feat(adapter): registry with detect filtering and duplicate guard

internal/merge/merge.go

@@ -0,0 +1,72 @@
+// Package merge wraps git merge-file for three-way file merges.
+package merge
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+// Inputs:
+//
+//	Base   = last-rendered snapshot (state/...)
+//	Ours   = new rendered output (what we'd write now)
+//	Theirs = current target file contents (what's on disk at the destination)
+type Inputs struct {
+	Base   []byte
+	Ours   []byte
+	Theirs []byte
+}
+
+type Result struct {
+	Body     []byte
+	Conflict bool
+}
+
+// ThreeWay runs `git merge-file -p ours base theirs` and returns the
+// merged body. When Conflict is true, Body contains conflict markers.
+func ThreeWay(in Inputs) (*Result, error) {
+	tmp, err := os.MkdirTemp("", "harness-sync-merge-")
+	if err != nil {
+		return nil, err
+	}
+	defer os.RemoveAll(tmp)
+
+	oursPath := filepath.Join(tmp, "ours")
+	basePath := filepath.Join(tmp, "base")
+	theirsPath := filepath.Join(tmp, "theirs")
+
+	if err := os.WriteFile(oursPath, in.Ours, 0o644); err != nil {
+		return nil, err
+	}
+	if err := os.WriteFile(basePath, in.Base, 0o644); err != nil {
+		return nil, err
+	}
+	if err := os.WriteFile(theirsPath, in.Theirs, 0o644); err != nil {
+		return nil, err
+	}
+
+	cmd := exec.Command(
+		"git", "merge-file",
+		"-L", "harness-sync (new)",
+		"-L", "harness-sync (base)",
+		"-L", "harness-sync (current)",
+		"-p",
+		oursPath, basePath, theirsPath,
+	)
+	out, runErr := cmd.Output()
+
+	res := &Result{Body: out}
+	if runErr != nil {
+		if ee, ok := runErr.(*exec.ExitError); ok {
+			// git merge-file exits with conflict-count on conflict; body still valid
+			if ee.ExitCode() > 0 {
+				res.Conflict = true
+				return res, nil
+			}
+		}
+		return nil, fmt.Errorf("git merge-file: %w", runErr)
+	}
+	return res, nil
+}