eden: make the EdenFS import helper use EdenAPI's /trees endpoint instead of /complete_trees

Summary:
The EdenFS import helper's `_fetch_tree_impl` method works by either calling `repo._prefetchtrees` or `repo.prefetchtrees`, both of which are methods from `treemanifest` extension's `treerepository` class. Unfortunately, these methods are lower-level than their names would suggest, and will always perform gettreepack style fetching (see [1]).

If HTTP fetching is enabled, this means that EdenFS will always query EdenAPI's `/complete_trees` endpoint instead of `/trees`, which is needlessly expensive given that EdenFS just wants the exact set of trees specified. As a somewhat hacky workaround, this diff just checks whether HTTP fetching is enabled, and if so, directly calls the appropriate HTTP fetch method.

This solution isn't ideal; it would be better for the import helper to request the trees from Mercurial's `remotetreestore` instead of via methods from `treerepository`. Unfortunately, with the current structure of Mercurial's storage layer, the `remotetreestore` isn't readily accessible because it gets passed into the Rust API's storage hierarchy upon construction.
 ---
[1]: The various `*prefetchtrees` methods  are usually called from Mercurial's `remotetreestore`, which is where the choice of tree fetching strategy is made (i.e., designated nodes vs gettreepack). The `remotetreestore` then calls `*prefetchtrees` for gettreepack-style fetching, or `*getdesignatednodes` for on-demand fetching. As such, a call to `*prefetchtrees` generally implies gettreepack-style fetching.

Reviewed By: quark-zju

Differential Revision: D26560451

fbshipit-source-id: 2eedf50a6e66fac78df77214b777544eb8049714

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

@@ -660,6 +660,22 @@ class HgServer(object):
 
     def _fetch_tree_impl(self, path, manifest_node):
         # type: (str, bytes) -> None
+
+        # The _prefetchtrees function called below will *always* perform
+        # gettreepack-style fetching (even if treemanifest.ondemandfetch is
+        # set to True) because the logic that determines the tree fetching
+        # strategy occurs in Mercurial's `remotetreestore` (which is not
+        # readily accessible here because it is passed into Mercurial's Rust
+        # storage API upon construction.
+        #
+        # As a workaround, let's just check if HTTP fetching is enabled, and
+        # if so, just directly call the appropriate treerepository method to
+        # send the request to EdenAPI's `/trees` endpoint (which only returns
+        # the exact set of trees requested, as desired here).
+        if self.repo._shouldusehttp():
+            self.repo._httpgetdesignatednodes([(path, manifest_node)])
+            return
+
         mfnodes = set([manifest_node])
         depth = str(self._treefetchdepth)
         with self.repo.ui.configoverride(

eden/scm/pystubs/edenscm/mercurial/localrepo.pyi

@@ -8,6 +8,7 @@
 import __future__
 
 import __builtin__
+import _weakref
 import contextlib
 from typing import (
     Any,
@@ -26,7 +27,6 @@ from typing import (
     Union,
 )
 
-import _weakref
 import edenscm.mercurial.connectionpool
 import edenscm.mercurial.context
 import edenscm.mercurial.filelog
@@ -179,6 +179,7 @@ class localrepository(object):
     def _filter(self, filterpats, filename, data) -> Any: ...
     def _getsvfsward(self, origfunc) -> Callable: ...
     def _getvfsward(self, origfunc) -> Callable: ...
+    def _httpgetdesignatednodes(self, keys: Iterable[(str, bytes)]): ...
     def _journalfiles(
         self,
     ) -> Tuple[
@@ -215,6 +216,7 @@ class localrepository(object):
     def _refreshfilecachestats(repo: localrepository, *args, **kwargs) -> None: ...
     def _restrictcapabilities(self, caps: Set[str]) -> Set[str]: ...
     def _rollback(repo: localrepository, *args, **kwargs) -> int: ...
+    def _shouldusehttp(self) -> bool: ...
     def _syncrevlogtozstore(self) -> None: ...
     def _wlockchecktransaction(self) -> None: ...
     def _writejournal(repo: localrepository, *args, **kwargs) -> None: ...