censor: extract the part about recomputing delta in a function

censor: extract the part about recomputing delta in a function

The v2_censor function is huge, now that its content has settled a bit it is a
good time to split individual part inside dedicated function.

It is now the turn of the logic that recompute new delta to replace the one
based on the revision that are going away.

Differential Revision: https://phab.mercurial-scm.org/D10900

--- a/mercurial/revlogutils/rewrite.py	Tue Jun 22 21:49:31 2021 +0200
+++ b/mercurial/revlogutils/rewrite.py	Tue Jun 22 22:10:22 2021 +0200
@@ -154,45 +154,14 @@
     data_cutoff = censored_entry[ENTRY_DATA_OFFSET] >> 16
     sidedata_cutoff = rl.sidedata_cut_off(censor_rev)
 
-    # rev → (new_base, data_start, data_end)
-    rewritten_entries = {}
-
-    dc = deltas.deltacomputer(rl)
-    excl = [censor_rev]
-
     with pycompat.unnamedtempfile(mode=b"w+b") as tmp_storage:
-        with rl._segmentfile._open_read() as dfh:
-            for rev in range(censor_rev + 1, len(old_index)):
-                entry = old_index[rev]
-                if censor_rev != entry[ENTRY_DELTA_BASE]:
-                    continue
-                # This is a revision that use the censored revision as the base
-                # for its delta. We need a need new deltas
-                if entry[ENTRY_DATA_UNCOMPRESSED_LENGTH] == 0:
-                    # this revision is empty, we can delta against nullrev
-                    rewritten_entries[rev] = (nullrev, 0, 0)
-                else:
-
-                    text = rl.rawdata(rev, _df=dfh)
-                    info = revlogutils.revisioninfo(
-                        node=entry[ENTRY_NODE_ID],
-                        p1=rl.node(entry[ENTRY_PARENT_1]),
-                        p2=rl.node(entry[ENTRY_PARENT_2]),
-                        btext=[text],
-                        textlen=len(text),
-                        cachedelta=None,
-                        flags=entry[ENTRY_DATA_OFFSET] & 0xFFFF,
-                    )
-                    d = dc.finddeltainfo(
-                        info, dfh, excluded_bases=excl, target_rev=rev
-                    )
-                    default_comp = rl._docket.default_compression_header
-                    comp_mode, d = deltas.delta_compression(default_comp, d)
-                    # using `tell` is a bit lazy, but we are not here for speed
-                    start = tmp_storage.tell()
-                    tmp_storage.write(d.data[1])
-                    end = tmp_storage.tell()
-                    rewritten_entries[rev] = (d.base, start, end, comp_mode)
+        # rev → (new_base, data_start, data_end, compression_mode)
+        rewritten_entries = _precompute_rewritten_delta(
+            rl,
+            old_index,
+            {censor_rev},
+            tmp_storage,
+        )
 
         old_index_filepath = rl.opener.join(docket.index_filepath())
         old_data_filepath = rl.opener.join(docket.data_filepath())
@@ -283,6 +252,59 @@
     docket.write(transaction=None, stripping=True)
 
 
+def _precompute_rewritten_delta(
+    revlog,
+    old_index,
+    excluded_revs,
+    tmp_storage,
+):
+    """Compute new delta for revisions whose delta is based on revision that
+    will not survive as is.
+
+    Return a mapping: {rev → (new_base, data_start, data_end, compression_mode)}
+    """
+    dc = deltas.deltacomputer(revlog)
+    rewritten_entries = {}
+    first_excl_rev = min(excluded_revs)
+    with revlog._segmentfile._open_read() as dfh:
+        for rev in range(first_excl_rev, len(old_index)):
+            if rev in excluded_revs:
+                # this revision will be preserved as is, so we don't need to
+                # consider recomputing a delta.
+                continue
+            entry = old_index[rev]
+            if entry[ENTRY_DELTA_BASE] not in excluded_revs:
+                continue
+            # This is a revision that use the censored revision as the base
+            # for its delta. We need a need new deltas
+            if entry[ENTRY_DATA_UNCOMPRESSED_LENGTH] == 0:
+                # this revision is empty, we can delta against nullrev
+                rewritten_entries[rev] = (nullrev, 0, 0, COMP_MODE_PLAIN)
+            else:
+
+                text = revlog.rawdata(rev, _df=dfh)
+                info = revlogutils.revisioninfo(
+                    node=entry[ENTRY_NODE_ID],
+                    p1=revlog.node(entry[ENTRY_PARENT_1]),
+                    p2=revlog.node(entry[ENTRY_PARENT_2]),
+                    btext=[text],
+                    textlen=len(text),
+                    cachedelta=None,
+                    flags=entry[ENTRY_DATA_OFFSET] & 0xFFFF,
+                )
+                d = dc.finddeltainfo(
+                    info, dfh, excluded_bases=excluded_revs, target_rev=rev
+                )
+                default_comp = revlog._docket.default_compression_header
+                comp_mode, d = deltas.delta_compression(default_comp, d)
+                # using `tell` is a bit lazy, but we are not here for speed
+                start = tmp_storage.tell()
+                tmp_storage.write(d.data[1])
+                end = tmp_storage.tell()
+                rewritten_entries[rev] = (d.base, start, end, comp_mode)
+    return rewritten_entries
+
+
 def _rewrite_simple(
     revlog,
     old_index,