scmutil: add option to register summary callbacks as transaction validators

We have a list of summary callbacks which are run after the transaction is
closed to show what has changed and what not. This patch makes it possible to
register those callbacks as transaction validators so that we can show summary
before committing the transaction and prompt user to accept the changes.

The goal of this is to implement `pull --confirm`.

Differential Revision: https://phab.mercurial-scm.org/D8199

mercurial/scmutil.py

@@ -1900,8 +1900,11 b' fileprefetchhooks = util.hooks()'
 _reportstroubledchangesets = True
 
 
-def registersummarycallback(repo, otr, txnname=b''):
+def registersummarycallback(repo, otr, txnname=b'', as_validator=False):
     """register a callback to issue a summary after the transaction is closed
+
+    If as_validator is true, then the callbacks are registered as transaction
+    validators instead
     """
 
     def txmatch(sources):
@@ -1927,6 +1930,9 b' def registersummarycallback(repo, otr, t'
             func(repo, tr)
 
         newcat = b'%02i-txnreport' % len(categories)
+        if as_validator:
+            otr.addvalidator(newcat, wrapped)
+        else:
         otr.addpostclose(newcat, wrapped)
         categories.append(newcat)
         return wrapped
@@ -1942,6 +1948,8 b' def registersummarycallback(repo, otr, t'
             if cgheads:
                 htext = _(b" (%+d heads)") % cgheads
             msg = _(b"added %d changesets with %d changes to %d files%s\n")
+            if as_validator:
+                msg = _(b"adding %d changesets with %d changes to %d files%s\n")
             assert repo is not None  # help pytype
             repo.ui.status(msg % (cgchangesets, cgrevisions, cgfiles, htext))
 
@@ -1954,7 +1962,10 b' def registersummarycallback(repo, otr, t'
             if newmarkers:
                 repo.ui.status(_(b'%i new obsolescence markers\n') % newmarkers)
             if obsoleted:
-                repo.ui.status(_(b'obsoleted %i changesets\n') % len(obsoleted))
+                msg = _(b'obsoleted %i changesets\n')
+                if as_validator:
+                    msg = _(b'obsoleting %i changesets\n')
+                repo.ui.status(msg % len(obsoleted))
 
     if obsolete.isenabled(
         repo, obsolete.createmarkersopt
@@ -2057,9 +2068,10 b' def registersummarycallback(repo, otr, t'
             ]
             if not published:
                 return
-            repo.ui.status(
-                _(b'%d local changesets published\n') % len(published)
-            )
+            msg = _(b'%d local changesets published\n')
+            if as_validator:
+                msg = _(b'%d local changesets will be published\n')
+            repo.ui.status(msg % len(published))
 
 
 def getinstabilitymessage(delta, instability):