delta-find: add a delta-reuse policy that blindly accepts incoming deltas

When this policy is set, incoming deltas are blindly accepted without regard
for the validity of the chain they build.

mercurial/helptext/config.txt

@@ -1948,6 +1948,13 b' The following sub-options can be defined'
     unbundling process, but can result in sub-optimal storage space if the
     remote peer is sending poor quality deltas.
 
+  - ``forced``: the deltas from the peer will be reused in all cases, even if
+    the resulting delta-chain is "invalid". This setting will ensure the bundle
+    is applied at minimal CPU cost, but it can result in longer delta chains
+    being created on the client, making revisions potentially slower to access
+    in the future. If you think you need this option, you should make sure you
+    are also talking to the Mercurial developer community to get confirmation.
+
   See `hg help config.storage.revlog.reuse-external-delta-parent` for a similar
   global option. That option defines the behavior of `default`.
 

mercurial/revlogutils/constants.py

@@ -315,3 +315,4 b' DELTA_BASE_REUSE_NO = 0'
 # The delta base will be tested for validy first. So that the cached deltas get
 # used when possible.
 DELTA_BASE_REUSE_TRY = 1
+DELTA_BASE_REUSE_FORCE = 2

mercurial/revlogutils/deltas.py

@@ -20,6 +20,7 b' from .constants import ('
     COMP_MODE_DEFAULT,
     COMP_MODE_INLINE,
     COMP_MODE_PLAIN,
+    DELTA_BASE_REUSE_FORCE,
     DELTA_BASE_REUSE_NO,
     KIND_CHANGELOG,
     KIND_FILELOG,
@@ -584,6 +585,13 b' def is_good_delta_info(revlog, deltainfo'
     if deltainfo is None:
         return False
 
+    if (
+        revinfo.cachedelta is not None
+        and deltainfo.base == revinfo.cachedelta[0]
+        and revinfo.cachedelta[2] == DELTA_BASE_REUSE_FORCE
+    ):
+        return True
+
     # - 'deltainfo.distance' is the distance from the base revision --
     #   bounding it limits the amount of I/O we need to do.
     # - 'deltainfo.compresseddeltalen' is the sum of the total size of
@@ -711,6 +719,16 b' def _candidategroups('
             # filter out revision we tested already
             if rev in tested:
                 continue
+
+            if (
+                cachedelta is not None
+                and rev == cachedelta[0]
+                and cachedelta[2] == DELTA_BASE_REUSE_FORCE
+            ):
+                # instructions are to forcibly consider/use this delta base
+                group.append(rev)
+                continue
+
             # an higher authority deamed the base unworthy (e.g. censored)
             if excluded_bases is not None and rev in excluded_bases:
                 tested.add(rev)

mercurial/utils/urlutil.py

@@ -775,6 +775,7 b' DELTA_REUSE_POLICIES = {'
     b'default': None,
     b'try-base': revlog_constants.DELTA_BASE_REUSE_TRY,
     b'no-reuse': revlog_constants.DELTA_BASE_REUSE_NO,
+    b'forced': revlog_constants.DELTA_BASE_REUSE_FORCE,
 }
 
 

tests/test-revlog-delta-find.t

@@ -260,3 +260,57 b' We requested to use the (bad) delta'
   DBG-DELTAS: CHANGELOG: * (glob)
   DBG-DELTAS: MANIFESTLOG: * (glob)
   DBG-DELTAS: FILELOG:my-file.txt: rev=3: delta-base=2 * (glob)
+
+Case where we force a "bad" delta to be applied
+===============================================
+
+We build a very different file content to force a full snapshot
+
+  $ cp -ar peer-bad-delta peer-bad-delta-with-full
+  $ cp -ar local-pre-pull local-pre-pull-full
+  $ echo '[paths]' >> local-pre-pull-full/.hg/hgrc
+  $ echo 'default=../peer-bad-delta-with-full' >> local-pre-pull-full/.hg/hgrc
+
+  $ hg -R peer-bad-delta-with-full update 'desc("merge")' --quiet
+  $ ($TESTDIR/seq.py 2000 2100; $TESTDIR/seq.py 500 510; $TESTDIR/seq.py 3000 3050) \
+  > | $PYTHON $TESTTMP/sha256line.py > peer-bad-delta-with-full/my-file.txt
+  $ hg -R peer-bad-delta-with-full commit -m 'trigger-full'
+  DBG-DELTAS: FILELOG:my-file.txt: rev=4: delta-base=4 * (glob)
+  DBG-DELTAS: MANIFESTLOG: * (glob)
+  DBG-DELTAS: CHANGELOG: * (glob)
+
+Check that "try-base" behavior challenge the delta
+--------------------------------------------------
+
+The bundling process creates a delta against the previous revision, however this
+is an invalid chain for the client, so it is not considered and we do a full
+snapshot again.
+
+  $ cp -ar local-pre-pull-full local-try-base-full
+  $ hg -R local-try-base-full pull --quiet \
+  > --config 'paths.default:delta-reuse-policy=try-base'
+  DBG-DELTAS: CHANGELOG: * (glob)
+  DBG-DELTAS: CHANGELOG: * (glob)
+  DBG-DELTAS: MANIFESTLOG: * (glob)
+  DBG-DELTAS: MANIFESTLOG: * (glob)
+  DBG-DELTAS: FILELOG:my-file.txt: rev=3: delta-base=2 * (glob)
+  DBG-DELTAS: FILELOG:my-file.txt: rev=4: delta-base=4 * (glob)
+
+Check that "forced" behavior do not challenge the delta, even if it is bad.
+---------------------------------------------------------------------------
+
+The client does not challenge anything and applies the bizarre delta directly.
+
+Note: If the bundling process becomes smarter, this test might no longer work
+(as the server won't be sending "bad" deltas anymore) and might need something
+more subtle to test this behavior.
+
+  $ cp -ar local-pre-pull-full local-forced-full
+  $ hg -R local-forced-full pull --quiet \
+  > --config 'paths.default:delta-reuse-policy=forced'
+  DBG-DELTAS: CHANGELOG: * (glob)
+  DBG-DELTAS: CHANGELOG: * (glob)
+  DBG-DELTAS: MANIFESTLOG: * (glob)
+  DBG-DELTAS: MANIFESTLOG: * (glob)
+  DBG-DELTAS: FILELOG:my-file.txt: rev=3: delta-base=2 * (glob)
+  DBG-DELTAS: FILELOG:my-file.txt: rev=4: delta-base=3 * (glob)