[PATCH 2 of 2 V3] treemanifest: make node reuse match flat manifest behavior

[Durham Goode]

# HG changeset patch
# User Durham Goode <durham at fb.com>
# Date 1488413981 28800
#      Wed Mar 01 16:19:41 2017 -0800
# Node ID f99c019063bea8a0b5f66c1e79dbd0b98b1326d8
# Parent  9c89185b63f574cad1d4009f6c9dd511a2cba986
treemanifest: make node reuse match flat manifest behavior

In a flat manifest, a node with the same content but different parents is still
considered a new node. In the current tree manifests however, if the content is
the same, we ignore the parents entirely and just reuse the existing node.

In our external treemanifest extension, we want to allow having one treemanifest
for every flat manifests, as a way of easeing the migration to treemanifests. To
make this possible, let's change the root node treemanifest behavior to match
the behavior for flat manifests, so we can have a 1:1 relationship.

While this sounds like a BC breakage, it's not actually a state users can
normally get in because: A) you can't make empty commits, and B) even if you try
to make an empty commit (by making a commit then amending it's changes away),
the higher level commit logic in localrepo.commitctx() forces the commit to use
the original p1 manifest node if no files were changed. So this would only
affect extensions and automation that reached passed the normal
localrepo.commit() logic straight into the manifest logic.

diff --git a/mercurial/manifest.py b/mercurial/manifest.py
--- a/mercurial/manifest.py
+++ b/mercurial/manifest.py
@@ -1233,7 +1233,7 @@ class manifestrevlog(revlog.revlog):
     def _addtree(self, m, transaction, link, m1, m2, readtree):
         # If the manifest is unchanged compared to one parent,
         # don't write a new revision
-        if m.unmodifiedsince(m1) or m.unmodifiedsince(m2):
+        if self._dir != '' and (m.unmodifiedsince(m1) or m.unmodifiedsince(m2)):
             return m.node()
         def writesubtree(subm, subp1, subp2):
             sublog = self.dirlog(subm.dir())
@@ -1241,13 +1241,17 @@ class manifestrevlog(revlog.revlog):
                        readtree=readtree)
         m.writesubtrees(m1, m2, writesubtree)
         text = m.dirtext(self._usemanifestv2)
-        # Double-check whether contents are unchanged to one parent
-        if text == m1.dirtext(self._usemanifestv2):
-            n = m1.node()
-        elif text == m2.dirtext(self._usemanifestv2):
-            n = m2.node()
-        else:
+        n = None
+        if self._dir != '':
+            # Double-check whether contents are unchanged to one parent
+            if text == m1.dirtext(self._usemanifestv2):
+                n = m1.node()
+            elif text == m2.dirtext(self._usemanifestv2):
+                n = m2.node()
+
+        if not n:
             n = self.addrevision(text, transaction, link, m1.node(), m2.node())
+
         # Save nodeid so parent manifest can calculate its nodeid
         m.setnode(n)
         return n
diff --git a/tests/test-treemanifest.t b/tests/test-treemanifest.t
--- a/tests/test-treemanifest.t
+++ b/tests/test-treemanifest.t
@@ -825,3 +825,13 @@ other branch
   added 1 changesets with 1 changes to 1 files (+1 heads)
   (run 'hg heads' to see heads, 'hg merge' to merge)
 
+Committing a empty commit does not duplicate root treemanifest
+  $ echo z >> z
+  $ hg commit -Aqm 'pre-empty commit'
+  $ hg rm z
+  $ hg commit --amend -m 'empty commit'
+  saved backup bundle to $TESTTMP/grafted-dir-repo-clone/.hg/strip-backup/cb99d5717cea-de37743b-amend-backup.hg (glob)
+  $ hg log -r 'tip + tip^' -T '{manifest}\n'
+  1:678d3574b88c
+  1:678d3574b88c
+  $ hg --config extensions.strip= strip -r . -q