fix: honor hardlinks across client mount topology

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", "\\")
+    )