--- a/mercurial/merge.py	Wed Nov 18 23:42:32 2015 -0800
+++ b/mercurial/merge.py	Wed Nov 18 15:46:45 2015 -0800
@@ -62,6 +62,8 @@
        (experimental)
     m: the external merge driver defined for this merge plus its run state
        (experimental)
+    X: unsupported mandatory record type (used in tests)
+    x: unsupported advisory record type (used in tests)
 
     Merge driver run states (experimental):
     u: driver-resolved files unmarked -- needs to be run next time we're about
@@ -231,6 +233,13 @@
         `type` is a single character, `length` is a 4 byte integer, and
         `content` is an arbitrary byte sequence of length `length`.
 
+        Mercurial versions prior to 3.7 have a bug where if there are
+        unsupported mandatory merge records, attempting to clear out the merge
+        state with hg update --clean or similar aborts. The 't' record type
+        works around that by writing out what those versions treat as an
+        advisory record, but later versions interpret as special: the first
+        character is the 'real' record type and everything onwards is the data.
+
         Returns list of records [(TYPE, data), ...]."""
         records = []
         try:
@@ -245,6 +254,8 @@
                 off += 4
                 record = data[off:(off + length)]
                 off += length
+                if rtype == 't':
+                    rtype, record = record[0], record[1:]
                 records.append((rtype, record))
             f.close()
         except IOError as err:
@@ -326,10 +337,16 @@
         f.close()
 
     def _writerecordsv2(self, records):
-        """Write current state on disk in a version 2 file"""
+        """Write current state on disk in a version 2 file
+
+        See the docstring for _readrecordsv2 for why we use 't'."""
+        # these are the records that all version 2 clients can read
+        whitelist = 'LOF'
         f = self._repo.vfs(self.statepathv2, 'w')
         for key, data in records:
             assert len(key) == 1
+            if key not in whitelist:
+                key, data = 't', '%s%s' % (key, data)
             format = '>sI%is' % len(data)
             f.write(_pack(format, key, len(data), data))
         f.close()

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/fakemergerecord.py	Wed Nov 18 15:46:45 2015 -0800
@@ -0,0 +1,25 @@
+# Extension to write out fake unsupported records into the merge state
+#
+#
+
+from __future__ import absolute_import
+
+from mercurial import (
+    cmdutil,
+    merge,
+)
+
+cmdtable = {}
+command = cmdutil.command(cmdtable)
+
+@command('fakemergerecord',
+         [('X', 'mandatory', None, 'add a fake mandatory record'),
+          ('x', 'advisory', None, 'add a fake advisory record')], '')
+def fakemergerecord(ui, repo, *pats, **opts):
+    ms = merge.mergestate.read(repo)
+    records = ms._makerecords()
+    if opts.get('mandatory'):
+        records.append(('X', 'mandatory record'))
+    if opts.get('advisory'):
+        records.append(('x', 'advisory record'))
+    ms._writerecords(records)

--- a/tests/test-histedit-non-commute-abort.t	Wed Nov 18 23:42:32 2015 -0800
+++ b/tests/test-histedit-non-commute-abort.t	Wed Nov 18 15:46:45 2015 -0800
@@ -75,10 +75,45 @@
   warning: conflicts while merging e! (edit, then use 'hg resolve --mark')
   Fix up the change and run hg histedit --continue
 
+insert unsupported advisory merge record
+  $ hg --config extensions.fakemergerecord=$TESTDIR/fakemergerecord.py fakemergerecord -x
+  $ hg debugmergestate
+  * version 2 records
+  local: 8f7551c7e4a2f2efe0bc8c741baf7f227d65d758
+  other: e860deea161a2f77de56603b340ebbb4536308ae
+  unrecognized entry: x	advisory record
+  file: e (record type "F", state "u", hash 58e6b3a414a1e090dfc6029add0f3555ccba127f)
+    local path: e (flags "")
+    ancestor path: e (node 0000000000000000000000000000000000000000)
+    other path: e (node 6b67ccefd5ce6de77e7ead4f5292843a0255329f)
+  $ hg resolve -l
+  U e
 
-abort the edit
+insert unsupported mandatory merge record
+  $ hg --config extensions.fakemergerecord=$TESTDIR/fakemergerecord.py fakemergerecord -X
+  $ hg debugmergestate
+  * version 2 records
+  local: 8f7551c7e4a2f2efe0bc8c741baf7f227d65d758
+  other: e860deea161a2f77de56603b340ebbb4536308ae
+  file: e (record type "F", state "u", hash 58e6b3a414a1e090dfc6029add0f3555ccba127f)
+    local path: e (flags "")
+    ancestor path: e (node 0000000000000000000000000000000000000000)
+    other path: e (node 6b67ccefd5ce6de77e7ead4f5292843a0255329f)
+  unrecognized entry: X	mandatory record
+  $ hg resolve -l
+  abort: unsupported merge state records: X
+  (see https://mercurial-scm.org/wiki/MergeStateRecords for more information)
+  [255]
+  $ hg resolve -ma
+  abort: unsupported merge state records: X
+  (see https://mercurial-scm.org/wiki/MergeStateRecords for more information)
+  [255]
+
+abort the edit (should clear out merge state)
   $ hg histedit --abort 2>&1 | fixbundle
   2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg debugmergestate
+  no merge state found
 
 log after abort
   $ hg resolve -l

--- a/tests/test-rebase-abort.t	Wed Nov 18 23:42:32 2015 -0800
+++ b/tests/test-rebase-abort.t	Wed Nov 18 15:46:45 2015 -0800
@@ -68,11 +68,49 @@
   unresolved conflicts (see hg resolve, then hg rebase --continue)
   [1]
 
-Abort:
+Insert unsupported advisory merge record:
+
+  $ hg --config extensions.fakemergerecord=$TESTDIR/fakemergerecord.py fakemergerecord -x
+  $ hg debugmergestate
+  * version 2 records
+  local: 3e046f2ecedb793b97ed32108086edd1a162f8bc
+  other: 46f0b057b5c061d276b91491c22151f78698abd2
+  unrecognized entry: x	advisory record
+  file: common (record type "F", state "u", hash 94c8c21d08740f5da9eaa38d1f175c592692f0d1)
+    local path: common (flags "")
+    ancestor path: common (node de0a666fdd9c1a0b0698b90d85064d8bd34f74b6)
+    other path: common (node 2f6411de53677f6f1048fef5bf888d67a342e0a5)
+  $ hg resolve -l
+  U common
+
+Insert unsupported mandatory merge record:
+
+  $ hg --config extensions.fakemergerecord=$TESTDIR/fakemergerecord.py fakemergerecord -X
+  $ hg debugmergestate
+  * version 2 records
+  local: 3e046f2ecedb793b97ed32108086edd1a162f8bc
+  other: 46f0b057b5c061d276b91491c22151f78698abd2
+  file: common (record type "F", state "u", hash 94c8c21d08740f5da9eaa38d1f175c592692f0d1)
+    local path: common (flags "")
+    ancestor path: common (node de0a666fdd9c1a0b0698b90d85064d8bd34f74b6)
+    other path: common (node 2f6411de53677f6f1048fef5bf888d67a342e0a5)
+  unrecognized entry: X	mandatory record
+  $ hg resolve -l
+  abort: unsupported merge state records: X
+  (see https://mercurial-scm.org/wiki/MergeStateRecords for more information)
+  [255]
+  $ hg resolve -ma
+  abort: unsupported merge state records: X
+  (see https://mercurial-scm.org/wiki/MergeStateRecords for more information)
+  [255]
+
+Abort (should clear out unsupported merge state):
 
   $ hg rebase --abort
   saved backup bundle to $TESTTMP/a/.hg/strip-backup/3e046f2ecedb-6beef7d5-backup.hg (glob)
   rebase aborted
+  $ hg debugmergestate
+  no merge state found
 
   $ hg tglog
   @  4:draft 'L2'

--- a/tests/test-resolve.t	Wed Nov 18 23:42:32 2015 -0800
+++ b/tests/test-resolve.t	Wed Nov 18 15:46:45 2015 -0800
@@ -249,9 +249,69 @@
   $ cat file1
   resolved
 
+insert unsupported advisory merge record
+
+  $ hg --config extensions.fakemergerecord=$TESTDIR/fakemergerecord.py fakemergerecord -x
+  $ hg debugmergestate
+  * version 2 records
+  local: 57653b9f834a4493f7240b0681efcb9ae7cab745
+  other: dc77451844e37f03f5c559e3b8529b2b48d381d1
+  unrecognized entry: x	advisory record
+  file: file1 (record type "F", state "r", hash 60b27f004e454aca81b0480209cce5081ec52390)
+    local path: file1 (flags "")
+    ancestor path: file1 (node 2ed2a3912a0b24502043eae84ee4b279c18b90dd)
+    other path: file1 (node 6f4310b00b9a147241b071a60c28a650827fb03d)
+  file: file2 (record type "F", state "u", hash cb99b709a1978bd205ab9dfd4c5aaa1fc91c7523)
+    local path: file2 (flags "")
+    ancestor path: file2 (node 2ed2a3912a0b24502043eae84ee4b279c18b90dd)
+    other path: file2 (node 6f4310b00b9a147241b071a60c28a650827fb03d)
+  $ hg resolve -l
+  R file1
+  U file2
+
+insert unsupported mandatory merge record
+
+  $ hg --config extensions.fakemergerecord=$TESTDIR/fakemergerecord.py fakemergerecord -X
+  $ hg debugmergestate
+  * version 2 records
+  local: 57653b9f834a4493f7240b0681efcb9ae7cab745
+  other: dc77451844e37f03f5c559e3b8529b2b48d381d1
+  file: file1 (record type "F", state "r", hash 60b27f004e454aca81b0480209cce5081ec52390)
+    local path: file1 (flags "")
+    ancestor path: file1 (node 2ed2a3912a0b24502043eae84ee4b279c18b90dd)
+    other path: file1 (node 6f4310b00b9a147241b071a60c28a650827fb03d)
+  file: file2 (record type "F", state "u", hash cb99b709a1978bd205ab9dfd4c5aaa1fc91c7523)
+    local path: file2 (flags "")
+    ancestor path: file2 (node 2ed2a3912a0b24502043eae84ee4b279c18b90dd)
+    other path: file2 (node 6f4310b00b9a147241b071a60c28a650827fb03d)
+  unrecognized entry: X	mandatory record
+  $ hg resolve -l
+  abort: unsupported merge state records: X
+  (see https://mercurial-scm.org/wiki/MergeStateRecords for more information)
+  [255]
+  $ hg resolve -ma
+  abort: unsupported merge state records: X
+  (see https://mercurial-scm.org/wiki/MergeStateRecords for more information)
+  [255]
+  $ hg summary
+  parent: 2:57653b9f834a 
+   append baz to files
+  parent: 1:dc77451844e3 
+   append bar to files
+  branch: default
+  warning: merge state has unsupported record types: X
+  commit: 2 modified, 2 unknown (merge)
+  update: 2 new changesets (update)
+  phases: 5 draft
+
+update --clean shouldn't abort on unsupported records
+
+  $ hg up -qC 1
+  $ hg debugmergestate
+  no merge state found
+
 test crashed merge with empty mergestate
 
-  $ hg up -qC 1
   $ mkdir .hg/merge
   $ touch .hg/merge/state