fastmanifest: allow creating a systemawarecachelimit without a reference to repo

Summary: This will be useful to refactor the cache

Test Plan:
We change the python test to give a vfs that makes sense, it will
be useful in the next patch when we refactor the cache. Indeed, with the new
cache we use systemawarecachelimit by default instead of no limit

Reviewers: ttung

Differential Revision: https://phabricator.intern.facebook.com/D3427796

fastmanifest/cachemanager.py

@@ -111,10 +111,18 @@ class _systemawarecachelimit(object):
                 repo.ui.warn((msg % strconfig))
         return configs
 
-    def __init__(self, repo):
+    def __init__(self, repo=None, opener=None, ui=None):
         # Probe the system root partition to know what is available
         try:
-            st = os.statvfs(repo.root)
+            if repo is None and (opener is None or ui is None):
+                raise error.Abort("Need to specify repo or (opener and ui)")
+            if repo is not None:
+                st = os.statvfs(repo.root)
+            else:
+                if util.safehasattr(opener, "vfs"):
+                    st = os.statvfs(opener.vfs.base)
+                else:
+                    st = os.statvfs(opener.base)
         except (OSError, IOError) as ex:
             if ex.errno == errno.EACCES:
                 self.free = 0
@@ -124,7 +132,10 @@ class _systemawarecachelimit(object):
         self.free = st.f_bavail * st.f_frsize
         self.total = st.f_blocks * st.f_frsize
         # Read parameters from config
-        self.config = self.parseconfig(repo)
+        if repo is not None:
+            self.config = self.parseconfig(repo.ui)
+        else:
+            self.config = self.parseconfig(ui)
 
     def bytes(self):
         return _systemawarecachelimit.cacheallocation(self.free, **self.config)

tests/test-fastmanifest.py

@@ -19,7 +19,7 @@ class HybridManifest(unittest.TestCase):
         breakage in prod
 
         """
-        vfs = scmutil.vfs('')
+        vfs = scmutil.vfs(os.getcwd())
         hd = fastmanifest.implementation.hybridmanifest(ui.ui(), vfs)
         ismagic = lambda x: x.startswith("__") and x.endswith("__")
         magicmethods = [k