hashindex: track unresolved pack location with F_PENDING flag

src/borg/archive.py

@@ -29,7 +29,6 @@
 from .crypto.low_level import IntegrityError as IntegrityErrorBase
 from .helpers import BackupError, BackupRaceConditionError, BackupItemExcluded
 from .helpers import BackupOSError, BackupPermissionError, BackupFileNotFoundError, BackupIOError
-from .hashindex import ChunkIndex, ChunkIndexEntry
 from .helpers import HardLinkManager
 from .helpers import ChunkIteratorFileWrapper, open_item
 from .helpers import Error, IntegrityError, set_ec
@@ -1968,13 +1967,7 @@ def add_reference(id_, size, cdata):
             # either we already have this chunk in repo and chunks index or we add it now
             if id_ not in self.chunks:
                 assert cdata is not None
-                self.chunks[id_] = ChunkIndexEntry(
-                    flags=ChunkIndex.F_USED,
-                    size=size,
-                    pack_id=UNKNOWN_BYTES32,
-                    obj_offset=UNKNOWN_INT32,
-                    obj_size=UNKNOWN_INT32,
-                )
+                self.chunks.add(id_, size)
                 if self.repair:
                     pack_results = self.repository.put(id_, cdata)
                     self.chunks.update_pack_info(pack_results)

src/borg/constants.py

@@ -55,7 +55,8 @@
 # Grep for UNKNOWN_INT32 to find every site that still needs updating.
 UNKNOWN_INT32 = 0xFFFFFFFF
 
-# Placeholder for pack_id (32-byte field) when the value is not yet known.
+# Filler for the pack_id (32-byte field) while the real value is unknown. Never interpreted;
+# an unresolved pack location is tracked by the ChunkIndex.F_PENDING flag.
 UNKNOWN_BYTES32 = b"\xff" * 32
 
 # MAX_OBJECT_SIZE = MAX_DATA_SIZE + len(PUT header)

src/borg/hashindex.pyx

@@ -52,6 +52,7 @@ class ChunkIndex(HTProxyMixin, MutableMapping):
     # user flags:
     F_USED = 2 ** 0  # chunk is used/referenced
     F_COMPRESS = 2 ** 1  # chunk shall get (re-)compressed
+    F_PENDING = 2 ** 2  # pack location (pack_id, obj_offset, obj_size) not resolved yet.
     # system flags (internal use, always 0 to user, not changeable by user):
     F_NEW = 2 ** 24  # a new chunk that is not present in repo index/* yet.
 
@@ -81,8 +82,10 @@ class ChunkIndex(HTProxyMixin, MutableMapping):
         else:
             flags = v.flags | self.F_USED
             assert v.size == 0 or v.size == size
+        # F_PENDING marks the pack location (pack_id, obj_offset, obj_size) as not yet set.
         self[key] = ChunkIndexEntry(
-            flags=flags, size=size, pack_id=UNKNOWN_BYTES32, obj_offset=UNKNOWN_INT32, obj_size=UNKNOWN_INT32
+            flags=flags | self.F_PENDING, size=size,
+            pack_id=UNKNOWN_BYTES32, obj_offset=UNKNOWN_INT32, obj_size=UNKNOWN_INT32
         )
 
     def __getitem__(self, key):
@@ -109,12 +112,19 @@ class ChunkIndex(HTProxyMixin, MutableMapping):
         self.ht[key] = value._replace(flags=system_flags | user_flags)
 
     def update_pack_info(self, pack_results):
-        """Update the on-disk location fields for a list of (chunk_id, pack_id, obj_offset, obj_size) tuples."""
+        """Set pack_id, obj_offset and obj_size from a list of (chunk_id, pack_id, obj_offset, obj_size)
+        tuples and clear F_PENDING."""
         if not pack_results:
             return
         for chunk_id, pack_id, obj_offset, obj_size in pack_results:
             existing = self[chunk_id]
-            self[chunk_id] = existing._replace(pack_id=pack_id, obj_offset=obj_offset, obj_size=obj_size)
+            self[chunk_id] = existing._replace(
+                flags=existing.flags & ~self.F_PENDING, pack_id=pack_id, obj_offset=obj_offset, obj_size=obj_size
+            )
+
+    def is_pending(self, key):
+        """Return whether the chunk's pack location (pack_id, obj_offset, obj_size) is unresolved."""
+        return bool(self[key].flags & self.F_PENDING)
 
     def clear_new(self):
         """Clears the F_NEW flag of all items."""

src/borg/repository.py

@@ -106,9 +106,9 @@ def build_rest_backend(location):
 class PackWriter:
     """Buffers chunks into a pack file and writes it to the store when full.
 
-    add() buffers a (chunk_id, cdata) pair and marks the chunk pending in the
-    ChunkIndex (pack_id=UNKNOWN_BYTES32); flush() writes the pack and sets each
-    entry's real pack_id and obj_offset.
+    add() buffers a (chunk_id, cdata) pair and marks the chunk pending (F_PENDING);
+    flush() writes the pack and sets each entry's pack_id, obj_offset and obj_size,
+    clearing F_PENDING.
 
     The ChunkIndex comes from the repository, or from an explicit chunks index when
     there is no repository (see the chunks property).
@@ -142,8 +142,6 @@ def chunks(self):
     def add(self, chunk_id, cdata):
         """Buffer a chunk.  Returns flush results if the pack is now full, else None."""
         self.chunks.add(chunk_id, 0)  # size: plaintext chunk size, set by the cache layer
-        # obj_size is final; pack_id and obj_offset get their real values in flush().
-        self.chunks.update_pack_info([(chunk_id, UNKNOWN_BYTES32, 0, len(cdata))])
         self._pieces.append((chunk_id, cdata))
         self._size += len(cdata)
         if (self.max_count is not None and len(self._pieces) >= self.max_count) or (
@@ -179,22 +177,19 @@ def flush(self):
             offset += obj_size
 
         key = "packs/" + bin_to_hex(pack_id)
-        # ids this flush pre-marked in the index via add() (pack_id still UNKNOWN_BYTES32).
         pending_ids = [chunk_id for chunk_id, _ in self._pieces]
         try:
             self.store.store(key, pack_data)
         except Exception:
-            # The pack is not stored, so drop the pending entries: keeping them would let
-            # seen_chunk() dedup later chunks against bytes that were never written.
+            # the pack was not stored: drop the index entries for its chunks.
             for chunk_id in pending_ids:
-                entry = self.chunks.get(chunk_id)
-                if entry is not None and entry.pack_id == UNKNOWN_BYTES32:
+                if chunk_id in self.chunks:  # a chunk_id may appear more than once in this pack
                     del self.chunks[chunk_id]
             raise
         finally:
             self._pieces = []  # cleared on success and on failure
             self._size = 0
-        self.chunks.update_pack_info(results)  # replace UNKNOWN_BYTES32 with real pack_id
+        self.chunks.update_pack_info(results)  # set the real location and clear F_PENDING
         return results
 
 
@@ -711,7 +706,7 @@ def list(self, limit=None, marker=None):
         collect = marker is None
         result = []
         for chunk_id, entry in self.chunks.iteritems():
-            if entry.pack_id == UNKNOWN_BYTES32:
+            if self.chunks.is_pending(chunk_id):
                 continue  # buffered in PackWriter, not flushed to a pack yet
             if collect:
                 result.append((chunk_id, entry.obj_size))
@@ -728,11 +723,9 @@ def get(self, id, read_data=True, raise_missing=True):
             if raise_missing:
                 raise self.ObjectNotFound(id, str(self._location))
             return None
-        if entry.pack_id == UNKNOWN_BYTES32:
-            # chunk is buffered in PackWriter, not yet flushed to a pack. Everything must be flushed
-            # before it can be read back, so reaching here points at a flush / index-update ordering
-            # bug, not a genuinely missing object. this is a code bug, so we crash loudly regardless
-            # of raise_missing instead of pretending the object is absent.
+        if self.chunks.is_pending(id):
+            # buffered but not flushed; a chunk must be flushed before any read, so this is a code
+            # bug (wrong flush/index ordering), not a missing object: raise regardless of raise_missing.
             raise self.PackLocationUnknown(id, str(self._location))
         pack_id, obj_offset, obj_size = entry.pack_id, entry.obj_offset, entry.obj_size
         id_hex = bin_to_hex(id)

src/borg/testsuite/hashindex_test.py

@@ -21,16 +21,18 @@ def test_chunkindex_add():
     chunks = ChunkIndex()
     x = H2(1)
     chunks.add(x, 0)
+    assert chunks.is_pending(x)
+    pending = ChunkIndex.F_USED | ChunkIndex.F_PENDING  # add() sets F_PENDING alongside F_USED
     assert chunks[x] == ChunkIndexEntry(
-        flags=ChunkIndex.F_USED, size=0, pack_id=UNKNOWN_BYTES32, obj_offset=UNKNOWN_INT32, obj_size=UNKNOWN_INT32
+        flags=pending, size=0, pack_id=UNKNOWN_BYTES32, obj_offset=UNKNOWN_INT32, obj_size=UNKNOWN_INT32
     )
     chunks.add(x, 2)  # updating size (we do not have a size yet)
     assert chunks[x] == ChunkIndexEntry(
-        flags=ChunkIndex.F_USED, size=2, pack_id=UNKNOWN_BYTES32, obj_offset=UNKNOWN_INT32, obj_size=UNKNOWN_INT32
+        flags=pending, size=2, pack_id=UNKNOWN_BYTES32, obj_offset=UNKNOWN_INT32, obj_size=UNKNOWN_INT32
     )
     chunks.add(x, 2)
     assert chunks[x] == ChunkIndexEntry(
-        flags=ChunkIndex.F_USED, size=2, pack_id=UNKNOWN_BYTES32, obj_offset=UNKNOWN_INT32, obj_size=UNKNOWN_INT32
+        flags=pending, size=2, pack_id=UNKNOWN_BYTES32, obj_offset=UNKNOWN_INT32, obj_size=UNKNOWN_INT32
     )
     with pytest.raises(AssertionError):
         chunks.add(x, 3)  # inconsistent size (we already have a different size)
@@ -43,10 +45,15 @@ def test_chunkindex_update_pack_info():
     chunks.add(x2, 20)
     assert chunks[x1].obj_offset == UNKNOWN_INT32
     assert chunks[x2].obj_offset == UNKNOWN_INT32
+    assert chunks.is_pending(x1)
+    assert chunks.is_pending(x2)
 
     pack_id = H2(3)
     # Both chunks land in the same pack: batch update in one call.
     chunks.update_pack_info([(x1, pack_id, 0, 50), (x2, pack_id, 50, 60)])
+    # resolving the location clears the pending flag
+    assert not chunks.is_pending(x1)
+    assert not chunks.is_pending(x2)
     # Location fields updated; flags and size must be unchanged.
     assert chunks[x1] == ChunkIndexEntry(flags=ChunkIndex.F_USED, size=10, pack_id=pack_id, obj_offset=0, obj_size=50)
     assert chunks[x2] == ChunkIndexEntry(flags=ChunkIndex.F_USED, size=20, pack_id=pack_id, obj_offset=50, obj_size=60)

src/borg/testsuite/repository_test.py

@@ -5,7 +5,6 @@
 import pytest
 from ..helpers import IntegrityError, Location, bin_to_hex
 from ..hashindex import ChunkIndex
-from ..constants import UNKNOWN_BYTES32
 from ..repository import Repository, MAX_DATA_SIZE, rest_serve_command, PackWriter, PackReader
 from ..repoobj import RepoObj, OBJ_MAGIC, OBJ_VERSION
 from .hashindex_test import H
@@ -187,11 +186,11 @@ def test_multi_object_pack_roundtrip(repo_fixtures, request):
     with get_repository_from_fixture(repo_fixtures, request) as repository:
         repository._pack_writer.max_count = 2  # this test is written for exactly two objects per pack
         repository.put(H(0), chunk0)
-        assert repository.chunks[H(0)].pack_id == UNKNOWN_BYTES32  # buffered: the pack is not full yet
+        assert repository.chunks.is_pending(H(0))  # buffered: the pack is not full yet
         repository.put(H(1), chunk1)  # fills the pack, flushing both objects at once
         # both objects share one pack, written exactly once, laid out in put() order
         pack_id = repository.chunks[H(0)].pack_id
-        assert pack_id != UNKNOWN_BYTES32
+        assert not repository.chunks.is_pending(H(0))
         assert repository.chunks[H(1)].pack_id == pack_id
         assert [info.name for info in repository.store_list("packs")] == [bin_to_hex(pack_id)]
         assert repository.chunks[H(0)].obj_offset == 0
@@ -509,20 +508,57 @@ def test_get_uses_chunk_index_location(tmp_path):
 
 
 def test_put_marks_id_in_chunk_index(tmp_path):
-    # put() marks the id pending (pack_id=UNKNOWN_BYTES32); flush() then fills in the
-    # real pack location for the current session.
+    # put() marks the id pending; flush() sets the real pack location and clears the pending flag.
     with Repository(str(tmp_path / "repo"), exclusive=True, create=True) as repository:
         id1 = H(1)
         repository.put(id1, fchunk(b"ZEROS"))
         entry = repository._chunks.get(id1)
         assert entry is not None
-        assert entry.pack_id == UNKNOWN_BYTES32  # buffered, not yet flushed
+        assert repository._chunks.is_pending(id1)  # buffered, not yet flushed
         repository.flush()
         entry = repository._chunks.get(id1)
+        assert not repository._chunks.is_pending(id1)
         assert entry.pack_id == sha256(fchunk(b"ZEROS")).digest()
         assert entry.size == 0  # uncompressed size filled in by cache layer
 
 
+def test_list_skips_pending_chunk(tmp_path):
+    # list() skips a pending chunk and yields it once flushed.
+    with Repository(str(tmp_path / "repo"), exclusive=True, create=True) as repository:
+        repository.put(H(1), fchunk(b"BUFFERED"))  # buffered: the pack is not full yet
+        assert repository._chunks.is_pending(H(1))
+        assert repository.list() == []
+        repository.flush()
+        assert [chunk_id for chunk_id, _ in repository.list()] == [H(1)]
+
+
+def test_get_pending_chunk_raises(tmp_path):
+    # get() on a pending chunk raises PackLocationUnknown, also with raise_missing=False.
+    with Repository(str(tmp_path / "repo"), exclusive=True, create=True) as repository:
+        repository.put(H(1), fchunk(b"BUFFERED"))  # buffered: the pack is not full yet
+        assert repository._chunks.is_pending(H(1))
+        with pytest.raises(Repository.PackLocationUnknown):
+            repository.get(H(1))
+        with pytest.raises(Repository.PackLocationUnknown):
+            repository.get(H(1), raise_missing=False)
+        repository.flush()  # close() requires the buffer to be empty
+
+
+def test_flush_store_failure_drops_pending_entries(tmp_path):
+    # flush() removes the pending index entries when storing the pack fails.
+    with Repository(str(tmp_path / "repo"), exclusive=True, create=True) as repository:
+        repository.put(H(1), fchunk(b"BUFFERED"))
+        assert repository._chunks.is_pending(H(1))
+
+        def boom(*args, **kwargs):
+            raise OSError("store failed")
+
+        repository._pack_writer.store.store = boom
+        with pytest.raises(OSError):
+            repository.flush()
+        assert H(1) not in repository._chunks
+
+
 def test_check_detects_corruption_in_later_object(tmp_path):
     # Corruption anywhere in a multi-object pack must be caught, not just in the first object: the pack
     # is named by sha256(content), so flipping any byte makes its stored hash differ from its name.