obsolete: experimental flag to get debug about obsmarkers exchange

The obsolescence markers exchange is still experimental. We (developer) need
more information about what is going on. I'm adding an experimental flag to add
display the amount of data exchanged during bundle2 exchanges.

mercurial/bundle2.py

@@ -1217,7 +1217,11 b' def handlepushkeyreply(op, inpart):'
 def handleobsmarker(op, inpart):
     """add a stream of obsmarkers to the repo"""
     tr = op.gettransaction()
-    new = op.repo.obsstore.mergemarkers(tr, inpart.read())
+    markerdata = inpart.read()
+    if op.ui.config('experimental', 'obsmarkers-exchange-debug', False):
+        op.ui.write(('obsmarker-exchange: %i bytes received\n')
+                    % len(markerdata))
+    new = op.repo.obsstore.mergemarkers(tr, markerdata)
     if new:
         op.repo.ui.status(_('%i new obsolescence markers\n') % new)
     op.records.add('obsmarkers', {'new': new})

tests/test-obsolete.t

@@ -616,6 +616,15 b' List of both'
 
 #if serve
 
+Test the debug output for exchange
+----------------------------------
+
+  $ hg pull ../tmpb --config 'experimental.obsmarkers-exchange-debug=True' --config 'experimental.bundle2-exp=True'
+  pulling from ../tmpb
+  searching for changes
+  no changes found
+  obsmarker-exchange: 346 bytes received
+
 check hgweb does not explode
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