fix(backports/apply): scope cherry-pick to target package, ignore other-package conflicts

dev/backports/apply/apply.go

@@ -305,9 +305,12 @@ func (a applier) prepareWorkingBranch(remote, branchName, workingBranch string)
 // cherryPickOrConflict attempts the cherry-pick. changelog.yml is always
 // restored to HEAD afterwards — its entries are fully regenerated by this
 // pipeline (see extractChangelogFields/InsertEntry), so cherry-picked changes
-// to it are redundant. manifest.yml is left as cherry-pick merges it: any
-// legitimate, non-version content change is preserved, and a conflict that is
-// purely a "version:" line difference (expected, since each backport branch
+// to it are redundant. All files outside the target package directory are also
+// reset to HEAD unconditionally, scoping the backport to the target package
+// only: changes (or conflicts) the source commit introduced in other packages
+// are irrelevant and discarded. manifest.yml is left as cherry-pick merges it:
+// any legitimate, non-version content change is preserved, and a conflict that
+// is purely a "version:" line difference (expected, since each backport branch
 // bumps its own version independently) is auto-resolved in favor of the
 // current branch — bumpPatchVersion recomputes the version afterwards. A
 // manifest.yml conflict block containing anything else is left as a genuine
@@ -340,6 +343,21 @@ func (a applier) cherryPickOrConflict(sha, branchName, pkg, changelogPath, manif
 		return nil, fmt.Errorf("restoring changelog after cherry-pick: %w", err)
 	}
 
+	// Scope the cherry-pick to the target package: reset every file outside
+	// pkgDir to HEAD, discarding both clean changes and conflicts that belong to
+	// other packages. This also resolves modify/delete conflicts in other
+	// packages (e.g. a package absent from this backport branch) in favor of
+	// the branch state, so conflictingFiles() below only surfaces issues that
+	// actually require the contributor's attention.
+	pkgDir := filepath.Dir(manifestPath)
+	relPkgDir, err := filepath.Rel(a.workDir, pkgDir)
+	if err != nil {
+		a.abortCherryPick()
+		return nil, fmt.Errorf("computing relative package dir: %w", err)
+	}
+	relPkgDir = filepath.ToSlash(relPkgDir)
+	a.resetNonPackageChanges(relPkgDir)
+
 	if conflict := a.manifestMissingConflict(sha, branchName, pkg, manifestPath); conflict != nil {
 		return conflict, nil
 	}
@@ -394,6 +412,49 @@ func (a applier) abortCherryPick() {
 	_ = a.git.RunToStderr("reset", "--hard", "HEAD")
 }
 
+// resetNonPackageChanges resets all index and working-tree changes outside
+// relPkgDir to HEAD. For each such file it first tries "git checkout HEAD --
+// <file>", which resolves both clean staged changes and most conflict types.
+// When that fails (the file does not exist in HEAD — i.e. a modify/delete
+// conflict where HEAD removed the file), it falls back to "git rm --force",
+// which removes the file from both the index and the working tree. Errors from
+// both commands are non-fatal: conflictingFiles() is authoritative about what
+// remains unresolved.
+func (a applier) resetNonPackageChanges(relPkgDir string) {
+	out, err := a.git.Output("status", "--porcelain")
+	if err != nil {
+		return
+	}
+	var outside []string
+	for _, line := range strings.Split(out, "\n") {
+		if len(line) < 4 {
+			continue
+		}
+		file := strings.TrimSpace(line[3:])
+		// Renamed entries look like "old -> new"; use only the new path.
+		if idx := strings.Index(file, " -> "); idx != -1 {
+			file = file[idx+4:]
+		}
+		fileFwd := filepath.ToSlash(file)
+		pkgPrefix := relPkgDir + "/"
+		if fileFwd == relPkgDir || strings.HasPrefix(fileFwd, pkgPrefix) {
+			continue
+		}
+		outside = append(outside, file)
+	}
+	if len(outside) == 0 {
+		return
+	}
+	fmt.Fprintf(os.Stderr, "note: cherry-pick touched %d file(s) outside %s — resetting to HEAD to scope backport to target package\n", len(outside), relPkgDir)
+	for _, file := range outside {
+		if err := a.git.RunToStderr("checkout", "HEAD", "--", file); err != nil {
+			// HEAD does not have this file (e.g. modify/delete conflict where the
+			// backport branch removed it): remove it from index and working tree.
+			_ = a.git.RunToStderr("rm", "--force", "--", file)
+		}
+	}
+}
+
 // manifestMissingConflict reports a conflict Result if manifestPath does not
 // exist in the working tree. Called both before the cherry-pick (the
 // package doesn't exist on the target backport branch at all yet) and after