fastannotate: add a method to quickly get the last node of the revmap

Summary:
Previously, to get changeset hashes in the revmap, we have to construct
the `revmap` object, which reads all content of a revmap.

In the following patches, we want to quickly get the last hash of the
revmap to test if the linelog / revmap is up-to-date or not. This patch
adds a method to do so.

Test Plan: Modified existing tests.

Reviewers: #sourcecontrol, stash

Reviewed By: stash

Subscribers: mjpieters

Differential Revision: https://phabricator.intern.facebook.com/D4103898

Signature: t1:4103898:1477988583:15869dfe9028068563b03d1f06d2337562049751

fastannotate/revmap.py

@@ -227,3 +227,17 @@ class revmap(object):
         if path is not None and path != self.rev2path(rev):
             return False
         return (self.rev2flag(rev) & sidebranchflag) == 0
+
+def getlastnode(path):
+    """return the last hash in a revmap, without loading its full content.
+    this is equivalent to `m = revmap(path); m.rev2hsh(m.maxrev)`, but faster.
+    """
+    hsh = None
+    try:
+        with open(path, 'rb') as f:
+            f.seek(-_hshlen, 2)
+            if f.tell() > len(revmap.HEADER):
+                hsh = f.read(_hshlen)
+    except IOError:
+        pass
+    return hsh

tests/test-fastannotate-revmap.py

@@ -175,7 +175,20 @@ def testcontains():
         ensure(fakefctx(genhsh(i), path=str(i // 2)) in rm)
         ensure(fakefctx(genhsh(i), path='a') not in rm)
 
+def testlastnode():
+    path = gettemppath()
+    ensure(revmap.getlastnode(path) is None)
+    rm = revmap.revmap(path)
+    ensure(revmap.getlastnode(path) is None)
+    for i in xrange(1, 10):
+        hsh = genhsh(i)
+        rm.append(hsh, path=str(i // 2), flush=True)
+        ensure(revmap.getlastnode(path) == hsh)
+        rm2 = revmap.revmap(path)
+        ensure(rm2.rev2hsh(rm2.maxrev) == hsh)
+
 testbasicreadwrite()
 testcorruptformat()
 testcopyfrom()
 testcontains()
+testlastnode()