transplant: handle non-empty patches doing nothing (issue2806)

If patch.patch() reports patched files when applying a changeset and the
following commit says nothing changed, transplant used to abort with a
RuntimeError, assuming something went wrong with patching.

The mismatch is patch.patch() reports patched files, not changed ones.
It could be modified to report changed files but it means duplicating
work from status, may be expensive in the case of binary files, and is
probably not that useful at API level. For instance, if two patches are
applied on the working directory, the outcome may be nothing changed
while each call would have returned modified files. The caller would
have to call status() itself again.

This patch fixes the issue by trusting patching code: if the patch
succeeded and commit reports nothing changed, then nothing changed,
patch() did not "dropped changes on the floor".

hgext/transplant.py

@@ -263,9 +263,6 b' class transplanter(object):'
                 files = set()
                 patch.patch(self.ui, repo, patchfile, files=files, eolmode=None)
                 files = list(files)
-                if not files:
-                    self.ui.warn(_('%s: empty changeset') % revlog.hex(node))
-                    return None
             except Exception, inst:
                 seriespath = os.path.join(self.path, 'series')
                 if os.path.exists(seriespath):
@@ -288,14 +285,8 b' class transplanter(object):'
         n = repo.commit(message, user, date, extra=extra, match=m,
                         editor=self.editor)
         if not n:
-            # Crash here to prevent an unclear crash later, in
-            # transplants.write().  This can happen if patch.patch()
-            # does nothing but claims success or if repo.status() fails
-            # to report changes done by patch.patch().  These both
-            # appear to be bugs in other parts of Mercurial, but dying
-            # here, as soon as we can detect the problem, is preferable
-            # to silently dropping changesets on the floor.
-            raise RuntimeError('nothing committed after transplant')
+            self.ui.warn(_('%s: empty changeset') % revlog.hex(node))
+            return None
         if not merge:
             self.transplants.set(n, node)
 

tests/test-transplant.t

@@ -603,3 +603,32 b' test transplant with merge changeset acc'
   applying be9f9b39483f
   be9f9b39483f transplanted to 9959e51f94d1
   $ cd ..
+
+test transplanting a patch turning into a no-op
+
+  $ hg init binarysource
+  $ cd binarysource
+  $ echo a > a
+  $ hg ci -Am adda a
+  >>> file('b', 'wb').write('\0b1')
+  $ hg ci -Am addb b
+  >>> file('b', 'wb').write('\0b2')
+  $ hg ci -m changeb b
+  $ cd ..
+
+  $ hg clone -r0 binarysource binarydest
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  updating to branch default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd binarydest
+  $ cp ../binarysource/b b
+  $ hg ci -Am addb2 b
+  $ hg transplant -s ../binarysource 2
+  searching for changes
+  applying 7a7d57e15850
+  7a7d57e158501e51588f5fd3288b491cac77e0d8: empty changeset (no-eol)
+  $ cd ..
+