build: rebase patches onto new upstream releases

README.md

@@ -181,10 +181,12 @@ cd ../..
 scripts/export-patches.sh           # rewrite patches/ from those commits
 ```
 
-Patches are applied with `git am --3way`, so small upstream movements around a hunk resolve by
-themselves. When one genuinely conflicts, `scripts/prepare.sh` stops and leaves the conflict staged
-in `build/upstream` to be resolved with `git am --continue`, after which `scripts/export-patches.sh`
-writes the fixed series back.
+Patches are applied to the release they were exported against (`patches/base.txt`), where they
+always fit, and then rebased onto the release in `upstream.txt` when that is newer. The rebase
+merges against the files the patches were written for, so upstream changes near a hunk resolve by
+themselves. When one genuinely conflicts, `scripts/prepare.sh` stops with the rebase in progress in
+`build/upstream`, to be resolved with `git rebase --continue`, after which
+`scripts/export-patches.sh` writes the fixed series back against the new release.
 
 To move to a newer upstream release:
 
@@ -199,7 +201,7 @@ scripts/prepare.sh
   reusable workflow the release job calls. It applies the patches, checks them, and builds on
   Linux, Windows and macOS.
 - **Upstream release** (`.github/workflows/upstream-release.yml`) runs daily. If Modrinth has
-  published a newer release than `upstream.txt`, it rebuilds against it and — only if every
+  published a newer release than `upstream.txt`, it rebases the patches onto it, rebuilds and — only if every
   platform built and every check passed — commits the bump, tags it with the upstream version and
   publishes a release with the installers.
 - **Revisions** of the same upstream release are published by running Upstream release by hand

patches/base.txt

@@ -0,0 +1 @@
+v0.21.2

scripts/common.sh

@@ -19,7 +19,18 @@ WORKTREE="${WORKTREE:-$REPO_ROOT/build/upstream}"
 # Branch the patches are applied on top of the upstream tag as.
 PATCH_BRANCH=enhanced
 
-PATCH_DIR="$REPO_ROOT/patches"
+PATCH_DIR="${PATCH_DIR:-$REPO_ROOT/patches}"
+
+# The upstream release the patches in patches/ were last exported against,
+# written there by export-patches.sh. upstream.txt cannot say: a release bumps
+# it without exporting the patches again.
+patch_base() {
+	if [ -f "$PATCH_DIR/base.txt" ]; then
+		tr -d '[:space:]' <"$PATCH_DIR/base.txt"
+	else
+		printf '%s\n' "$UPSTREAM_REF"
+	fi
+}
 
 log() {
 	printf '\033[1;32m==>\033[0m %s\n' "$*"

scripts/export-patches.sh

@@ -26,5 +26,8 @@ git -C "$WORKTREE" format-patch \
 	--output-directory "$PATCH_DIR" \
 	"$UPSTREAM_REF..HEAD"
 
+# So that prepare.sh applies them where they fit and rebases from there.
+printf '%s\n' "$UPSTREAM_REF" >"$PATCH_DIR/base.txt"
+
 log "patches/ now contains:"
 ls -1 "$PATCH_DIR"

scripts/prepare.sh

@@ -4,11 +4,23 @@
 # The result lands in build/upstream on the `enhanced` branch and is what all
 # other scripts build from. Running this again always starts from a clean
 # upstream tree, so it is safe to repeat.
+#
+# The patches are applied to the release they were exported against, where they
+# always fit, and rebased onto $UPSTREAM_REF when that is a different release.
+# A rebase merges against the files the patches were written for, so upstream
+# changing something near a patch resolves by itself. `git am` straight onto the
+# new release cannot do that: a shallow checkout does not have those files.
 
 . "$(dirname "${BASH_SOURCE[0]}")/common.sh"
 
+BASE_REF="$(patch_base)"
+
 log "Upstream $UPSTREAM_REF from $UPSTREAM_REPO"
 
+fetch_tag() {
+	git -C "$WORKTREE" fetch --depth 1 --force origin "refs/tags/$1:refs/tags/$1"
+}
+
 if [ ! -d "$WORKTREE/.git" ]; then
 	log "Cloning into $WORKTREE"
 	mkdir -p "$(dirname "$WORKTREE")"
@@ -16,21 +28,27 @@ if [ ! -d "$WORKTREE/.git" ]; then
 else
 	log "Fetching $UPSTREAM_REF into the existing checkout"
 	git -C "$WORKTREE" remote set-url origin "$UPSTREAM_REPO"
-	git -C "$WORKTREE" fetch --depth 1 --force origin "refs/tags/$UPSTREAM_REF:refs/tags/$UPSTREAM_REF"
+	fetch_tag "$UPSTREAM_REF"
 fi
 
-# `git am` refuses to run with a rebase or merge in progress, and a previous
-# run may have stopped on a conflict.
-git -C "$WORKTREE" am --abort 2>/dev/null || true
+if [ "$BASE_REF" != "$UPSTREAM_REF" ]; then
+	log "Fetching $BASE_REF, which the patches were exported against"
+	fetch_tag "$BASE_REF"
+fi
 
-log "Resetting to $UPSTREAM_REF"
-git -C "$WORKTREE" checkout --detach --force "$UPSTREAM_REF"
-git -C "$WORKTREE" branch -f "$PATCH_BRANCH" "$UPSTREAM_REF"
+# Neither `git am` nor `git rebase` runs while the other is in progress, and a
+# previous run may have stopped on a conflict in either.
+git -C "$WORKTREE" am --abort 2>/dev/null || true
+git -C "$WORKTREE" rebase --abort 2>/dev/null || true
+
+log "Resetting to $BASE_REF"
+git -C "$WORKTREE" checkout --detach --force "$BASE_REF"
+git -C "$WORKTREE" branch -f "$PATCH_BRANCH" "$BASE_REF"
 git -C "$WORKTREE" checkout --force "$PATCH_BRANCH"
-git -C "$WORKTREE" reset --hard "$UPSTREAM_REF"
+git -C "$WORKTREE" reset --hard "$BASE_REF"
 git -C "$WORKTREE" clean -fdx -e node_modules -e target
 
-# `git am` needs an identity for the commits it creates.
+# `git am` and `git rebase` need an identity for the commits they create.
 git -C "$WORKTREE" config user.name "Modrinth Enhanced"
 git -C "$WORKTREE" config user.email "patches@modrinth-enhanced.invalid"
 git -C "$WORKTREE" config commit.gpgsign false
@@ -41,11 +59,11 @@ shopt -u nullglob
 
 [ ${#patches[@]} -gt 0 ] || die "No patches found in $PATCH_DIR"
 
-log "Applying ${#patches[@]} patches"
+log "Applying ${#patches[@]} patches to $BASE_REF"
 if ! git -C "$WORKTREE" am --3way --whitespace=nowarn "${patches[@]}"; then
 	cat >&2 <<EOF
 
-A patch did not apply to $UPSTREAM_REF.
+A patch did not apply to $BASE_REF.
 
 The failed patch is left staged in $WORKTREE so it can be fixed by hand:
 
@@ -60,5 +78,26 @@ EOF
 	exit 1
 fi
 
+if [ "$BASE_REF" != "$UPSTREAM_REF" ]; then
+	log "Rebasing the patches onto $UPSTREAM_REF"
+	if ! git -C "$WORKTREE" rebase --onto "$UPSTREAM_REF" "$BASE_REF" "$PATCH_BRANCH"; then
+		cat >&2 <<EOF
+
+A patch conflicts with what changed between $BASE_REF and $UPSTREAM_REF.
+
+The rebase is left stopped in $WORKTREE so it can be resolved by hand:
+
+  cd $WORKTREE
+  git status                 # see the conflicts
+  # ...resolve them, then:
+  git add -A && git rebase --continue
+  # once every patch is in, with upstream.txt set to $UPSTREAM_REF:
+  $REPO_ROOT/scripts/export-patches.sh
+
+EOF
+		exit 1
+	fi
+fi
+
 log "Patched checkout ready at $WORKTREE"
 git -C "$WORKTREE" --no-pager log --oneline "$UPSTREAM_REF..$PATCH_BRANCH"