defrag: deal with short/long reads, fix #194

docs/changes.rst

@@ -12,6 +12,9 @@ Fixes:
   request (giving up with a BackendError if it can't be reestablished), #167
 - rest (stdio-over-ssh): raise BackendConnectionError on unexpected EOF during response headers
   or response body transfer, preventing silent data truncation.
+- defrag: check for short/long reads in range requests and raise ReadRangeError exception in
+  case of a size mismatch, #194
+
 
 
 Version 0.5.4 (2026-07-09)

src/borgstore/backends/_base.py

@@ -131,8 +131,17 @@ def defrag(self, sources, *, target=None, algorithm=None, namespace=None, levels
         # default implementation: slow, but works for all backends.
         # might be overridden for performance.
         from ..utils.nesting import nest
-
-        data = b"".join(self.load(source, offset=offset, size=size) for source, offset, size in sources)
+        from .errors import ReadRangeError
+
+        data_parts = []
+        for source, offset, size in sources:
+            chunk = self.load(source, offset=offset, size=size)
+            if len(chunk) != size:
+                raise ReadRangeError(
+                    f"Read range error from {source} (requested {size} bytes at offset {offset}, got {len(chunk)})"
+                )
+            data_parts.append(chunk)
+        data = b"".join(data_parts)
         if target is None:
             if algorithm is None:
                 raise ValueError("Either target or algorithm must be given for defrag")

src/borgstore/backends/errors.py

@@ -45,3 +45,7 @@ class PermissionDenied(BackendError):
 
 class QuotaExceeded(BackendError):
     """Quota exceeded for the requested operation."""
+
+
+class ReadRangeError(BackendError):
+    """Read range error occurred (e.g. short read)."""

src/borgstore/backends/rest.py

@@ -41,6 +41,7 @@
     BackendError,
     BackendMustBeOpen,
     BackendMustNotBeOpen,
+    ReadRangeError,
 )
 
 logger = logging.getLogger(__name__)
@@ -451,6 +452,8 @@ def _handle_response(self, response, name=None):
             raise PermissionDenied(name or self.base_url)
         if response.status_code == HTTP.INSUFFICIENT_STORAGE:
             raise QuotaExceeded(response.text)
+        if response.status_code == HTTP.REQUESTED_RANGE_NOT_SATISFIABLE:
+            raise ReadRangeError(response.text)
         if response.status_code == HTTP.BAD_REQUEST:
             raise ValueError(response.text)
         response.raise_for_status()

src/borgstore/server/rest.py

@@ -22,6 +22,7 @@
     BackendError,
     BackendMustBeOpen,
     BackendMustNotBeOpen,
+    ReadRangeError,
 )
 from ..backends._utils import parse_range_header
 from ..store import get_backend
@@ -153,6 +154,8 @@ def _handle_exception(self, e, name=None):
             self.send_error(HTTP.FORBIDDEN, msg)
         elif isinstance(e, QuotaExceeded):
             self.send_error(HTTP.INSUFFICIENT_STORAGE, msg)
+        elif isinstance(e, ReadRangeError):
+            self.send_error(HTTP.REQUESTED_RANGE_NOT_SATISFIABLE, msg)
         elif isinstance(e, (ValueError, TypeError)):
             self.send_error(HTTP.BAD_REQUEST, msg)
             logger.exception("Exception for %s", name or self.path)

src/borgstore/store.py

@@ -20,7 +20,7 @@
 
 from .utils.nesting import nest, unnest
 from .backends._base import ItemInfo, BackendBase
-from .backends.errors import ObjectNotFound, NoBackendGiven, BackendURLInvalid  # noqa
+from .backends.errors import ObjectNotFound, NoBackendGiven, BackendURLInvalid, ReadRangeError  # noqa
 from .backends.posixfs import get_file_backend
 from .backends.rclone import get_rclone_backend
 from .backends.sftp import get_sftp_backend

tests/test_server_rest.py

@@ -14,7 +14,7 @@
 from borgstore.server.rest import BorgStoreRESTServer
 from borgstore.backends.rest import get_rest_backend
 from borgstore.backends.posixfs import get_file_backend
-from borgstore.backends.errors import ObjectNotFound, BackendAlreadyExists, QuotaExceeded
+from borgstore.backends.errors import ObjectNotFound, BackendAlreadyExists, QuotaExceeded, ReadRangeError
 from borgstore.store import get_backend, Store
 
 
@@ -422,6 +422,12 @@ def test_rest_backend_defrag(rest_server_with_auth):
         with pytest.raises(ValueError, match="Unsupported hash algorithm"):
             be.defrag(sources, algorithm="invalid")
 
+        # Test error: short read
+        with pytest.raises(ReadRangeError) as exc_info:
+            be.defrag([("file1", 2, 20)], target="target2")
+        assert "Read range error from" in str(exc_info.value)
+        assert "requested 20 bytes" in str(exc_info.value)
+
         # Test defrag with levels=1 and algorithm
         from borgstore.utils.nesting import nest
 

tests/test_store.py

@@ -13,7 +13,7 @@
 from .test_backends import get_s3_test_backend, s3_is_available  # noqa
 
 from borgstore.constants import ROOTNS
-from borgstore.store import Store, ItemInfo
+from borgstore.store import Store, ItemInfo, ReadRangeError
 
 CONFIG = {"zero/": {"levels": [0]}, "one/": {"levels": [1]}, "two/": {"levels": [2]}}  # Layout used for most tests
 
@@ -121,6 +121,12 @@ def test_defrag_nested(posixfs_store_created):
         assert res == expected_hash
         assert store.load(ns + "/" + res) == expected_data
 
+        # 3. Test defrag with short read
+        with pytest.raises(ReadRangeError) as exc_info:
+            store.defrag([("file1", 2, 20)], target="target2", namespace=ns)
+        assert "Read range error from" in str(exc_info.value)
+        assert "requested 20 bytes" in str(exc_info.value)
+
 
 def test_hash(posixfs_store_created):
     ns = "two"