Phase 19: compose validate (docker compose config) + diff vs deployed in editor (0.25.0)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/services/compose_service.py

@@ -9,6 +9,7 @@ import asyncio
 import os
 import re
 import shutil
+import tempfile
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from typing import Optional
@@ -313,6 +314,37 @@ async def run_compose(
     }
 
 
+async def validate_yaml(content: str, env_content: str = "") -> dict:
+    """Validate compose YAML by running ``docker compose config -q`` against it
+    in a throwaway directory. Returns ``{"ok": bool, "errors": str}`` — the
+    errors string carries compose's own diagnostics (parse errors, unknown
+    keys, bad interpolation) so the editor can surface them before saving."""
+    with tempfile.TemporaryDirectory(prefix="sp-validate-") as tmp:
+        compose_file = os.path.join(tmp, DEFAULT_COMPOSE_NAME)
+        with open(compose_file, "w", encoding="utf-8") as fh:
+            fh.write(content)
+        if env_content:
+            with open(os.path.join(tmp, ".env"), "w", encoding="utf-8") as fh:
+                fh.write(env_content)
+        cmd = [
+            "docker", "compose",
+            "--project-directory", tmp,
+            "-f", compose_file,
+            "config", "-q",
+        ]
+        try:
+            proc = await asyncio.create_subprocess_exec(
+                *cmd,
+                stdout=asyncio.subprocess.PIPE,
+                stderr=asyncio.subprocess.PIPE,
+            )
+            _out, err = await asyncio.wait_for(proc.communicate(), timeout=30.0)
+        except asyncio.TimeoutError as exc:
+            raise StackFileError("compose config validation timed out") from exc
+        ok = proc.returncode == 0
+        return {"ok": ok, "errors": "" if ok else err.decode("utf-8", "replace").strip()}
+
+
 async def stream_compose(
     stack_id: str, args: list[str], override: Optional[str] = None
 ):