make the EdenFS tree prefetch depth configurable

Summary:
D18263067 updated the `CMD_FETCH_TREE` handler to fetch tree with a depth
of 1, rather than using fetching the entire recursive tree structure.
I think this has contributed to some of the recent user-reported slowness for
fetch operations, as fetching a large directory structure now triggers many
more individual requests for each separate tree.

This diff adds a configuration parameter to control the tree fetch depth.  I
have set the default value at 3, which should hopefully provide a reasonable
level of tree prefetching without completely fetching all contents of deep
trees.

Reviewed By: wez

Differential Revision: D18942585

fbshipit-source-id: 6a8d749434520baee25a4277712c44b916adcb3f

eden/scm/edenscm/mercurial/commands/eden.py

@@ -215,6 +215,13 @@ class HgServer(object):
                 continue
             self._commands[value.__COMMAND_ID__] = value
 
+        # Fetch the configuration for the depth to use when fetching trees.
+        # Setting this to a value larger than 1 causes the CMD_FETCH_TREE code to
+        # pre-fetch more children trees in addition to the specific tree that was
+        # requested.  This helps avoiding multiple round-trips when traversing into a
+        # directory.
+        self._treefetchdepth = self.repo.ui.configint("edenfs", "tree-fetch-depth")
+
     def serve(self):
         # Send a CMD_STARTED response to indicate we have started,
         # and include some information about the repository configuration.
@@ -497,12 +504,12 @@ class HgServer(object):
         if path:
             # We have to call repo._prefetchtrees() directly if we have a path.
             # We cannot compute the set of base nodes in this case.
-            self.repo._prefetchtrees(path, mfnodes, [], [], depth=1)
+            self.repo._prefetchtrees(path, mfnodes, [], [], depth=self._treefetchdepth)
             self.repo.commitpending()
         else:
             # When querying the top-level node use repo.prefetchtrees()
             # It will compute a reasonable set of base nodes to send in the query.
-            self.repo.prefetchtrees(mfnodes, depth=1)
+            self.repo.prefetchtrees(mfnodes, depth=self._treefetchdepth)
             self.repo.commitpending()
 
     def send_chunk(self, request, *data, **kwargs):

eden/scm/edenscm/mercurial/configitems.py

@@ -225,6 +225,7 @@ coreconfigitem("diff", "nobinary", default=False)
 coreconfigitem("diff", "noprefix", default=False)
 coreconfigitem("discovery", "fastdiscovery", False)
 coreconfigitem("discovery", "knownserverbookmarks", default=[])
+coreconfigitem("edenfs", "tree-fetch-depth", default=3)
 coreconfigitem("email", "bcc", default=None)
 coreconfigitem("email", "cc", default=None)
 coreconfigitem("email", "charsets", default=list)

eden/scm/edenscm/mercurial/helptext.py

@@ -965,6 +965,17 @@ The following options apply to all hosts.
     Optional. Bookmarks that should normally be present on client and server.
     Can be used to make fastdiscovery more precise
 
+``edenfs``
+---------
+
+Options that control the behavior of EdenFS.
+
+``tree-fetch-depth``
+    How many layers of children trees to fetch when downloading a directory listing from
+    the servers.  Higher values increase the latency of individual fetch operations, but
+    potentially help save having to send separate fetch requests later to download any
+    child trees that are needed.
+
 ``email``
 ---------