diff --git a/mercurial/state.py b/mercurial/state.py
--- a/mercurial/state.py
+++ b/mercurial/state.py
@@ -19,6 +19,8 @@
 
 from __future__ import absolute_import
 
+import contextlib
+
 from .i18n import _
 
 from . import (
@@ -119,6 +121,7 @@
         reportonly,
         continueflag,
         stopflag,
+        childopnames,
         cmdmsg,
         cmdhint,
         statushint,
@@ -132,6 +135,8 @@
         self._reportonly = reportonly
         self._continueflag = continueflag
         self._stopflag = stopflag
+        self._childopnames = childopnames
+        self._delegating = False
         self._cmdmsg = cmdmsg
         self._cmdhint = cmdhint
         self._statushint = statushint
@@ -181,12 +186,15 @@
         """
         if self._opname == b'merge':
             return len(repo[None].parents()) > 1
+        elif self._delegating:
+            return False
         else:
             return repo.vfs.exists(self._fname)
 
 
 # A list of statecheck objects for multistep operations like graft.
 _unfinishedstates = []
+_unfinishedstatesbyname = {}
 
 
 def addunfinished(
@@ -197,6 +205,7 @@
     reportonly=False,
     continueflag=False,
     stopflag=False,
+    childopnames=[],
     cmdmsg=b"",
     cmdhint=b"",
     statushint=b"",
@@ -218,6 +227,8 @@
     `--continue` option or not.
     stopflag is a boolean that determines whether or not a command supports
     --stop flag
+    childopnames is a list of other opnames this op uses as sub-steps of its
+    own execution. They must already be added.
     cmdmsg is used to pass a different status message in case standard
     message of the format "abort: cmdname in progress" is not desired.
     cmdhint is used to pass a different hint message in case standard
@@ -238,17 +249,98 @@
         reportonly,
         continueflag,
         stopflag,
+        childopnames,
         cmdmsg,
         cmdhint,
         statushint,
         abortfunc,
         continuefunc,
     )
+
     if opname == b'merge':
         _unfinishedstates.append(statecheckobj)
     else:
+        # This check enforces that for any op 'foo' which depends on op 'bar',
+        # 'foo' comes before 'bar' in _unfinishedstates. This ensures that
+        # getrepostate() always returns the most specific applicable answer.
+        for childopname in childopnames:
+            if childopname not in _unfinishedstatesbyname:
+                raise error.ProgrammingError(
+                    _(b'op %s depends on unknown op %s') % (opname, childopname)
+                )
+
         _unfinishedstates.insert(0, statecheckobj)
 
+    if opname in _unfinishedstatesbyname:
+        raise error.ProgrammingError(_(b'op %s registered twice') % opname)
+    _unfinishedstatesbyname[opname] = statecheckobj
+
+
+def _getparentandchild(opname, childopname):
+    p = _unfinishedstatesbyname.get(opname, None)
+    if not p:
+        raise error.ProgrammingError(_(b'unknown op %s') % opname)
+    if childopname not in p._childopnames:
+        raise error.ProgrammingError(
+            _(b'op %s does not delegate to %s') % (opname, childopname)
+        )
+    c = _unfinishedstatesbyname[childopname]
+    return p, c
+
+
+@contextlib.contextmanager
+def delegating(repo, opname, childopname):
+    """context wrapper for delegations from opname to childopname.
+
+    requires that childopname was specified when opname was registered.
+
+    Usage:
+      def my_command_foo_that_uses_rebase(...):
+        ...
+        with state.delegating(repo, 'foo', 'rebase'):
+          _run_rebase(...)
+        ...
+    """
+
+    p, c = _getparentandchild(opname, childopname)
+    if p._delegating:
+        raise error.ProgrammingError(
+            _(b'cannot delegate from op %s recursively') % opname
+        )
+    p._delegating = True
+    try:
+        yield
+    except error.ConflictResolutionRequired as e:
+        # Rewrite conflict resolution advice for the parent opname.
+        if e.opname == childopname:
+            raise error.ConflictResolutionRequired(opname)
+        raise e
+    finally:
+        p._delegating = False
+
+
+def ischildunfinished(repo, opname, childopname):
+    """Returns true if both opname and childopname are unfinished."""
+
+    p, c = _getparentandchild(opname, childopname)
+    return (p._delegating or p.isunfinished(repo)) and c.isunfinished(repo)
+
+
+def continuechild(ui, repo, opname, childopname):
+    """Checks that childopname is in progress, and continues it."""
+
+    p, c = _getparentandchild(opname, childopname)
+    if not ischildunfinished(repo, opname, childopname):
+        raise error.ProgrammingError(
+            _(b'child op %s of parent %s is not unfinished')
+            % (childopname, opname)
+        )
+    if not c.continuefunc:
+        raise error.ProgrammingError(
+            _(b'op %s has no continue function') % childopname
+        )
+    return c.continuefunc(ui, repo)
+
 
 addunfinished(
     b'update',
diff --git a/tests/test-state-extension.t b/tests/test-state-extension.t
new file mode 100644
--- /dev/null
+++ b/tests/test-state-extension.t
@@ -0,0 +1,136 @@
+Test extension of unfinished states support.
+  $ mkdir chainify
+  $ cd chainify
+  $ cat >> chainify.py <<EOF
+  > from mercurial import cmdutil, error, extensions, exthelper, node, scmutil, state
+  > from hgext import rebase
+  > 
+  > eh = exthelper.exthelper()
+  > 
+  > extsetup = eh.finalextsetup
+  > cmdtable = eh.cmdtable
+  > 
+  > # Rebase calls addunfinished in uisetup, so we have to call it in extsetup.
+  > # Ideally there'd by an 'extensions.afteruisetup()' just like
+  > # 'extensions.afterloaded()' to allow nesting multiple commands.
+  > @eh.extsetup
+  > def _extsetup(ui):
+  >     state.addunfinished(
+  >         b'chainify',
+  >         b'chainify.state',
+  >         continueflag=True,
+  >         childopnames=[b'rebase'])
+  > 
+  > def _node(repo, arg):
+  >     return node.hex(scmutil.revsingle(repo, arg).node())
+  > 
+  > @eh.command(
+  >     b'chainify',
+  >     [(b'r', b'revs', [], b'revs to chain', b'REV'),
+  >      (b'', b'continue', False, b'continue op')],
+  >     b'chainify [-r REV] +',
+  >     inferrepo=True)
+  > def chainify(ui, repo, **opts):
+  >     """Rebases r1, r2, r3, etc. into a chain."""
+  >     with repo.wlock(), repo.lock():
+  >         cmdstate = state.cmdstate(repo, b'chainify.state')
+  >         if opts['continue']:
+  >             if not cmdstate.exists():
+  >                 raise error.Abort(b'no chainify in progress')
+  >         else:
+  >             cmdutil.checkunfinished(repo)
+  >             data = {
+  >                 b'tip': _node(repo, opts['revs'][0]),
+  >                 b'revs': b','.join(_node(repo, r) for r in opts['revs'][1:]),
+  >             }
+  >             cmdstate.save(1, data)
+  > 
+  >         data = cmdstate.read()
+  >         while data[b'revs']:
+  >             tip = data[b'tip']
+  >             revs = data[b'revs'].split(b',')
+  >             with state.delegating(repo, b'chainify', b'rebase'):
+  >                 ui.status(b'rebasing %s onto %s\n' % (revs[0][:12], tip[:12]))
+  >                 if state.ischildunfinished(repo, b'chainify', b'rebase'):
+  >                     rc = state.continuechild(ui, repo, b'chainify', b'rebase')
+  >                 else:
+  >                     rc = rebase.rebase(ui, repo, rev=[revs[0]], dest=tip)
+  >                 if rc and rc != 0:
+  >                     raise error.Abort(b'rebase failed (rc: %d)' % rc)
+  >             data[b'tip'] = _node(repo, b'tip')
+  >             data[b'revs'] = b','.join(revs[1:])
+  >             cmdstate.save(1, data)
+  >         cmdstate.delete()
+  >         ui.status(b'done chainifying\n')
+  > EOF
+
+  $ chainifypath=`pwd`/chainify.py
+  $ echo '[extensions]' >> $HGRCPATH
+  $ echo "chainify = $chainifypath" >> $HGRCPATH
+  $ echo "rebase =" >> $HGRCPATH
+
+  $ cd $TESTTMP
+  $ hg init a
+  $ cd a
+  $ echo base > base.txt
+  $ hg commit -Aqm 'base commit'
+  $ echo foo > file1
+  $ hg commit -Aqm 'add file'
+  $ hg co -q .^
+  $ echo bar > file2
+  $ hg commit -Aqm 'add other file'
+  $ hg co -q .^
+  $ echo foo2 > file1
+  $ hg commit -Aqm 'add conflicting file'
+  $ hg co -q .^
+  $ hg log --graph --template '{rev} {files}'
+  o  3 file1
+  |
+  | o  2 file2
+  |/
+  | o  1 file1
+  |/
+  @  0 base.txt
+  
+  $ hg chainify -r 8430cfdf77c2 -r f8596309dff8 -r a858b338b3e9
+  rebasing f8596309dff8 onto 8430cfdf77c2
+  rebasing 2:f8596309dff8 "add other file"
+  saved backup bundle to $TESTTMP/* (glob)
+  rebasing a858b338b3e9 onto 83c722183a8e
+  rebasing 2:a858b338b3e9 "add conflicting file"
+  merging file1
+  warning: conflicts while merging file1! (edit, then use 'hg resolve --mark')
+  unresolved conflicts (see 'hg resolve', then 'hg chainify --continue')
+  [1]
+  $ hg status --config commands.status.verbose=True
+  M file1
+  ? file1.orig
+  # The repository is in an unfinished *chainify* state.
+  
+  # Unresolved merge conflicts:
+  # 
+  #     file1
+  # 
+  # To mark files as resolved:  hg resolve --mark FILE
+  
+  # To continue:    hg chainify --continue
+  # To abort:       hg chainify --abort
+  
+  $ echo foo3 > file1
+  $ hg resolve --mark file1
+  (no more unresolved files)
+  continue: hg chainify --continue
+  $ hg chainify --continue
+  rebasing a858b338b3e9 onto 83c722183a8e
+  rebasing 2:a858b338b3e9 "add conflicting file"
+  saved backup bundle to $TESTTMP/* (glob)
+  done chainifying
+  $ hg log --graph --template '{rev} {files}'
+  o  3 file1
+  |
+  o  2 file2
+  |
+  o  1 file1
+  |
+  @  0 base.txt
+