revlog: move censor logic out of censor extension The censor extension is doing very low-level things with revlogs. It is fundamentally impossible for this logic to remain in the censor extension while support multiple storage backends: we need each storage backend to implement censor in its own storage-specific way. This commit effectively moves the revlog-specific censoring code to be a method of revlogs themselves. We've defined a new API on the file storage interface for censoring an individual node. Even though the current censoring code doesn't use it, the API requires a transaction instance because it logically makes sense for storage backends to require an active transaction (which implies a held write lock) in order to rewrite storage. After this commit, the censor extension has been reduced to boilerplate precondition checking before invoking the generic storage API. I tried to keep the code as similar as possible. But some minor changes were made: * We use self._io instead of instantiating a new revlogio instance. * We compare self.version against REVLOGV0 instead of != REVLOGV1 because presumably all future revlog versions will support censoring. * We use self.opener instead of going through repo.svfs (we don't have a handle on the repo instance from a revlog). * "revlog" dropped * Replace "flog" with "self". Differential Revision: https://phab.mercurial-scm.org/D4656

--- a/mercurial/repository.py	Tue Sep 18 16:47:09 2018 -0700
+++ b/mercurial/repository.py	Tue Sep 18 17:51:43 2018 -0700
@@ -691,6 +691,23 @@
         even if it existed in the store previously.
         """
 
+    def censorrevision(tr, node, tombstone=b''):
+        """Remove the content of a single revision.
+
+        The specified ``node`` will have its content purged from storage.
+        Future attempts to access the revision data for this node will
+        result in failure.
+
+        A ``tombstone`` message can optionally be stored. This message may be
+        displayed to users when they attempt to access the missing revision
+        data.
+
+        Storage backends may have stored deltas against the previous content
+        in this revision. As part of censoring a revision, these storage
+        backends are expected to rewrite any internally stored deltas such
+        that they no longer reference the deleted content.
+        """
+
     def getstrippoint(minlink):
         """Find the minimum revision that must be stripped to strip a linkrev.