diff --git a/mercurial/exchange.py b/mercurial/exchange.py
--- a/mercurial/exchange.py
+++ b/mercurial/exchange.py
@@ -1441,7 +1441,8 @@ def unbundle(repo, cg, heads, source, ur
     If the push was raced as PushRaced exception is raised."""
     r = 0
     # need a transaction when processing a bundle2 stream
-    wlock = lock = tr = None
+    # [wlock, lock, tr] - needs to be an array so nested functions can modify it
+    lockandtr = [None, None, None]
     recordout = None
     # quick fix for output mismatch with bundle2 in 3.4
     captureoutput = repo.ui.configbool('experimental', 'bundle2-output-capture',
@@ -1454,13 +1455,22 @@ def unbundle(repo, cg, heads, source, ur
         if util.safehasattr(cg, 'params'):
             r = None
             try:
-                wlock = repo.wlock()
-                lock = repo.lock()
-                tr = repo.transaction(source)
-                tr.hookargs['source'] = source
-                tr.hookargs['url'] = url
-                tr.hookargs['bundle2'] = '1'
-                op = bundle2.bundleoperation(repo, lambda: tr,
+                def gettransaction():
+                    if not lockandtr[2]:
+                        lockandtr[0] = repo.wlock()
+                        lockandtr[1] = repo.lock()
+                        lockandtr[2] = repo.transaction(source)
+                        lockandtr[2].hookargs['source'] = source
+                        lockandtr[2].hookargs['url'] = url
+                        lockandtr[2].hookargs['bundle2'] = '1'
+                    return lockandtr[2]
+
+                # Do greedy locking by default until we're satisfied with lazy
+                # locking.
+                if not repo.ui.configbool('experimental', 'bundle2lazylocking'):
+                    gettransaction()
+
+                op = bundle2.bundleoperation(repo, gettransaction,
                                              captureoutput=captureoutput)
                 try:
                     op = bundle2.processbundle(repo, cg, op=op)
@@ -1470,7 +1480,8 @@ def unbundle(repo, cg, heads, source, ur
                         repo.ui.pushbuffer(error=True, subproc=True)
                         def recordout(output):
                             r.newpart('output', data=output, mandatory=False)
-                tr.close()
+                if lockandtr[2] is not None:
+                    lockandtr[2].close()
             except BaseException as exc:
                 exc.duringunbundle2 = True
                 if captureoutput and r is not None:
@@ -1481,10 +1492,10 @@ def unbundle(repo, cg, heads, source, ur
                         parts.append(part)
                 raise
         else:
-            lock = repo.lock()
+            lockandtr[1] = repo.lock()
             r = changegroup.addchangegroup(repo, cg, source, url)
     finally:
-        lockmod.release(tr, lock, wlock)
+        lockmod.release(lockandtr[2], lockandtr[1], lockandtr[0])
         if recordout is not None:
             recordout(repo.ui.popbuffer())
     return r
diff --git a/tests/test-bundle2-exchange.t b/tests/test-bundle2-exchange.t
--- a/tests/test-bundle2-exchange.t
+++ b/tests/test-bundle2-exchange.t
@@ -7,6 +7,7 @@ Test exchange of common information usin
 
 enable obsolescence
 
+  $ cp $HGRCPATH $TESTTMP/hgrc.orig
   $ cat > $TESTTMP/bundle2-pushkey-hook.sh << EOF
   > echo pushkey: lock state after \"\$HG_NAMESPACE\"
   > hg debuglock
@@ -897,3 +898,47 @@ Check abort from mandatory pushkey
   abort: Clown phase push failed
   [255]
 
+Test lazily acquiring the lock during unbundle
+  $ cp $TESTTMP/hgrc.orig $HGRCPATH
+  $ cat >> $HGRCPATH <<EOF
+  > [ui]
+  > ssh=python "$TESTDIR/dummyssh"
+  > EOF
+
+  $ cat >> $TESTTMP/locktester.py <<EOF
+  > import os
+  > from mercurial import extensions, bundle2, util
+  > def checklock(orig, repo, *args, **kwargs):
+  >     if repo.svfs.lexists("lock"):
+  >         raise util.Abort("Lock should not be taken")
+  >     return orig(repo, *args, **kwargs)
+  > def extsetup(ui):
+  >    extensions.wrapfunction(bundle2, 'processbundle', checklock)
+  > EOF
+
+  $ hg init lazylock
+  $ cat >> lazylock/.hg/hgrc <<EOF
+  > [extensions]
+  > locktester=$TESTTMP/locktester.py
+  > EOF
+
+  $ hg clone -q ssh://user@dummy/lazylock lazylockclient
+  $ cd lazylockclient
+  $ touch a && hg ci -Aqm a
+  $ hg push
+  pushing to ssh://user@dummy/lazylock
+  searching for changes
+  abort: Lock should not be taken
+  [255]
+
+  $ cat >> ../lazylock/.hg/hgrc <<EOF
+  > [experimental]
+  > bundle2lazylocking=True
+  > EOF
+  $ hg push
+  pushing to ssh://user@dummy/lazylock
+  searching for changes
+  remote: adding changesets
+  remote: adding manifests
+  remote: adding file changes
+  remote: added 1 changesets with 1 changes to 1 files