--- a/mercurial/bundle2.py	Thu Dec 17 09:35:07 2020 -0800
+++ b/mercurial/bundle2.py	Fri Apr 24 16:36:04 2020 +0200
@@ -1739,7 +1739,11 @@
 
     if opts.get(b'obsolescence', False):
         obsmarkers = repo.obsstore.relevantmarkers(outgoing.missing)
-        buildobsmarkerspart(bundler, obsmarkers)
+        buildobsmarkerspart(
+            bundler,
+            obsmarkers,
+            mandatory=opts.get(b'obsolescence-mandatory', True),
+        )
 
     if opts.get(b'phases', False):
         headsbyphase = phases.subsetphaseheads(repo, outgoing.missing)
@@ -1862,7 +1866,7 @@
     part.addparam(b'requirements', requirements, mandatory=True)
 
 
-def buildobsmarkerspart(bundler, markers):
+def buildobsmarkerspart(bundler, markers, mandatory=True):
     """add an obsmarker part to the bundler with <markers>
 
     No part is created if markers is empty.
@@ -1876,7 +1880,7 @@
     if version is None:
         raise ValueError(b'bundler does not support common obsmarker format')
     stream = obsolete.encodemarkers(markers, True, version=version)
-    return bundler.newpart(b'obsmarkers', data=stream)
+    return bundler.newpart(b'obsmarkers', data=stream, mandatory=mandatory)
 
 
 def writebundle(

--- a/mercurial/commands.py	Thu Dec 17 09:35:07 2020 -0800
+++ b/mercurial/commands.py	Fri Apr 24 16:36:04 2020 +0200
@@ -1648,12 +1648,17 @@
     if complevel is not None:
         compopts[b'level'] = complevel
 
-    # Allow overriding the bundling of obsmarker in phases through
-    # configuration while we don't have a bundle version that include them
-    if repo.ui.configbool(b'experimental', b'evolution.bundle-obsmarker'):
-        bundlespec.contentopts[b'obsolescence'] = True
-    if repo.ui.configbool(b'experimental', b'bundle-phases'):
-        bundlespec.contentopts[b'phases'] = True
+    # Bundling of obsmarker and phases is optional as not all clients
+    # support the necessary features.
+    cfg = ui.configbool
+    contentopts = {
+        b'obsolescence': cfg(b'experimental', b'evolution.bundle-obsmarker'),
+        b'obsolescence-mandatory': cfg(
+            b'experimental', b'evolution.bundle-obsmarker:mandatory'
+        ),
+        b'phases': cfg(b'experimental', b'bundle-phases'),
+    }
+    bundlespec.contentopts.update(contentopts)
 
     bundle2.writenewbundle(
         ui,

--- a/mercurial/configitems.py	Thu Dec 17 09:35:07 2020 -0800
+++ b/mercurial/configitems.py	Fri Apr 24 16:36:04 2020 +0200
@@ -900,6 +900,11 @@
 )
 coreconfigitem(
     b'experimental',
+    b'evolution.bundle-obsmarker:mandatory',
+    default=True,
+)
+coreconfigitem(
+    b'experimental',
     b'log.topo',
     default=False,
 )

--- a/tests/test-obsolete-bundle-strip.t	Thu Dec 17 09:35:07 2020 -0800
+++ b/tests/test-obsolete-bundle-strip.t	Fri Apr 24 16:36:04 2020 +0200
@@ -1444,3 +1444,35 @@
   # unbundling: new changesets 9ac430e15fca (1 drafts)
   # unbundling: (1 other changesets obsolete on arrival)
   # unbundling: (run 'hg update' to get a working copy)
+
+Test that advisory obsolescence markers in bundles are ignored if unsupported
+
+  $ hg init repo-with-obs
+  $ cd repo-with-obs
+  $ hg debugbuilddag +1
+  $ hg debugobsolete `getid 0`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  $ hg bundle --config experimental.evolution.bundle-obsmarker=true --config experimental.evolution.bundle-obsmarker:mandatory=false --all --hidden bundle-with-obs
+  1 changesets found
+  $ cd ..
+  $ hg init repo-without-obs
+  $ cd repo-without-obs
+  $ hg --config experimental.evolution=False unbundle ../repo-with-obs/bundle-with-obs --debug
+  bundle2-input-bundle: 1 params with-transaction
+  bundle2-input-part: "changegroup" (params: 1 mandatory 1 advisory) supported
+  adding changesets
+  add changeset 1ea73414a91b
+  adding manifests
+  adding file changes
+  bundle2-input-part: total payload size 190
+  bundle2-input-part: "cache:rev-branch-cache" (advisory) supported
+  bundle2-input-part: total payload size 39
+  bundle2-input-part: "obsmarkers" (advisory) supported
+  bundle2-input-part: total payload size 50
+  ignoring obsolescence markers, feature not enabled
+  bundle2-input-bundle: 3 parts total
+  updating the branch cache
+  added 1 changesets with 0 changes to 0 files
+  new changesets 1ea73414a91b (1 drafts)
+  (run 'hg update' to get a working copy)

--- a/tests/test-obsolete-distributed.t	Thu Dec 17 09:35:07 2020 -0800
+++ b/tests/test-obsolete-distributed.t	Fri Apr 24 16:36:04 2020 +0200
@@ -138,14 +138,42 @@
 
   $ hg up 'desc("ROOT")'
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
-  $ hg pull --confirm --config ui.interactive=True << EOF
+  $ hg pull --debug --confirm --config ui.interactive=True << EOF
   > n
   > EOF
   pulling from $TESTTMP/distributed-chain-building/server
+  query 1; heads
   searching for changes
+  taking quick initial sample
+  query 2; still undecided: 1, sample size is: 1
+  2 total queries in *.*s (glob)
+  1 changesets found
+  list of changesets:
+  391a2bf12b1b8b05a72400ae36b26d50a091dc22
+  listing keys for "bookmarks"
+  bundle2-output-bundle: "HG20", 5 parts total
+  bundle2-output-part: "changegroup" (params: 1 mandatory 1 advisory) streamed payload
+  bundle2-output-part: "listkeys" (params: 1 mandatory) empty payload
+  bundle2-output-part: "obsmarkers" streamed payload
+  bundle2-output-part: "phase-heads" 48 bytes payload
+  bundle2-output-part: "cache:rev-branch-cache" (advisory) streamed payload
+  bundle2-input-bundle: with-transaction
+  bundle2-input-part: "changegroup" (params: 1 mandatory 1 advisory) supported
   adding changesets
+  add changeset 391a2bf12b1b
   adding manifests
   adding file changes
+  adding c_B1 revisions
+  bundle2-input-part: total payload size 485
+  bundle2-input-part: "listkeys" (params: 1 mandatory) supported
+  bundle2-input-part: "obsmarkers" supported
+  bundle2-input-part: total payload size 143
+  bundle2-input-part: "phase-heads" supported
+  bundle2-input-part: total payload size 48
+  bundle2-input-part: "cache:rev-branch-cache" (advisory) supported
+  bundle2-input-part: total payload size 39
+  bundle2-input-bundle: 5 parts total
+  checking for updated bookmarks
   adding 1 changesets with 1 changes to 1 files (+1 heads)
   1 new obsolescence markers
   obsoleting 1 changesets

--- a/tests/test-obsolete.t	Thu Dec 17 09:35:07 2020 -0800
+++ b/tests/test-obsolete.t	Fri Apr 24 16:36:04 2020 +0200
@@ -1682,6 +1682,24 @@
   |
   @  0:a78f55e5508c (draft) [ ] 0
   
+
+
+Test that bundles can ship the markers without making them mandatory
+for non-obsmarker enabled clients:
+
+  $ hg --config experimental.evolution.bundle-obsmarker=1 --config experimental.evolution.bundle-obsmarker:mandatory=0 bundle --base 0 -r 1:: obslog-bundle.hg
+  2 changesets found
+  $ hg debugbundle obslog-bundle.hg
+  Stream params: {Compression: BZ}
+  changegroup -- {nbchanges: 2, version: 02} (mandatory: True)
+      e016b03fd86fcccc54817d120b90b751aaf367d6
+      b0551702f918510f01ae838ab03a463054c67b46
+  cache:rev-branch-cache -- {} (mandatory: False)
+  obsmarkers -- {} (mandatory: False)
+      version: 1 (92 bytes)
+      e008cf2834908e5d6b0f792a9d4b0e2272260fb8 b0551702f918510f01ae838ab03a463054c67b46 0 (Thu Jan 01 00:00:00 1970 +0000) {'ef1': '8', 'operation': 'amend', 'user': 'test'}
+
+
 Test that 'hg debugobsolete --index --rev' can show indices of obsmarkers when
 only a subset of those are displayed (because of --rev option)
   $ hg init doindexrev