storage: introduce a `revlog.reuse-external-delta-parent` config

As pointed in c59987ab96b4, we had no simple way to get a client to not blindly
reuse the delta parent from a bundle. Instead one had to rely on a side effect
of the deprecated `format.generaldelta` configuration.

We introduce an explicit `revlog.reuse-external-delta-parent` configuration
option (default to True) to control this behavior. If the option is not set,
`format.generaldelta` still control this behavior.

To test the new option, we convert a couple of place where `generaldelta` have
been used for its side effects.

mercurial/configitems.py

@@ -983,6 +983,9 coreconfigitem('storage', 'revlog.optimi
     default=True,
     alias=[('format', 'aggressivemergedeltas')],
 )
+coreconfigitem('storage', 'revlog.reuse-external-delta-parent',
+    default=None,
+)
 coreconfigitem('server', 'bookmarks-pushkey-compat',
     default=True,
 )

mercurial/help/config.txt

@@ -1843,6 +1843,28 category impact performance and reposito
     Turning this option off can result in large increase of repository size for
     repository with many merges.
 
+``revlog.reuse-external-delta-parent``
+    Control the order in which delta parents are considered when adding new
+    revisions from an external source.
+    (typically: apply bundle from `hg pull` or `hg push`).
+
+    New revisions are usually provided as a delta against other revisions. By
+    default, Mercurial will try to reuse this delta first, therefore using the
+    same "delta parent" as the source. Directly using delta's from the source
+    reduces CPU usage and usually speeds up operation. However, in some case,
+    the source might have sub-optimal delta bases and forcing their reevaluation
+    is useful. For example, pushes from an old client could have sub-optimal
+    delta's parent that the server want to optimize. (lack of general delta, bad
+    parents, choice, lack of sparse-revlog, etc).
+
+    This option is enabled by default. Turning it off will ensure bad delta
+    parent choices from older client do not propagate to this repository, at
+    the cost of a small increase in CPU consumption.
+
+    Note: this option only control the order in which delta parents are
+    considered.  Even when disabled, the existing delta from the source will be
+    reused if the same delta parent is selected.
+
 ``server``
 ----------
 

mercurial/localrepo.py

@@ -752,7 +752,11 def resolverevlogstorevfsoptions(ui, req
                                      b'revlog.optimize-delta-parent-choice')
     options[b'deltabothparents'] = deltabothparents
 
-    options[b'lazydeltabase'] = not scmutil.gddeltaconfig(ui)
+    lazydeltabase = ui.configbool(b'storage',
+                                  b'revlog.reuse-external-delta-parent')
+    if lazydeltabase is None:
+        lazydeltabase = not scmutil.gddeltaconfig(ui)
+    options[b'lazydeltabase'] = lazydeltabase
 
     chainspan = ui.configbytes(b'experimental', b'maxdeltachainspan')
     if 0 <= chainspan:

tests/test-generaldelta.t

@@ -339,7 +339,7 test maxdeltachainspan
        52       5        1       -1    base        369        640        369   0.57656       369         0    0.00000
        53       6        1       -1    base          0          0          0   0.00000         0         0    0.00000
        54       7        1       -1    base        369        640        369   0.57656       369         0    0.00000
-  $ hg clone --pull source-repo --config experimental.maxdeltachainspan=0 noconst-chain --config format.generaldelta=yes
+  $ hg clone --pull source-repo --config experimental.maxdeltachainspan=0 noconst-chain --config format.usegeneraldelta=yes --config storage.revlog.reuse-external-delta-parent=no
   requesting all changes
   adding changesets
   adding manifests

tests/test-sparse-revlog.t

@@ -40,8 +40,7 repeatedly while some of it changes rare
   > maxchainlen = 15
   > [storage]
   > revlog.optimize-delta-parent-choice = yes
-  > [format]
-  > generaldelta = yes
+  > revlog.reuse-external-delta-parent = no
   > EOF
   $ hg init sparse-repo
   $ cd sparse-repo