)
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The fallback for failed cherry-picks used `git diff HEAD $sha` which
computes the diff between the linearize branch and the commit's full
tree. This captures ALL tree differences, not just the changes the
commit introduced — causing unrelated file deletions.

Fix: diff from the commit's own parent (`${sha}^` for regular commits,
`${sha}^1` for merge commits). Also detect and skip empty merges
(where cherry-pick -m 1 produces nothing because the changes were
already applied by a prior cherry-pick).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 lib/sync.sh | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/lib/sync.sh b/lib/sync.sh
index 596ae65..5c80de2 100644
--- a/lib/sync.sh
+++ b/lib/sync.sh
@@ -339,14 +339,16 @@ reverse_sync() {
         if git cherry-pick "$sha" >/dev/null 2>&1; then
           log "INFO" "  cherry-pick ${sha:0:7}: ${short_msg}"
         else
-          log "WARN" "  conflict ${sha:0:7} — applying tree directly"
+          log "WARN" "  conflict ${sha:0:7} — applying patch from parent"
           git cherry-pick --abort 2>/dev/null
           local msg author_name author_email author_date
           msg=$(git log -1 --format="%B" "$sha")
           author_name=$(git log -1 --format="%an" "$sha")
           author_email=$(git log -1 --format="%ae" "$sha")
           author_date=$(git log -1 --format="%aI" "$sha")
-          git diff HEAD "$sha" | git apply --index >/dev/null 2>&1 \
+          # Diff from the commit's own parent — NOT from HEAD (which is
+          # the linearize branch and may have a different base).
+          git diff "${sha}^" "$sha" | git apply --index >/dev/null 2>&1 \
             || die "Failed to apply diff for ${sha:0:7}"
           GIT_AUTHOR_NAME="$author_name" GIT_AUTHOR_EMAIL="$author_email" \
             GIT_AUTHOR_DATE="$author_date" \
@@ -362,16 +364,26 @@ reverse_sync() {
         author_date=$(git log -1 --format="%aI" "$sha")
 
         if git cherry-pick -m 1 "$sha" >/dev/null 2>&1; then
-          : # success
+          log "INFO" "  squash    ${sha:0:7}: ${short_msg}"
         else
           git cherry-pick --abort 2>/dev/null
-          git diff HEAD "$sha" | git apply --index >/dev/null 2>&1 \
-            || die "Failed to apply merge diff for ${sha:0:7}"
-          GIT_AUTHOR_NAME="$author_name" GIT_AUTHOR_EMAIL="$author_email" \
-            GIT_AUTHOR_DATE="$author_date" \
-            git commit -m "$msg" >/dev/null 2>&1 || die "Failed to commit merge ${sha:0:7}"
+          # Diff from first parent — what the merge actually introduced.
+          # NOT from HEAD (which would capture all tree differences, not
+          # just this merge's changes).
+          if git diff "${sha}^1" "$sha" --quiet 2>/dev/null; then
+            # Empty diff from first parent — the merge introduced no new
+            # changes (e.g., a fast-forward merge or changes already applied
+            # by a prior cherry-pick).  Skip silently.
+            log "INFO" "  skip      ${sha:0:7}: ${short_msg} (empty — already applied)"
+          else
+            git diff "${sha}^1" "$sha" | git apply --index >/dev/null 2>&1 \
+              || die "Failed to apply merge diff for ${sha:0:7}"
+            GIT_AUTHOR_NAME="$author_name" GIT_AUTHOR_EMAIL="$author_email" \
+              GIT_AUTHOR_DATE="$author_date" \
+              git commit -m "$msg" >/dev/null 2>&1 || die "Failed to commit merge ${sha:0:7}"
+            log "INFO" "  squash    ${sha:0:7}: ${short_msg}"
+          fi
         fi
-        log "INFO" "  squash    ${sha:0:7}: ${short_msg}"
       fi
     done