--- a/mercurial/changelog.py	Mon May 03 12:35:14 2021 +0200
+++ b/mercurial/changelog.py	Mon May 03 12:35:25 2021 +0200
@@ -395,7 +395,6 @@
         ``concurrencychecker`` will be passed to the revlog init function, see
         the documentation there.
         """
-
         revlog.revlog.__init__(
             self,
             opener,
@@ -484,6 +483,8 @@
     def _writepending(self, tr):
         """create a file containing the unfinalized state for
         pretxnchangegroup"""
+        if self._docket:
+            return self._docket.write(tr, pending=True)
         if self._delaybuf:
             # make a temporary copy of the index
             fp1 = self._realopener(self._indexfile)

--- a/mercurial/configitems.py	Mon May 03 12:35:14 2021 +0200
+++ b/mercurial/configitems.py	Mon May 03 12:35:25 2021 +0200
@@ -1150,7 +1150,6 @@
 )
 # "out of experimental" todo list.
 #
-# * expose transaction content hooks during pre-commit validation
 # * include management of a persistent nodemap in the main docket
 # * enforce a "no-truncate" policy for mmap safety
 #      - for censoring operation

--- a/mercurial/revlog.py	Mon May 03 12:35:14 2021 +0200
+++ b/mercurial/revlog.py	Mon May 03 12:35:25 2021 +0200
@@ -529,7 +529,9 @@
             if self._initempty:
                 self._docket = docketutil.default_docket(self, header)
             else:
-                self._docket = docketutil.parse_docket(self, entry_data)
+                self._docket = docketutil.parse_docket(
+                    self, entry_data, use_pending=self._trypending
+                )
             self._indexfile = self._docket.index_filepath()
             index_data = b''
             index_size = self._docket.index_end

--- a/mercurial/revlogutils/docket.py	Mon May 03 12:35:14 2021 +0200
+++ b/mercurial/revlogutils/docket.py	Mon May 03 12:35:25 2021 +0200
@@ -19,6 +19,10 @@
 
 import struct
 
+from .. import (
+    error,
+)
+
 from . import (
     constants,
 )
@@ -29,19 +33,35 @@
 #          |   This is mandatory as docket must be compatible with the previous
 #          |   revlog index header.
 # * 8 bytes: size of index data
-S_HEADER = struct.Struct(constants.INDEX_HEADER.format + 'L')
+# * 8 bytes: pending size of index data
+S_HEADER = struct.Struct(constants.INDEX_HEADER.format + 'LL')
 
 
 class RevlogDocket(object):
     """metadata associated with revlog"""
 
-    def __init__(self, revlog, version_header=None, index_end=0):
+    def __init__(
+        self,
+        revlog,
+        use_pending=False,
+        version_header=None,
+        index_end=0,
+        pending_index_end=0,
+    ):
         self._version_header = version_header
+        self._read_only = bool(use_pending)
         self._dirty = False
         self._radix = revlog.radix
         self._path = revlog._docket_file
         self._opener = revlog.opener
-        self._index_end = index_end
+        # this assert should be True as long as we have a single index filename
+        assert index_end <= pending_index_end
+        self._initial_index_end = index_end
+        self._pending_index_end = pending_index_end
+        if use_pending:
+            self._index_end = self._pending_index_end
+        else:
+            self._index_end = self._initial_index_end
 
     def index_filepath(self):
         """file path to the current index file associated to this docket"""
@@ -58,22 +78,38 @@
             self._index_end = new_size
             self._dirty = True
 
-    def write(self, transaction, stripping=False):
+    def write(self, transaction, pending=False, stripping=False):
         """write the modification of disk if any
 
         This make the new content visible to all process"""
-        if self._dirty:
+        if not self._dirty:
+            return False
+        else:
+            if self._read_only:
+                msg = b'writing read-only docket: %s'
+                msg %= self._path
+                raise error.ProgrammingError(msg)
             if not stripping:
                 # XXX we could, leverage the docket while stripping. However it
                 # is not powerfull enough at the time of this comment
                 transaction.addbackup(self._path, location=b'store')
             with self._opener(self._path, mode=b'w', atomictemp=True) as f:
-                f.write(self._serialize())
-            self._dirty = False
+                f.write(self._serialize(pending=pending))
+            # if pending we still need to the write final data eventually
+            self._dirty = pending
+            return True
 
-    def _serialize(self):
+    def _serialize(self, pending=False):
+        if pending:
+            official_index_end = self._initial_index_end
+        else:
+            official_index_end = self._index_end
+
+        # this assert should be True as long as we have a single index filename
+        assert official_index_end <= self._index_end
         data = (
             self._version_header,
+            official_index_end,
             self._index_end,
         )
         return S_HEADER.pack(*data)
@@ -88,13 +124,15 @@
     return docket
 
 
-def parse_docket(revlog, data):
+def parse_docket(revlog, data, use_pending=False):
     """given some docket data return a docket object for the given revlog"""
     header = S_HEADER.unpack(data[: S_HEADER.size])
-    version_header, index_size = header
+    version_header, index_size, pending_index_size = header
     docket = RevlogDocket(
         revlog,
+        use_pending=use_pending,
         version_header=version_header,
         index_end=index_size,
+        pending_index_end=pending_index_size,
     )
     return docket

--- a/tests/test-transaction-safety.t	Mon May 03 12:35:14 2021 +0200
+++ b/tests/test-transaction-safety.t	Mon May 03 12:35:25 2021 +0200
@@ -46,13 +46,13 @@
   $ cat << EOF > script/external.sh
   > #!/bin/sh
   > $RUNTESTDIR/testlib/wait-on-file 5 $HG_TEST_FILE_EXT_UNLOCK $HG_TEST_FILE_EXT_WAITING
-  > hg log --rev 'tip' -T 'external: {rev} {desc}\n' > $TESTTMP/output/external.out 2>/dev/null
+  > hg log --rev 'tip' -T 'external: {rev} {desc}\n' > $TESTTMP/output/external.out
   > touch $HG_TEST_FILE_EXT_DONE
   > EOF
   $ chmod +x script/external.sh
   $ cat << EOF > script/internal.sh
   > #!/bin/sh
-  > hg log --rev 'tip' -T 'internal: {rev} {desc}\n' > $TESTTMP/output/internal.out 2>/dev/null
+  > hg log --rev 'tip' -T 'internal: {rev} {desc}\n' > $TESTTMP/output/internal.out
   > $RUNTESTDIR/testlib/wait-on-file 5 $HG_TEST_FILE_EXT_DONE $HG_TEST_FILE_EXT_UNLOCK
   > EOF
   $ chmod +x script/internal.sh
@@ -124,8 +124,7 @@
   $ make_one_commit first
   pre-commit: -1 
   external: -1 
-  internal: 0 first (revlogv1 !)
-  internal: -1  (revlogv2 known-bad-output !)
+  internal: 0 first
   post-tr:  0 first
 
 #if revlogv1
@@ -150,8 +149,7 @@
   $ make_one_commit second
   pre-commit: 0 first
   external: 0 first
-  internal: 1 second (revlogv1 !)
-  internal: 0 first (revlogv2 known-bad-output !)
+  internal: 1 second
   post-tr:  1 second
 
 #if revlogv1
@@ -177,8 +175,7 @@
   pre-commit: 1 second
   warning: repository is unrelated
   external: 1 second
-  internal: 5 r3 (revlogv1 !)
-  internal: 1 second (revlogv2 known-bad-output !)
+  internal: 5 r3
   post-tr:  5 r3
 
 #if revlogv1
@@ -203,8 +200,7 @@
   $ make_one_pull 400
   pre-commit: 5 r3
   external: 5 r3
-  internal: 402 r400 (revlogv1 !)
-  internal: 5 r3 (revlogv2 known-bad-output !)
+  internal: 402 r400
   post-tr:  402 r400
 
 #if revlogv1
@@ -229,8 +225,7 @@
   $ make_one_commit third
   pre-commit: 402 r400
   external: 402 r400
-  internal: 403 third (revlogv1 !)
-  internal: 402 r400 (revlogv2 known-bad-output !)
+  internal: 403 third
   post-tr:  403 third
 
 #if revlogv1
@@ -256,8 +251,7 @@
   $ make_one_pull tip
   pre-commit: 403 third
   external: 403 third
-  internal: 503 r500 (revlogv1 !)
-  internal: 403 third (revlogv2 known-bad-output !)
+  internal: 503 r500
   post-tr:  503 r500
 
 #if revlogv1