fastannotate: deal with non-writable fastannotate directory

Summary:
Previously, fastannotate requires `.hg/fastannotate` to be writable. However,
this cannot be guaranteed in cases like running hg in other's repo.

This patch adds some exception handling to make fastannotate running in "fctx"
mode just work even if `.hg/fastannotate` is not writable.

Note: the `fastannotate` command ("fastannotate" mode) is not changed as if
that command is triggered explicitly, the user should be aware of it works
differently from the vanilla annotate command.

Test Plan: Added a test

Reviewers: #sourcecontrol, durham

Reviewed By: durham

Subscribers: mjpieters

Differential Revision: https://phabricator.intern.facebook.com/D4481427

Tasks: 15489272

Signature: t1:4481427:1485775004:ccc0b99b38146034c5b235445d762e55dbb78dbe

fastannotate/protocol.py

@@ -233,13 +233,17 @@ def localreposetup(ui, repo):
             master = _getmaster(self.ui)
             needupdatepaths = []
             lastnodemap = {}
-            for path in _filterfetchpaths(self, paths):
-                with context.annotatecontext(self, path) as actx:
-                    if not actx.isuptodate(master, strict=False):
-                        needupdatepaths.append(path)
-                        lastnodemap[path] = actx.lastnode
-            if needupdatepaths:
-                clientfetch(self, needupdatepaths, lastnodemap, peer)
+            try:
+                for path in _filterfetchpaths(self, paths):
+                    with context.annotatecontext(self, path) as actx:
+                        if not actx.isuptodate(master, strict=False):
+                            needupdatepaths.append(path)
+                            lastnodemap[path] = actx.lastnode
+                if needupdatepaths:
+                    clientfetch(self, needupdatepaths, lastnodemap, peer)
+            except Exception as ex:
+                # could be directory not writable or so, not fatal
+                self.ui.debug('fastannotate: prefetch failed: %r\n' % ex)
     repo.__class__ = fastannotaterepo
 
 def clientreposetup(ui, repo):

tests/test-fastannotate-protocol.t

@@ -190,6 +190,21 @@ without downloading from the server
   3: 4
   4: 5
 
+if the fastannotate directory is not writable, the fctx mode still works
+
+  $ rm -rf $p1
+  $ touch $p1
+  $ hg annotate a --debug --traceback --config fastannotate.modes=fctx
+  fastannotate: a: cache broken and deleted
+  fastannotate: prefetch failed: * (glob)
+  fastannotate: a: cache broken and deleted
+  fastannotate: falling back to the vanilla annotate: * (glob)
+  0: 1
+  1: 2
+  2: 3
+  3: 4
+  4: 5
+
 with serverbuildondemand=False, the server will not build anything
 
   $ cat >> ../repo-server/.hg/hgrc <<EOF