revlog: overwrite revlog config through copy of the config object

The new objects allow for this kind of blanket approach that make things
cleaner.

If we have more cases, it would probably deserve a context manager, but since we
only have two usage, I don't think it is worth it.

mercurial/revlog.py

@@ -241,8 +241,13 b' FILE_TOO_SHORT_MSG = _('
 hexdigits = b'0123456789abcdefABCDEF'
 
 
+class _Config:
+    def copy(self):
+        return self.__class__(**self.__dict__)
+
+
 @attr.s()
-class FeatureConfig:
+class FeatureConfig(_Config):
     """Hold configuration values about the available revlog features"""
 
     # the default compression engine
@@ -265,9 +270,14 b' class FeatureConfig:'
     # can ellipsis commit be used
     enable_ellipsis = attr.ib(default=False)
 
+    def copy(self):
+        new = super().copy()
+        new.compression_engine_options = self.compression_engine_options.copy()
+        return new
+
 
 @attr.s()
-class DataConfig:
+class DataConfig(_Config):
     """Hold configuration value about how the revlog data are read"""
 
     # should we try to open the "pending" version of the revlog
@@ -297,7 +307,7 b' class DataConfig:'
 
 
 @attr.s()
-class DeltaConfig:
+class DeltaConfig(_Config):
     """Hold configuration value about how new delta are computed
 
     Some attributes are duplicated from DataConfig to help havign each object
@@ -3375,9 +3385,8 b' class revlog:'
 
         # lazydelta and lazydeltabase controls whether to reuse a cached delta,
         # if possible.
-        oldlazydelta = destrevlog._lazydelta
-        oldlazydeltabase = destrevlog._lazydeltabase
-        oldamd = destrevlog._deltabothparents
+        old_delta_config = destrevlog.delta_config
+        destrevlog.delta_config = destrevlog.delta_config.copy()
 
         try:
             if deltareuse == self.DELTAREUSEALWAYS:
@@ -3390,7 +3399,9 b' class revlog:'
                 destrevlog.delta_config.lazy_delta_base = False
                 destrevlog.delta_config.lazy_delta = False
 
-            delta_both_parents = forcedeltabothparents or oldamd
+            delta_both_parents = (
+                forcedeltabothparents or old_delta_config.delta_both_parents
+            )
             destrevlog.delta_config.delta_both_parents = delta_both_parents
 
             with self.reading():
@@ -3404,9 +3415,7 b' class revlog:'
                 )
 
         finally:
-            destrevlog.delta_config.lazy_delta = oldlazydelta
-            destrevlog.delta_config.lazy_delta_base = oldlazydeltabase
-            destrevlog.delta_config.delta_both_parents = oldamd
+            destrevlog.delta_config = old_delta_config
 
     def _clone(
         self,

tests/test-revlog-raw.py

@@ -371,7 +371,10 b' slicingdata = ['
 
 
 def slicingtest(rlog):
-    oldmin = rlog._srmingapsize
+    old_delta_config = rlog.delta_config
+    old_data_config = rlog.data_config
+    rlog.delta_config = rlog.delta_config.copy()
+    rlog.data_config = rlog.data_config.copy()
     try:
         # the test revlog is small, we remove the floor under which we
         # slicing is diregarded.
@@ -388,8 +391,8 b' def slicingtest(rlog):'
                 print('  expected: %s' % expected)
                 print('  result:   %s' % result)
     finally:
-        rlog.data_config.sr_min_gap_size = oldmin
-        rlog.delta_config.sr_min_gap_size = oldmin
+        rlog.delta_config = old_delta_config
+        rlog.data_config = old_data_config
 
 
 def md5sum(s):