diff --git a/mercurial/exchange.py b/mercurial/exchange.py
--- a/mercurial/exchange.py
+++ b/mercurial/exchange.py
@@ -8,7 +8,7 @@
 from i18n import _
 from node import hex, nullid
 import errno, urllib
-import util, scmutil, changegroup, base85, error
+import util, scmutil, changegroup, base85, error, store
 import discovery, phases, obsolete, bookmarks as bookmod, bundle2, pushkey
 import lock as lockmod
 
@@ -1332,3 +1332,68 @@ def unbundle(repo, cg, heads, source, ur
         if recordout is not None:
             recordout(repo.ui.popbuffer())
     return r
+
+# This is it's own function so extensions can override it.
+def _walkstreamfiles(repo):
+    return repo.store.walk()
+
+def generatestreamclone(repo):
+    """Emit content for a streaming clone.
+
+    This is a generator of raw chunks that constitute a streaming clone.
+
+    The stream begins with a line of 2 space-delimited integers containing the
+    number of entries and total bytes size.
+
+    Next, are N entries for each file being transferred. Each file entry starts
+    as a line with the file name and integer size delimited by a null byte.
+    The raw file data follows. Following the raw file data is the next file
+    entry, or EOF.
+
+    When used on the wire protocol, an additional line indicating protocol
+    success will be prepended to the stream. This function is not responsible
+    for adding it.
+
+    This function will obtain a repository lock to ensure a consistent view of
+    the store is captured. It therefore may raise LockError.
+    """
+    entries = []
+    total_bytes = 0
+    # Get consistent snapshot of repo, lock during scan.
+    lock = repo.lock()
+    try:
+        repo.ui.debug('scanning\n')
+        for name, ename, size in _walkstreamfiles(repo):
+            if size:
+                entries.append((name, size))
+                total_bytes += size
+    finally:
+            lock.release()
+
+    repo.ui.debug('%d files, %d bytes to transfer\n' %
+                  (len(entries), total_bytes))
+    yield '%d %d\n' % (len(entries), total_bytes)
+
+    sopener = repo.svfs
+    oldaudit = sopener.mustaudit
+    debugflag = repo.ui.debugflag
+    sopener.mustaudit = False
+
+    try:
+        for name, size in entries:
+            if debugflag:
+                repo.ui.debug('sending %s (%d bytes)\n' % (name, size))
+            # partially encode name over the wire for backwards compat
+            yield '%s\0%d\n' % (store.encodedir(name), size)
+            if size <= 65536:
+                fp = sopener(name)
+                try:
+                    data = fp.read(size)
+                finally:
+                    fp.close()
+                yield data
+            else:
+                for chunk in util.filechunkiter(sopener(name), limit=size):
+                    yield chunk
+    finally:
+        sopener.mustaudit = oldaudit
diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -744,73 +744,27 @@ def pushkey(repo, proto, namespace, key,
 def _allowstream(ui):
     return ui.configbool('server', 'uncompressed', True, untrusted=True)
 
-def _walkstreamfiles(repo):
-    # this is it's own function so extensions can override it
-    return repo.store.walk()
-
 @wireprotocommand('stream_out')
 def stream(repo, proto):
     '''If the server supports streaming clone, it advertises the "stream"
     capability with a value representing the version and flags of the repo
     it is serving. Client checks to see if it understands the format.
-
-    The format is simple: the server writes out a line with the amount
-    of files, then the total amount of bytes to be transferred (separated
-    by a space). Then, for each file, the server first writes the filename
-    and file size (separated by the null character), then the file contents.
     '''
-
     if not _allowstream(repo.ui):
         return '1\n'
 
-    entries = []
-    total_bytes = 0
-    try:
-        # get consistent snapshot of repo, lock during scan
-        lock = repo.lock()
-        try:
-            repo.ui.debug('scanning\n')
-            for name, ename, size in _walkstreamfiles(repo):
-                if size:
-                    entries.append((name, size))
-                    total_bytes += size
-        finally:
-            lock.release()
-    except error.LockError:
-        return '2\n' # error: 2
-
-    def streamer(repo, entries, total):
-        '''stream out all metadata files in repository.'''
-        yield '0\n' # success
-        repo.ui.debug('%d files, %d bytes to transfer\n' %
-                      (len(entries), total_bytes))
-        yield '%d %d\n' % (len(entries), total_bytes)
+    def getstream(it):
+        yield '0\n'
+        for chunk in it:
+            yield chunk
 
-        sopener = repo.svfs
-        oldaudit = sopener.mustaudit
-        debugflag = repo.ui.debugflag
-        sopener.mustaudit = False
-
-        try:
-            for name, size in entries:
-                if debugflag:
-                    repo.ui.debug('sending %s (%d bytes)\n' % (name, size))
-                # partially encode name over the wire for backwards compat
-                yield '%s\0%d\n' % (store.encodedir(name), size)
-                if size <= 65536:
-                    fp = sopener(name)
-                    try:
-                        data = fp.read(size)
-                    finally:
-                        fp.close()
-                    yield data
-                else:
-                    for chunk in util.filechunkiter(sopener(name), limit=size):
-                        yield chunk
-        finally:
-            sopener.mustaudit = oldaudit
-
-    return streamres(streamer(repo, entries, total_bytes))
+    try:
+        # LockError may be raised before the first result is yielded. Don't
+        # emit output until we're sure we got the lock successfully.
+        it = exchange.generatestreamclone(repo)
+        return streamres(getstream(it))
+    except error.LockError:
+        return '2\n'
 
 @wireprotocommand('unbundle', 'heads')
 def unbundle(repo, proto, heads):