changelog: load pending file directly

When changelogs are written, a copy of the index (or inline revlog)
may be written to an 00changelog.i.a file to facilitate hooks and
other processes having access to the pending data before it is
finalized.

The way it works today, the localrepo class loads the changelog
like normal. Then, if it detects a pending transaction, it asks
the changelog class to load a pending changelog. The changelog
class looks for a 00changelog.i.a file. If it exists, it is
loaded and internal data structures on the new revlog class are
copied to the original instance.

The existing mechanism is inefficient because it loads 2 revlog
files. The index, node map, and chunk cache for 00changelog.i
are thrown away and replaced by those for 00changelog.i.a.

The existing mechanism is also brittle because it is a layering
violation to access the data structures being accessed. For example,
the code copies the "chunk cache" because for inline revlogs
this cache contains the raw revision chunks and allows the original
changelog/revlog instance to access revision data for these pending
revisions. This whole behavior of course relies on the revlog
constructor reading the entirety of an inline revlog into memory
and caching it. That's why it is brittle. (I discovered all this
as part of modifying behavior of the chunk cache.)

This patch streamlines the loading of a pending 00changelog.i.a
revlog by doing it directly in the changelog constructor if told
to do so. When this code path is active, we no longer load the
00changelog.i file at all.

The only negative outcome I see from this change is if loading
00changelog.i was somehow facilitating a role. But I can't imagine
what that would be because we throw away its data (the index data
structures are replaced and inline revision data is replaced via
the chunk cache) and since 00changelog.i.a is a copy of
00changelog.i, file content should be identical, so there should
be no meaninful file integrity checking at play. I think this was
all just sub-optimal code.

mercurial/changelog.py

@@ -258,9 +258,23 b' class changelogrevision(object):'
         return encoding.tolocal(self._text[self._offsets[3] + 2:])
 
 class changelog(revlog.revlog):
-    def __init__(self, opener):
-        revlog.revlog.__init__(self, opener, "00changelog.i",
-                               checkambig=True)
+    def __init__(self, opener, trypending=False):
+        """Load a changelog revlog using an opener.
+
+        If ``trypending`` is true, we attempt to load the index from a
+        ``00changelog.i.a`` file instead of the default ``00changelog.i``.
+        The ``00changelog.i.a`` file contains index (and possibly inline
+        revision) data for a transaction that hasn't been finalized yet.
+        It exists in a separate file to facilitate readers (such as
+        hooks processes) accessing data before a transaction is finalized.
+        """
+        if trypending and opener.exists('00changelog.i.a'):
+            indexfile = '00changelog.i.a'
+        else:
+            indexfile = '00changelog.i'
+
+        revlog.revlog.__init__(self, opener, indexfile, checkambig=True)
+
         if self._initempty:
             # changelogs don't benefit from generaldelta
             self.version &= ~revlog.REVLOGGENERALDELTA
@@ -401,27 +415,6 b' class changelog(revlog.revlog):'
         # split when we're done
         self.checkinlinesize(tr)
 
-    def readpending(self, file):
-        """read index data from a "pending" file
-
-        During a transaction, the actual changeset data is already stored in the
-        main file, but not yet finalized in the on-disk index. Instead, a
-        "pending" index is written by the transaction logic. If this function
-        is running, we are likely in a subprocess invoked in a hook. The
-        subprocess is informed that it is within a transaction and needs to
-        access its content.
-
-        This function will read all the index data out of the pending file and
-        overwrite the main index."""
-
-        if not self.opener.exists(file):
-            return # no pending data for changelog
-        r = revlog.revlog(self.opener, file)
-        self.index = r.index
-        self.nodemap = r.nodemap
-        self._nodecache = r._nodecache
-        self._chunkcache = r._chunkcache
-
     def _writepending(self, tr):
         "create a file containing the unfinalized state for pretxnchangegroup"
         if self._delaybuf:

mercurial/localrepo.py

@@ -528,10 +528,8 b' class localrepository(object):'
 
     @storecache('00changelog.i')
     def changelog(self):
-        c = changelog.changelog(self.svfs)
-        if txnutil.mayhavepending(self.root):
-            c.readpending('00changelog.i.a')
-        return c
+        return changelog.changelog(self.svfs,
+                                   trypending=txnutil.mayhavepending(self.root))
 
     def _constructmanifest(self):
         # This is a temporary function while we migrate from manifest to