Implement chunk comparison and selective extraction

Add tests

Change logic for chunk comaprison and extraction

Refactor `compare_and_extract_chunks` and improve test coverage

To do:
- Remove additional comments after approval.

Thank you for helping me with my first file system operations contribution!

Refactor `compare_and_extract_chunks`

remove unnecessary `st` parameter

Fix file state tracking with st = None after unlink/rmdir

src/borg/archive.py

@@ -719,6 +719,63 @@ def extract_helper(self, item, path, hlm, *, dry_run=False):
                 # In this case, we *want* to extract twice, because there is no other way.
                 pass
 
+    def compare_and_extract_chunks(self, item, fs_path, *, st, pi=None):
+        """Compare file chunks and patch if needed. Returns True if patching succeeded."""
+        if st is None:
+            return False
+        try:
+            # First pass: Build fs chunks list
+            fs_chunks = []
+            with backup_io("open"):
+                fs_file = open(fs_path, "rb")
+            with fs_file:
+                for chunk in item.chunks:
+                    with backup_io("read"):
+                        data = fs_file.read(chunk.size)
+
+                    fs_chunks.append(ChunkListEntry(id=self.key.id_hash(data), size=len(data)))
+
+            # Compare chunks and collect needed chunk IDs
+            needed_chunks = []
+            for fs_chunk, item_chunk in zip(fs_chunks, item.chunks):
+                if fs_chunk != item_chunk:
+                    needed_chunks.append(item_chunk)
+
+            # Fetch all needed chunks and iterate through ALL of them
+            chunk_data_iter = self.pipeline.fetch_many([chunk.id for chunk in needed_chunks], ro_type=ROBJ_FILE_STREAM)
+
+            # Second pass: Update file
+            with backup_io("open"):
+                fs_file = open(fs_path, "rb+")
+            with fs_file:
+                for fs_chunk, item_chunk in zip(fs_chunks, item.chunks):
+                    if fs_chunk == item_chunk:
+                        with backup_io("seek"):
+                            fs_file.seek(item_chunk.size, 1)
+                    else:
+                        chunk_data = next(chunk_data_iter)
+
+                        with backup_io("write"):
+                            fs_file.write(chunk_data)
+                    if pi:
+                        pi.show(increase=len(chunk_data), info=[remove_surrogates(item.path)])
+
+                final_size = fs_file.tell()
+                with backup_io("truncate_and_attrs"):
+                    fs_file.truncate(item.size)
+                    fs_file.flush()
+                    self.restore_attrs(fs_path, item, fd=fs_file.fileno())
+
+            if "size" in item and item.size != final_size:
+                raise BackupError(f"Size inconsistency detected: size {item.size}, chunks size {final_size}")
+
+            if "chunks_healthy" in item and not item.chunks_healthy:
+                raise BackupError("File has damaged (all-zero) chunks. Try running borg check --repair.")
+
+            return True
+        except OSError:
+            return False
+
     def extract_item(
         self,
         item,
@@ -802,12 +859,14 @@ def same_item(item, st):
                 return  # done! we already have fully extracted this file in a previous run.
             elif stat.S_ISDIR(st.st_mode):
                 os.rmdir(path)
+                st = None
             else:
                 os.unlink(path)
+                st = None
         except UnicodeEncodeError:
             raise self.IncompatibleFilesystemEncodingError(path, sys.getfilesystemencoding()) from None
         except OSError:
-            pass
+            st = None
 
         def make_parent(path):
             parent_dir = os.path.dirname(path)
@@ -821,6 +880,9 @@ def make_parent(path):
             with self.extract_helper(item, path, hlm) as hardlink_set:
                 if hardlink_set:
                     return
+                if self.compare_and_extract_chunks(item, path, st=st, pi=pi):
+                    return
+
                 with backup_io("open"):
                     fd = open(path, "wb")
                 with fd:

src/borg/testsuite/archive_test.py

@@ -12,7 +12,7 @@
 from ..archive import Archive, CacheChunkBuffer, RobustUnpacker, valid_msgpacked_dict, ITEM_KEYS, Statistics
 from ..archive import BackupOSError, backup_io, backup_io_iter, get_item_uid_gid
 from ..helpers import msgpack
-from ..item import Item, ArchiveItem
+from ..item import Item, ArchiveItem, ChunkListEntry
 from ..manifest import Manifest
 from ..platform import uid2user, gid2group, is_win32
 
@@ -132,6 +132,11 @@ def add_chunk(self, id, meta, data, stats=None, wait=True, ro_type=None):
         self.objects[id] = data
         return id, len(data)
 
+    def fetch_many(self, ids, ro_type=None):
+        """Mock implementation of fetch_many"""
+        for id in ids:
+            yield self.objects[id]
+
 
 def test_cache_chunk_buffer():
     data = [Item(path="p1"), Item(path="p2")]
@@ -402,3 +407,161 @@ def test_reject_non_sanitized_item():
     for path in rejected_dotdot_paths:
         with pytest.raises(ValueError, match="unexpected '..' element in path"):
             Item(path=path, user="root", group="root")
+
+
+@pytest.fixture
+def setup_extractor(tmpdir):
+    """Setup common test infrastructure"""
+
+    class MockCache:
+        def __init__(self):
+            self.objects = {}
+
+    repository = Mock()
+    key = PlaintextKey(repository)
+    manifest = Manifest(key, repository)
+    cache = MockCache()
+
+    extractor = Archive(manifest=manifest, name="test", create=True)
+    extractor.pipeline = cache
+    extractor.key = key
+    extractor.cwd = str(tmpdir)
+    extractor.restore_attrs = Mock()
+
+    # Track fetched chunks across tests
+    fetched_chunks = []
+
+    def create_mock_chunks(item_data, chunk_size=4):
+        chunks = []
+        for i in range(0, len(item_data), chunk_size):
+            chunk_data = item_data[i : i + chunk_size]
+            chunk_id = key.id_hash(chunk_data)
+            chunks.append(ChunkListEntry(id=chunk_id, size=len(chunk_data)))
+            cache.objects[chunk_id] = chunk_data
+
+        item = Mock(spec=["chunks", "size", "__contains__", "get"])
+        item.chunks = chunks
+        item.size = len(item_data)
+        item.__contains__ = lambda self, item: item == "size"
+
+        return item, str(tmpdir.join("test.txt"))
+
+    def mock_fetch_many(chunk_ids, ro_type=None):
+        fetched_chunks.extend(chunk_ids)
+        return iter([cache.objects[chunk_id] for chunk_id in chunk_ids])
+
+    def clear_fetched_chunks():
+        fetched_chunks.clear()
+
+    def get_fetched_chunks():
+        return fetched_chunks
+
+    cache.fetch_many = mock_fetch_many
+
+    return extractor, key, cache, tmpdir, create_mock_chunks, get_fetched_chunks, clear_fetched_chunks
+
+
+@pytest.mark.parametrize(
+    "name, item_data, fs_data, expected_fetched_chunks",
+    [
+        (
+            "no_changes",
+            b"1111",  # One complete chunk, no changes needed
+            b"1111",  # Identical content
+            0,  # No chunks should be fetched
+        ),
+        (
+            "single_chunk_change",
+            b"11112222",  # Two chunks
+            b"1111XXXX",  # Second chunk different
+            1,  # Only second chunk should be fetched
+        ),
+        (
+            "cross_boundary_change",
+            b"11112222",  # Two chunks
+            b"111XX22",  # Change crosses chunk boundary
+            2,  # Both chunks need update
+        ),
+        (
+            "exact_multiple_chunks",
+            b"11112222333",  # Three chunks (last one partial)
+            b"1111XXXX333",  # Middle chunk different
+            1,  # Only middle chunk fetched
+        ),
+        (
+            "first_chunk_change",
+            b"11112222",  # Two chunks
+            b"XXXX2222",  # First chunk different
+            1,  # Only first chunk should be fetched
+        ),
+        (
+            "all_chunks_different",
+            b"11112222",  # Two chunks
+            b"XXXXYYYY",  # Both chunks different
+            2,  # Both chunks should be fetched
+        ),
+        (
+            "partial_last_chunk",
+            b"111122",  # One full chunk + partial
+            b"1111XX",  # Partial chunk different
+            1,  # Only second chunk should be fetched
+        ),
+        (
+            "fs_file_shorter",
+            b"11112222",  # Two chunks in archive
+            b"111122",  # Shorter on disk - missing part of second chunk
+            1,  # Should fetch second chunk
+        ),
+        (
+            "fs_file_longer",
+            b"11112222",  # Two chunks in archive
+            b"1111222233",  # Longer on disk
+            0,  # Should fetch no chunks since content matches up to archive length
+        ),
+        (
+            "empty_archive_file",
+            b"",  # Empty in archive
+            b"11112222",  # Content on disk
+            0,  # No chunks to compare = no chunks to fetch
+        ),
+        (
+            "empty_fs_file",
+            b"11112222",  # Two chunks in archive
+            b"",  # Empty on disk
+            2,  # Should fetch all chunks since file is empty
+        ),
+    ],
+)
+def test_compare_and_extract_chunks(setup_extractor, name, item_data, fs_data, expected_fetched_chunks):
+    """Test chunk comparison and extraction"""
+    extractor, key, cache, tmpdir, create_mock_chunks, get_fetched_chunks, clear_fetched_chunks = setup_extractor
+    clear_fetched_chunks()
+
+    chunk_size = 4
+    item, target_path = create_mock_chunks(item_data, chunk_size=chunk_size)
+
+    original_chunk_ids = [chunk.id for chunk in item.chunks]
+
+    with open(target_path, "wb") as f:
+        f.write(fs_data)
+
+    st = os.stat(target_path)
+    result = extractor.compare_and_extract_chunks(item, target_path, st=st)
+    assert result
+
+    fetched_chunks = get_fetched_chunks()
+    assert len(fetched_chunks) == expected_fetched_chunks
+
+    # For single chunk changes, verify it's the correct chunk
+    if expected_fetched_chunks == 1:
+        item_chunks = [item_data[i : i + chunk_size] for i in range(0, len(item_data), chunk_size)]
+        fs_chunks = [fs_data[i : i + chunk_size] for i in range(0, len(fs_data), chunk_size)]
+
+        # Find which chunk should have changed by comparing item_data with fs_data
+        for i, (item_chunk, fs_chunk) in enumerate(zip(item_chunks, fs_chunks)):
+            if item_chunk != fs_chunk:
+                assert fetched_chunks[0] == original_chunk_ids[i]
+                break
+
+    with open(target_path, "rb") as f:
+        assert f.read() == item_data