Enhance backend configuration and sanitization logic

- Updated the `recover_errored_backends` function to preserve `ExtraArgs` during backend reconfiguration, improving error recovery.
- Refined the `sanitize_backends_fds` function to prevent newline merging issues with bare `\\x` entries, ensuring correct FDS formatting.
- Enhanced the `configure_comfy_backend` function to accept and manage `ExtraArgs`, preventing loss of important configuration during recovery.
- Added tests to validate the new sanitization behavior and ensure that bare `\\x` entries remain intact, improving overall backend management.

src/gpu_rent/remote/install_swarm_comfy.py

@@ -263,10 +263,12 @@ def recover_errored_backends(*, wait_sec: float = 180.0) -> str:
             return "errored"
     else:
         for bid, meta in entries:
-            script = str((meta.get("settings") or {}).get("StartScript") or "")
+            settings = meta.get("settings") or {}
+            script = str(settings.get("StartScript") or "")
+            extra = str(settings.get("ExtraArgs") or "")
             print(f"reconfigure errored id={bid} old StartScript={script!r}", flush=True)
             try:
-                configure_comfy_backend(sid, bid)
+                configure_comfy_backend(sid, bid, extra_args=extra)
             except Exception as exc:
                 print(f"configure FAIL id={bid}: {exc}", flush=True)
 
@@ -328,7 +330,11 @@ def recover_errored_backends(*, wait_sec: float = 180.0) -> str:
 
 
 def sanitize_backends_fds() -> bool:
-    """Repair ExtraArgs: \\x --flag (FDS empty + appended flag). Return True if changed."""
+    """Repair ExtraArgs: \\x --flag (FDS empty + appended flag). Return True if changed.
+
+    Do not use ``\\s`` after ``\\x`` — newline would be eaten and merge FDS keys.
+    Bare ``ExtraArgs: \\x`` is valid empty and must be left alone.
+    """
     path = DATA / "Data" / "Backends.fds"
     if not path.is_file():
         return False
@@ -336,8 +342,8 @@ def sanitize_backends_fds() -> bool:
         text = path.read_text(encoding="utf-8")
     except OSError:
         return False
-    new, n = re.subn(r"^(\s*ExtraArgs:\s*)\\x(\s+)", r"\1", text, flags=re.M)
+    new, n = re.subn(r"^(\s*ExtraArgs:\s*)\\x[ \t]+", r"\1", text, flags=re.M)
-    if not n:
+    if not n or new == text:
         return False
     path.write_text(new, encoding="utf-8")
     print(f"sanitized Backends.fds ExtraArgs \\x corruption ({n} line(s))", flush=True)
@@ -449,12 +455,18 @@ def toggle_backend(sid: str, backend_id: int, *, enabled: bool) -> dict:
     )
 
 
-def configure_comfy_backend(sid: str, backend_id: int) -> None:
+def configure_comfy_backend(
+    sid: str, backend_id: int, *, extra_args: str | None = None
+) -> None:
     """Set StartScript after AddNewBackend (defaults are empty → stays disabled)."""
     script = comfy_start_script()
+    # Preserve ExtraArgs (e.g. --use-sage-attention); "" wipes sage every recover.
+    extra = "" if extra_args is None else str(extra_args).strip()
+    if extra in {"\\x", "x"}:
+        extra = ""
     settings = {
         "StartScript": script,
-        "ExtraArgs": "",
+        "ExtraArgs": extra,
         "DisableInternalArgs": False,
         "AutoUpdate": "false",
         "UpdateManagedNodes": "false",
@@ -551,7 +563,8 @@ def recover_empty_backends() -> str:
                     flush=True,
                 )
                 try:
-                    configure_comfy_backend(sid, bid)
+                    extra = str(settings.get("ExtraArgs") or "")
+                    configure_comfy_backend(sid, bid, extra_args=extra)
                 except Exception as exc:
                     print(f"configure FAIL id={bid}: {exc}", flush=True)
 

src/gpu_rent/remote/tune_swarm_perf.py

@@ -84,26 +84,21 @@ def find_pip() -> Path | None:
 
 
 def sanitize_backends_fds() -> bool:
-    """Repair ``ExtraArgs: \\x --flag`` and bare ``ExtraArgs: \\x`` → clean values."""
+    """Repair ``ExtraArgs: \\x --flag`` only. Bare ``\\x`` is valid FDS empty.
+
+    Important: do not use ``\\s+`` after ``\\x`` — that eats the newline and
+    merges the next FDS key onto the ExtraArgs line (breaks SwarmUI load).
+    """
     if not BACKENDS.is_file():
         return False
     text = BACKENDS.read_text(encoding="utf-8")
-    new = text
+    # Only horizontal whitespace after \x (spaces/tabs), never newline.
-    # ``ExtraArgs: \x --flags`` → ``ExtraArgs: --flags``
+    new, n = re.subn(
-    new, n1 = re.subn(
+        r"^(\s*ExtraArgs:\s*)\\x[ \t]+",
-        r"^(\s*ExtraArgs:\s*)\\x(\s+)",
         r"\1",
-        new,
+        text,
         flags=re.M,
     )
-    # Bare empty marker left alone is OK for Swarm, but normalize trailing junk.
-    new, n2 = re.subn(
-        r"^(\s*ExtraArgs:\s*)\\x\s*$",
-        r"\1",
-        new,
-        flags=re.M,
-    )
-    n = n1 + n2
     if n and new != text:
         BACKENDS.write_text(new, encoding="utf-8")
         print(f"sanitized Backends.fds ExtraArgs \\x corruption ({n} line(s))")

tests/test_tune_swarm_perf.py

@@ -178,6 +178,17 @@ def test_sanitize_backends_fds_corruption(tmp_path, monkeypatch):
     assert backends.read_text(encoding="utf-8") == "ExtraArgs: --use-sage-attention\n"
 
 
+def test_sanitize_does_not_eat_newline_on_bare_empty(tmp_path, monkeypatch):
+    """Bare FDS empty ``\\x`` must not match — ``\\s+`` would merge next key."""
+    mod = _load()
+    backends = tmp_path / "Backends.fds"
+    original = "\tExtraArgs: \\x\n\tStartScript: /mnt/swarm_data/dlbackend/ComfyUI/main.py\n"
+    backends.write_text(original, encoding="utf-8")
+    monkeypatch.setattr(mod, "BACKENDS", backends)
+    assert mod.sanitize_backends_fds() is False
+    assert backends.read_text(encoding="utf-8") == original
+
+
 def test_ensure_absolute_start_script(tmp_path, monkeypatch):
     mod = _load()
     data = tmp_path