revlog: add a way to keep track of older uids in the docket

revlog: add a way to keep track of older uids in the docket

When the revlog content is rewritten, we will use new files, to avoid truncating
the previous ones. We need some way to keep track of the older files before we
clean them up.

Differential Revision: https://phab.mercurial-scm.org/D10866

--- a/mercurial/revlogutils/docket.py	Mon Jun 21 15:00:53 2021 +0200
+++ b/mercurial/revlogutils/docket.py	Thu May 20 21:47:09 2021 +0200
@@ -91,8 +91,11 @@
 #          |   This is mandatory as docket must be compatible with the previous
 #          |   revlog index header.
 # * 1 bytes: size of index uuid
+# * 1 bytes: number of outdated index uuid
 # * 1 bytes: size of data uuid
+# * 1 bytes: number of outdated data uuid
 # * 1 bytes: size of sizedata uuid
+# * 1 bytes: number of outdated data uuid
 # * 8 bytes: size of index-data
 # * 8 bytes: pending size of index-data
 # * 8 bytes: size of data
@@ -100,7 +103,10 @@
 # * 8 bytes: pending size of data
 # * 8 bytes: pending size of sidedata
 # * 1 bytes: default compression header
-S_HEADER = struct.Struct(constants.INDEX_HEADER_FMT + b'BBBLLLLLLc')
+S_HEADER = struct.Struct(constants.INDEX_HEADER_FMT + b'BBBBBBLLLLLLc')
+# * 1 bytes: size of index uuid
+# * 8 bytes: size of file
+S_OLD_UID = struct.Struct('>BL')
 
 
 class RevlogDocket(object):
@@ -112,8 +118,11 @@
         use_pending=False,
         version_header=None,
         index_uuid=None,
+        older_index_uuids=(),
         data_uuid=None,
+        older_data_uuids=(),
         sidedata_uuid=None,
+        older_sidedata_uuids=(),
         index_end=0,
         pending_index_end=0,
         data_end=0,
@@ -129,8 +138,14 @@
         self._path = revlog._docket_file
         self._opener = revlog.opener
         self._index_uuid = index_uuid
+        self._older_index_uuids = older_index_uuids
         self._data_uuid = data_uuid
+        self._older_data_uuids = older_data_uuids
         self._sidedata_uuid = sidedata_uuid
+        self._older_sidedata_uuids = older_sidedata_uuids
+        assert not set(older_index_uuids) & set(older_data_uuids)
+        assert not set(older_data_uuids) & set(older_sidedata_uuids)
+        assert not set(older_index_uuids) & set(older_sidedata_uuids)
         # thes asserts should be True as long as we have a single index filename
         assert index_end <= pending_index_end
         assert data_end <= pending_data_end
@@ -239,8 +254,11 @@
         data = (
             self._version_header,
             len(self._index_uuid),
+            len(self._older_index_uuids),
             len(self._data_uuid),
+            len(self._older_data_uuids),
             len(self._sidedata_uuid),
+            len(self._older_sidedata_uuids),
             official_index_end,
             self._index_end,
             official_data_end,
@@ -251,9 +269,24 @@
         )
         s = []
         s.append(S_HEADER.pack(*data))
+
         s.append(self._index_uuid)
+        for u, size in self._older_index_uuids:
+            s.append(S_OLD_UID.pack(len(u), size))
+        for u, size in self._older_index_uuids:
+            s.append(u)
+
         s.append(self._data_uuid)
+        for u, size in self._older_data_uuids:
+            s.append(S_OLD_UID.pack(len(u), size))
+        for u, size in self._older_data_uuids:
+            s.append(u)
+
         s.append(self._sidedata_uuid)
+        for u, size in self._older_sidedata_uuids:
+            s.append(S_OLD_UID.pack(len(u), size))
+        for u, size in self._older_sidedata_uuids:
+            s.append(u)
         return b''.join(s)
 
 
@@ -272,6 +305,19 @@
     return docket
 
 
+def _parse_old_uids(get_data, count):
+    all_sizes = []
+    all_uids = []
+    for i in range(0, count):
+        raw = get_data(S_OLD_UID.size)
+        all_sizes.append(S_OLD_UID.unpack(raw))
+
+    for uid_size, file_size in all_sizes:
+        uid = get_data(uid_size)
+        all_uids.append((uid, file_size))
+    return all_uids
+
+
 def parse_docket(revlog, data, use_pending=False):
     """given some docket data return a docket object for the given revlog"""
     header = S_HEADER.unpack(data[: S_HEADER.size])
@@ -297,12 +343,21 @@
     index_uuid_size = next(iheader)
     index_uuid = get_data(index_uuid_size)
 
+    older_index_uuid_count = next(iheader)
+    older_index_uuids = _parse_old_uids(get_data, older_index_uuid_count)
+
     data_uuid_size = next(iheader)
     data_uuid = get_data(data_uuid_size)
 
+    older_data_uuid_count = next(iheader)
+    older_data_uuids = _parse_old_uids(get_data, older_data_uuid_count)
+
     sidedata_uuid_size = next(iheader)
     sidedata_uuid = get_data(sidedata_uuid_size)
 
+    older_sidedata_uuid_count = next(iheader)
+    older_sidedata_uuids = _parse_old_uids(get_data, older_sidedata_uuid_count)
+
     index_size = next(iheader)
 
     pending_index_size = next(iheader)
@@ -322,8 +377,11 @@
         use_pending=use_pending,
         version_header=version_header,
         index_uuid=index_uuid,
+        older_index_uuids=older_index_uuids,
         data_uuid=data_uuid,
+        older_data_uuids=older_data_uuids,
         sidedata_uuid=sidedata_uuid,
+        older_sidedata_uuids=older_sidedata_uuids,
         index_end=index_size,
         pending_index_end=pending_index_size,
         data_end=data_size,