sqlitestore: support for storing revisions without their parents

This commit kinda/sorta implements the equivalent of ellipsis nodes for
the SQLite storage backend.

Without implementing full blown ellipsis nodes (and the necessary support for
them in the wire protocol), we instead teach the store to rewrite the p1 and
p2 nodes to nullid when the incoming parent isn't in the local store. This allows
servers to remain dumb and send the real parent and have the clients deal
with the missing parent problem.

This obviously isn't ideal because a benefit of ellipsis nodes is we can
insert a fake parent to ellide missing changesets. But neither solution is
ideal because it drops the original parent from storage. We could probably
teach the SQLite store to retain the original parent and handle missing
parents at read time. However, parent revisions are stored as integers and
it isn't trivial to store an "empty" revision in the store yet, which would
be necessary to represent the "missing" parent.

The store is somewhat intelligent in trying to remove the missing parents
metadata when the revision is re-added. But, revision numbers will be all
messed up in that case, so I'm not sure it is worth it. At some point we'll
likely want to remove the concept of revision numbers from the database and
have the store invent them at index generation time. Or even better, we can
do away with revision numbers from the file storage interface completely.
We'll get there eventually...

Differential Revision: https://phab.mercurial-scm.org/D5168

hgext/sqlitestore.py

@@ -110,6 +110,8 b' COMPRESSION_ZSTD = 2'
 COMPRESSION_ZLIB = 3
 
 FLAG_CENSORED = 1
+FLAG_MISSING_P1 = 2
+FLAG_MISSING_P2 = 4
 
 CREATE_SCHEMA = [
     # Deltas are stored as content-indexed blobs.
@@ -535,6 +537,11 b' class sqlitefilestore(object):'
                                      self._revisioncache, stoprids,
                                      zstddctx=self._dctx)
 
+        # Don't verify hashes if parent nodes were rewritten, as the hash
+        # wouldn't verify.
+        if self._revisions[node].flags & (FLAG_MISSING_P1 | FLAG_MISSING_P2):
+            _verifyhash = False
+
         if _verifyhash:
             self._checkhash(fulltext, node)
             self._revisioncache[node] = fulltext
@@ -618,10 +625,6 b' class sqlitefilestore(object):'
 
     def addgroup(self, deltas, linkmapper, transaction, addrevisioncb=None,
                  maybemissingparents=False):
-        if maybemissingparents:
-            raise error.Abort(_('SQLite storage does not support missing '
-                                'parents write mode'))
-
         nodes = []
 
         for node, p1, p2, linknode, deltabase, delta, wireflags in deltas:
@@ -633,6 +636,15 b' class sqlitefilestore(object):'
             if wireflags & ~repository.REVISION_FLAG_CENSORED:
                 raise SQLiteStoreError('unhandled revision flag')
 
+            if maybemissingparents:
+                if p1 != nullid and not self.hasnode(p1):
+                    p1 = nullid
+                    storeflags |= FLAG_MISSING_P1
+
+                if p2 != nullid and not self.hasnode(p2):
+                    p2 = nullid
+                    storeflags |= FLAG_MISSING_P2
+
             baserev = self.rev(deltabase)
 
             # If base is censored, delta must be full replacement in a single
@@ -657,6 +669,29 b' class sqlitefilestore(object):'
             nodes.append(node)
 
             if node in self._revisions:
+                # Possibly reset parents to make them proper.
+                entry = self._revisions[node]
+
+                if entry.flags & FLAG_MISSING_P1 and p1 != nullid:
+                    entry.p1node = p1
+                    entry.p1rev = self._nodetorev[p1]
+                    entry.flags &= ~FLAG_MISSING_P1
+
+                    self._db.execute(
+                        r'UPDATE fileindex SET p1rev=?, flags=? '
+                        r'WHERE id=?',
+                        (self._nodetorev[p1], entry.flags, entry.rid))
+
+                if entry.flags & FLAG_MISSING_P2 and p2 != nullid:
+                    entry.p2node = p2
+                    entry.p2rev = self._nodetorev[p2]
+                    entry.flags &= ~FLAG_MISSING_P2
+
+                    self._db.execute(
+                        r'UPDATE fileindex SET p2rev=?, flags=? '
+                        r'WHERE id=?',
+                        (self._nodetorev[p1], entry.flags, entry.rid))
+
                 continue
 
             if deltabase == nullid: