fix: honor hardlinks across client mount topology

src/archive_clients/config.py

@@ -27,17 +27,28 @@ _ENV = re.compile(r"\$\{([A-Za-z_][A-Za-z0-9_]*)\}")
 class RootMapping:
     api_root: PurePosixPath
     local_root: Path
+    local_path_overrides: tuple[tuple[PurePosixPath, Path], ...] = ()
 
     def api_to_local(self, api_path: str) -> Path:
         candidate = PurePosixPath(api_path)
-        try:
-            relative = candidate.relative_to(self.api_root)
-        except ValueError as exc:
-            raise ConfigError("API path is outside its configured root") from exc
-        if any(part in {"", ".", ".."} for part in relative.parts):
-            raise ConfigError("API path contains an unsafe component")
+        for api_root, local_root in self.local_path_overrides:
+            relative = _safe_relative(candidate, api_root)
+            if relative is not None:
+                return local_root.joinpath(*relative.parts)
+        relative = _safe_relative(candidate, self.api_root)
+        if relative is None:
+            raise ConfigError("API path is outside its configured root")
         return self.local_root.joinpath(*relative.parts)
 
+    def local_root_for_api(self, api_path: str) -> Path:
+        candidate = PurePosixPath(api_path)
+        for api_root, local_root in self.local_path_overrides:
+            if _safe_relative(candidate, api_root) is not None:
+                return local_root
+        if _safe_relative(candidate, self.api_root) is None:
+            raise ConfigError("API path is outside its configured root")
+        return self.local_root
+
 
 @dataclass(frozen=True)
 class ServiceConfig:
@@ -48,10 +59,13 @@ class ServiceConfig:
     password_file: Path | None = None
     api_key_file: Path | None = None
     advertised_addresses: tuple[str, ...] = ()
+    local_path_overrides: tuple[tuple[PurePosixPath, Path], ...] = ()
 
     @property
     def roots(self) -> RootMapping:
-        return RootMapping(self.api_root, self.local_root)
+        return RootMapping(
+            self.api_root, self.local_root, self.local_path_overrides
+        )
 
     def read_password(self) -> str | None:
         return (
@@ -160,7 +174,7 @@ def _service(value: Any, name: str) -> ServiceConfig:
         raise ConfigError(f"{name} must be a table")
     allowed = {
         "endpoint", "api_root", "local_root", "username", "password_file",
-        "api_key_file", "advertised_addresses",
+        "api_key_file", "advertised_addresses", "local_path_overrides",
     }
     _keys(value, allowed, name)
     api_root = PurePosixPath(_string(value, "api_root"))
@@ -182,6 +196,7 @@ def _service(value: Any, name: str) -> ServiceConfig:
         raise ConfigError("qbittorrent username and password_file are required")
     if name == "syncthing" and "api_key_file" not in value:
         raise ConfigError("syncthing api_key_file is required")
+    overrides = _local_path_overrides(value, api_root, name)
     return ServiceConfig(
         _endpoint(value, "endpoint", {"http", "https"}), api_root,
         _absolute_path(value, "local_root"), username,
@@ -189,10 +204,46 @@ def _service(value: Any, name: str) -> ServiceConfig:
         if "password_file" in value else None,
         _absolute_path(value, "api_key_file")
         if "api_key_file" in value else None,
-        tuple(addresses),
+        tuple(addresses), overrides,
     )
 
 
+def _local_path_overrides(
+    value: dict[str, Any], api_root: PurePosixPath, name: str
+) -> tuple[tuple[PurePosixPath, Path], ...]:
+    raw = value.get("local_path_overrides", {})
+    if name != "syncthing" and raw:
+        raise ConfigError(f"{name}.local_path_overrides is unsupported")
+    if not isinstance(raw, dict):
+        raise ConfigError(f"{name}.local_path_overrides must be a table")
+    parsed: list[tuple[PurePosixPath, Path]] = []
+    for raw_api_path, raw_local_path in raw.items():
+        if not isinstance(raw_api_path, str) or not isinstance(raw_local_path, str):
+            raise ConfigError(f"{name}.local_path_overrides entries must be strings")
+        candidate = PurePosixPath(raw_api_path)
+        if not candidate.is_absolute() or ".." in candidate.parts:
+            raise ConfigError(f"{name}.local_path_overrides API path is invalid")
+        if _safe_relative(candidate, api_root) is None:
+            raise ConfigError(f"{name}.local_path_overrides API path is outside root")
+        local = Path(raw_local_path)
+        if not local.is_absolute():
+            raise ConfigError(f"{name}.local_path_overrides local path is invalid")
+        parsed.append((candidate, local))
+    return tuple(sorted(parsed, key=lambda item: len(item[0].parts), reverse=True))
+
+
+def _safe_relative(
+    candidate: PurePosixPath, root: PurePosixPath
+) -> PurePosixPath | None:
+    try:
+        relative = candidate.relative_to(root)
+    except ValueError:
+        return None
+    if any(part in {"", ".", ".."} for part in relative.parts):
+        raise ConfigError("API path contains an unsafe component")
+    return relative
+
+
 def _connection(value: Any) -> ConnectionConfig:
     if not isinstance(value, dict):
         raise ConfigError("connection must be a table")

src/archive_clients/jobs.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import hashlib
 import json
+import os
 import shutil
 import stat
 import threading
@@ -893,6 +894,7 @@ class ClientJobExecutor:
             if (
                 not stat.S_ISREG(metadata.st_mode)
                 or metadata.st_dev != destination_device
+                or _mount_id(source) != _mount_id(destination_root)
             ):
                 required += logical_bytes
         return required
@@ -1150,3 +1152,41 @@ def _job_error_code(error: Exception) -> int:
     if isinstance(error, EvictionError):
         return common_pb2.ERROR_CODE_PRECONDITION_FAILED
     return common_pb2.ERROR_CODE_INTERNAL
+
+
+def _mount_id(path: Path) -> str | None:
+    """Return Linux's effective mount ID for a path when procfs is available.
+
+    Bind mounts can share ``st_dev`` while still rejecting ``link(2)`` with
+    ``EXDEV``.  Mount IDs distinguish that case without creating probe files
+    inside a Syncthing folder.
+    """
+
+    try:
+        target = os.path.realpath(path)
+        best: tuple[int, str] | None = None
+        with open("/proc/self/mountinfo", encoding="utf-8") as source:
+            for line in source:
+                fields = line.rstrip("\n").split(" ")
+                if len(fields) < 5:
+                    continue
+                mountpoint = _unescape_mount_path(fields[4])
+                if target != mountpoint and not target.startswith(
+                    mountpoint.rstrip("/") + "/"
+                ):
+                    continue
+                candidate = (len(mountpoint), fields[0])
+                if best is None or candidate[0] > best[0]:
+                    best = candidate
+        return None if best is None else best[1]
+    except OSError:
+        return None
+
+
+def _unescape_mount_path(value: str) -> str:
+    return (
+        value.replace("\\040", " ")
+        .replace("\\011", "\t")
+        .replace("\\012", "\n")
+        .replace("\\134", "\\")
+    )

src/archive_clients/routes.py

@@ -37,7 +37,9 @@ def discover_routes(
             relative = normalized_api_path.relative_to(roots.api_root)
         except (ConfigError, ValueError):
             continue
-        if not _lexically_within(local_path, roots.local_root):
+        if not _lexically_within(
+            local_path, roots.local_root_for_api(normalized_api_path.as_posix())
+        ):
             continue
         if relative == PurePosixPath("."):
             continue

src/archive_clients/syncthing.py

@@ -356,7 +356,9 @@ class SyncthingRouteManager:
             local_path = self.config.roots.api_to_local(api_path)
         except ConfigError as exc:
             raise RoutePathConflict("route path is outside the sync root") from exc
-        resolved_root = self.config.local_root.resolve(strict=False)
+        resolved_root = self.config.roots.local_root_for_api(api_path).resolve(
+            strict=False
+        )
         try:
             local_path.resolve(strict=False).relative_to(resolved_root)
         except ValueError as exc: