borg extract: add --continue flag

src/borg/archive.py

@@ -1,4 +1,5 @@
 import errno
+import io
 import os
 import socket
 import stat
@@ -22,7 +23,7 @@
 from .compress import Compressor
 from .constants import *  # NOQA
 from .hashindex import ChunkIndex, ChunkIndexEntry
-from .helpers import Manifest
+from .helpers import Manifest, slice_chunks
 from .helpers import Chunk, ChunkIteratorFileWrapper, open_item
 from .helpers import Error, IntegrityError
 from .helpers import uid2user, user2uid, gid2group, group2gid
@@ -428,7 +429,7 @@ def add_file_chunks(chunks):
         return stats
 
     def extract_item(self, item, restore_attrs=True, dry_run=False, stdout=False, sparse=False,
-                     hardlink_masters=None, stripped_components=0, original_path=None, pi=None):
+                     complete_partial=False, stripped_components=0, hardlink_masters=None, original_path=None, pi=None):
         """
         Extract archive item.
 
@@ -437,6 +438,7 @@ def extract_item(self, item, restore_attrs=True, dry_run=False, stdout=False, sp
         :param dry_run: do not write any data
         :param stdout: write extracted data to stdout
         :param sparse: write sparse files (chunk-granularity, independent of the original being sparse)
+        :param complete_partial: False: replace files, True: existing files are completed
         :param hardlink_masters: maps paths to (chunks, link_target) for extracting subtrees with hardlinks correctly
         :param stripped_components: stripped leading path components to correct hard link extraction
         :param original_path: 'path' key as stored in archive
@@ -466,10 +468,11 @@ def extract_item(self, item, restore_attrs=True, dry_run=False, stdout=False, sp
         # Attempt to remove existing files, ignore errors on failure
         try:
             st = os.lstat(path)
-            if stat.S_ISDIR(st.st_mode):
-                os.rmdir(path)
-            else:
-                os.unlink(path)
+            if not complete_partial:
+                if stat.S_ISDIR(st.st_mode):
+                    os.rmdir(path)
+                else:
+                    os.unlink(path)
         except UnicodeEncodeError:
             raise self.IncompatibleFilesystemEncodingError(path, sys.getfilesystemencoding()) from None
         except OSError:
@@ -496,9 +499,21 @@ def extract_item(self, item, restore_attrs=True, dry_run=False, stdout=False, sp
                     return
                 # Extract chunks, since the item which had the chunks was not extracted
             with backup_io():
-                fd = open(path, 'wb')
+                fd = open(path, 'ab')
             with fd:
-                ids = [c.id for c in item.chunks]
+                chunks = item.chunks
+                if complete_partial:
+                    with backup_io():
+                        fd.seek(0, io.SEEK_END)
+                        existing_length = fd.tell()
+                        # Slice chunks by current length of the existing file.
+                        chunks, prefix_length = slice_chunks(chunks, maximum_length=existing_length)
+                        # We don't bother extracting fractional chunks. Just seek to a chunk boundary.
+                        fd.seek(prefix_length)
+                        fd.truncate()
+                    if pi:
+                        pi.show(increase=prefix_length)
+                ids = [c.id for c in chunks]
                 for _, data in self.pipeline.fetch_many(ids, is_preloaded=True):
                     if pi:
                         pi.show(increase=len(data))
@@ -509,8 +524,6 @@ def extract_item(self, item, restore_attrs=True, dry_run=False, stdout=False, sp
                         else:
                             fd.write(data)
                 with backup_io():
-                    pos = fd.tell()
-                    fd.truncate(pos)
                     fd.flush()
                     self.restore_attrs(path, item, fd=fd.fileno())
             if has_damaged_chunks:

src/borg/archiver.py

@@ -487,11 +487,16 @@ def do_extract(self, args, repository, manifest, key, archive):
         dry_run = args.dry_run
         stdout = args.stdout
         sparse = args.sparse
+        continue_ = args.continue_
         strip_components = args.strip_components
         dirs = []
         partial_extract = not matcher.empty() or strip_components
         hardlink_masters = {} if partial_extract else None
 
+        if stdout and continue_:
+            self.print_error('Cannot combine --stdout and --continue.')
+            return self.exit_code
+
         def peek_and_store_hardlink_masters(item, matched):
             if (partial_extract and not matched and stat.S_ISREG(item.mode) and
                     item.get('hardlink_master', True) and 'source' not in item):
@@ -527,8 +532,9 @@ def peek_and_store_hardlink_masters(item, matched):
                         dirs.append(item)
                         archive.extract_item(item, restore_attrs=False)
                     else:
-                        archive.extract_item(item, stdout=stdout, sparse=sparse, hardlink_masters=hardlink_masters,
-                                             stripped_components=strip_components, original_path=orig_path, pi=pi)
+                        archive.extract_item(item, stdout=stdout, complete_partial=continue_, sparse=sparse,
+                                             hardlink_masters=hardlink_masters, original_path=orig_path, pi=pi,
+                                             stripped_components=strip_components)
             except BackupOSError as e:
                 self.print_warning('%s: %s', remove_surrogates(orig_path), e)
 
@@ -1867,6 +1873,9 @@ def build_parser(self, prog=None):
         subparser.add_argument('--sparse', dest='sparse',
                                action='store_true', default=False,
                                help='create holes in output sparse file from all-zero chunks')
+        subparser.add_argument('--continue', dest='continue_',
+                               action='store_true', default=False,
+                               help='continue interrupted extraction')
         subparser.add_argument('location', metavar='ARCHIVE',
                                type=location_validator(archive=True),
                                help='archive to extract')

src/borg/helpers.py

@@ -23,7 +23,7 @@
 from datetime import datetime, timezone, timedelta
 from fnmatch import translate
 from functools import wraps, partial, lru_cache
-from itertools import islice
+from itertools import islice, dropwhile
 from operator import attrgetter
 from string import Formatter
 
@@ -1769,3 +1769,21 @@ def swidth_slice(string, max_width):
     if reverse:
         result.reverse()
     return ''.join(result)
+
+
+def slice_chunks(chunks, maximum_length):
+    """
+    Slice *chunks* (list(ChunkListEntry)) to remove a prefix of *maximum_length*.
+
+    Return (sliced_chunks, prefix_length).
+    """
+    def should_drop(chunk):
+        nonlocal current_length
+        dropped = (current_length + chunk.size) <= maximum_length
+        if dropped:
+            current_length += chunk.size
+        return dropped
+    current_length = 0
+    sliced_chunks = list(dropwhile(should_drop, chunks))
+    assert current_length >= 0
+    return sliced_chunks, current_length

src/borg/testsuite/archiver.py

@@ -736,6 +736,22 @@ def test_extract_with_pattern(self):
             self.cmd("extract", self.repository_location + "::test", "fm:input/file1", "fm:*file33*", "input/file2")
         self.assert_equal(sorted(os.listdir("output/input")), ["file1", "file2", "file333"])
 
+    def test_extract_continue(self):
+        self.cmd('init', self.repository_location)
+        self.create_regular_file('file1', size=1024 * 80)
+        self.create_regular_file('file2', size=1024 * 100)
+        self.cmd('create', self.repository_location + '::test', 'input')
+
+        with changedir('output'):
+            self.cmd('extract', self.repository_location + '::test')
+            with open('input/file1', 'ab') as file:
+                file.truncate(1234)
+            with open('input/file2', 'ab') as file:
+                file.write(b'extra bytes')
+            print(self.cmd('extract', self.repository_location + '::test', '--continue'))
+            assert os.path.getsize('input/file1') == 1024 * 80
+            assert os.path.getsize('input/file2') == 1024 * 100
+
     def test_extract_list_output(self):
         self.cmd('init', self.repository_location)
         self.create_regular_file('file', size=1024 * 80)

src/borg/testsuite/helpers.py

@@ -10,7 +10,8 @@
 import msgpack.fallback
 
 from .. import platform
-from ..helpers import Location
+from ..cache import ChunkListEntry
+from ..helpers import Location, slice_chunks
 from ..helpers import Buffer
 from ..helpers import partial_format, format_file_size, parse_file_size, format_timedelta, format_line, PlaceholderError, replace_placeholders
 from ..helpers import make_path_safe, clean_lines
@@ -1081,3 +1082,29 @@ def test_swidth_slice_mixed_characters():
     string = '나윤a선나윤선나윤선나윤선나윤선'
     assert swidth_slice(string, 5) == '나윤a'
     assert swidth_slice(string, 6) == '나윤a'
+
+
+class TestSliceChunks:
+    @staticmethod
+    def chunk(size):
+        return ChunkListEntry(None, size, 0)
+
+    @pytest.mark.parametrize('chunks, offset, expected_chunks, expected_prefix_length', (
+        # Edge case: offset exactly on chunk boundary
+        ([(1, 1000), (2, 500)], 1000, [(2, 500)], 1000),
+        ([(1, 1000), (2, 500)], 999, [(1, 1000), (2, 500)], 0),
+        ([(1, 1000), (2, 500)], 1001, [(2, 500)], 1000),
+
+        # Edge case: offset completely consumes
+        ([(1, 1000), (2, 500)], 1500, [], 1500),
+        ([(1, 1000), (2, 500)], 1499, [(2, 500)], 1000),
+
+        # Edge case: offset > length of chunks
+        ([(1, 1000), (2, 500)], 1501, [], 1500),
+    ))
+    def test_basic(self, chunks, offset, expected_chunks, expected_prefix_length):
+        chunks = [ChunkListEntry(id, size, 0) for id, size in chunks]
+        expected_chunks = [ChunkListEntry(id, size, 0) for id, size in expected_chunks]
+        chunks, remaing_offset = slice_chunks(chunks, offset)
+        assert chunks == expected_chunks
+        assert remaing_offset == expected_prefix_length