hooks: add preupdate and update hooks.

preupdate run before working dir updated. can prevent update.
update run after working dir updated.

doc/hgrc.5.txt

@@ -219,10 +219,20 b' hooks::'
     commit to proceed.  Non-zero status will cause the transaction to
     be rolled back.  ID of changeset is in $HG_NODE.  Parent changeset
     IDs are in $HG_PARENT1 and $HG_PARENT2.
+  preupdate;;
+    Run before updating the working directory.  Exit status 0 allows
+    the update to proceed.  Non-zero status will prevent the update.
+    Changeset ID of first new parent is in $HG_PARENT1.  If merge, ID
+    of second new parent is in $HG_PARENT2.
   tag;;
     Run after a tag is created.  ID of tagged changeset is in
     $HG_NODE.  Name of tag is in $HG_TAG.  Tag is local if
     $HG_LOCAL=1, in repo if $HG_LOCAL=0.
+  update;;
+    Run after updating the working directory.  Changeset ID of first
+    new parent is in $HG_PARENT1.  If merge, ID of second new parent
+    is in $HG_PARENT2.  If update succeeded, $HG_ERROR=0.  If update
+    failed (e.g. because conflicts not resolved), $HG_ERROR=1.
 
   In earlier releases, the names of hook environment variables did not
   have a "HG_" prefix.  These unprefixed names are still provided in

mercurial/localrepo.py

@@ -1592,6 +1592,7 b' class localrepository(object):'
         if allow and not forcemerge:
             if modified or added or removed:
                 raise util.Abort(_("outstanding uncommitted changes"))
+
         if not forcemerge and not force:
             for f in unknown:
                 if f in m2:
@@ -1765,6 +1766,13 b' class localrepository(object):'
                 return 1
             branch_merge = True
 
+        xp1 = hex(p1)
+        xp2 = hex(p2)
+        if p2 == nullid: xxp2 = ''
+        else: xxp2 = xp2
+
+        self.hook('preupdate', throw=True, parent1=xp1, parent2=xxp2)
+
         # get the files we don't need to change
         files = get.keys()
         files.sort()
@@ -1785,8 +1793,6 b' class localrepository(object):'
         failedmerge = []
         files = merge.keys()
         files.sort()
-        xp1 = hex(p1)
-        xp2 = hex(p2)
         for f in files:
             self.ui.status(_("merging %s\n") % f)
             my, other, flag = merge[f]
@@ -1850,6 +1856,7 b' class localrepository(object):'
                 self.ui.status(_("There are unresolved merges with"
                                  " locally modified files.\n"))
 
+        self.hook('update', parent1=xp1, parent2=xxp2, error=int(err))
         return err
 
     def merge3(self, fn, my, other, p1, p2):

tests/test-hook

@@ -61,6 +61,14 b" echo 'precommit.forbid = echo precommit."
 hg commit -m 'fail' -d '4 0'
 hg -q tip
 
+# preupdate hook can prevent update
+echo 'preupdate = echo preupdate hook: p1=$HG_PARENT1 p2=$HG_PARENT2' >> .hg/hgrc
+hg update 1
+
+# update hook
+echo 'update = echo update hook: p1=$HG_PARENT1 p2=$HG_PARENT2 err=$HG_ERROR' >> .hg/hgrc
+hg update
+
 # prechangegroup hook can prevent incoming changes
 cd ../b
 hg -q tip

tests/test-hook.out

@@ -62,6 +62,11 b' precommit hook: p1=4f92e785b90ae8995dfe1'
 precommit.forbid hook
 abort: precommit.forbid hook exited with status 1
 4:4f92e785b90a
+preupdate hook: p1=b702efe9688826e3a91283852b328b84dbf37bc2 p2=
+0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+preupdate hook: p1=4f92e785b90ae8995dfe156e39dd4fbc3b346a24 p2=
+update hook: p1=4f92e785b90ae8995dfe156e39dd4fbc3b346a24 p2= err=0
+2 files updated, 0 files merged, 0 files removed, 0 files unresolved
 3:4c52fb2e4022
 prechangegroup.forbid hook
 pulling from ../a