manifest: introduce new exception to signal unavailability of fastdelta()

manifest: introduce new exception to signal unavailability of fastdelta()

I've spent some time reflecting on this, and I think this is the best
we can do in this API contract for now. This opens the door to adding
treemanifest's implementation to the list of implementers of
imanifestdict.

Differential Revision: https://phab.mercurial-scm.org/D8365

--- a/mercurial/interfaces/repository.py	Sat Mar 07 00:30:33 2020 +0100
+++ b/mercurial/interfaces/repository.py	Thu Apr 02 13:04:41 2020 -0400
@@ -1052,6 +1052,9 @@
 
         Returns a 2-tuple containing ``bytearray(self.text())`` and the
         delta between ``base`` and this manifest.
+
+        If this manifest implementation can't support ``fastdelta()``,
+        raise ``mercurial.manifest.FastdeltaUnavailable``.
         """
 
 

--- a/mercurial/manifest.py	Sat Mar 07 00:30:33 2020 +0100
+++ b/mercurial/manifest.py	Thu Apr 02 13:04:41 2020 -0400
@@ -1212,6 +1212,9 @@
             ret._dirty = True
         return ret
 
+    def fastdelta(self, base, changes):
+        raise FastdeltaUnavailable()
+
     def diff(self, m2, match=None, clean=False):
         '''Finds changes between the current manifest and m2.
 
@@ -1488,6 +1491,10 @@
 MAXCOMPRESSION = 3
 
 
+class FastdeltaUnavailable(Exception):
+    """Exception raised when fastdelta isn't usable on a manifest."""
+
+
 @interfaceutil.implementer(repository.imanifeststorage)
 class manifestrevlog(object):
     '''A revlog that stores manifest texts. This is responsible for caching the
@@ -1614,7 +1621,9 @@
         readtree=None,
         match=None,
     ):
-        if p1 in self.fulltextcache and util.safehasattr(m, b'fastdelta'):
+        try:
+            if p1 not in self.fulltextcache:
+                raise FastdeltaUnavailable()
             # If our first parent is in the manifest cache, we can
             # compute a delta here using properties we know about the
             # manifest up-front, which may save time later for the
@@ -1633,11 +1642,12 @@
             n = self._revlog.addrevision(
                 text, transaction, link, p1, p2, cachedelta
             )
-        else:
-            # The first parent manifest isn't already loaded, so we'll
-            # just encode a fulltext of the manifest and pass that
-            # through to the revlog layer, and let it handle the delta
-            # process.
+        except FastdeltaUnavailable:
+            # The first parent manifest isn't already loaded or the
+            # manifest implementation doesn't support fastdelta, so
+            # we'll just encode a fulltext of the manifest and pass
+            # that through to the revlog layer, and let it handle the
+            # delta process.
             if self._treeondisk:
                 assert readtree, b"readtree must be set for treemanifest writes"
                 assert match, b"match must be specified for treemanifest writes"