--- a/mercurial/repository.py	Tue Oct 23 21:11:13 2018 +0900
+++ b/mercurial/repository.py	Tue Oct 09 22:02:01 2018 +0200
@@ -39,6 +39,10 @@
 REVISION_FLAGS_KNOWN = (
     REVISION_FLAG_CENSORED | REVISION_FLAG_ELLIPSIS | REVISION_FLAG_EXTSTORED)
 
+CG_DELTAMODE_STD = b'default'
+CG_DELTAMODE_PREV = b'previous'
+CG_DELTAMODE_FULL = b'fulltext'
+
 class ipeerconnection(interfaceutil.Interface):
     """Represents a "connection" to a repository.
 

--- a/mercurial/revlog.py	Tue Oct 23 21:11:13 2018 +0900
+++ b/mercurial/revlog.py	Tue Oct 09 22:02:01 2018 +0200
@@ -2213,6 +2213,12 @@
         if nodesorder is None and not self._generaldelta:
             nodesorder = 'storage'
 
+        deltamode = repository.CG_DELTAMODE_STD
+        if deltaprevious:
+            deltamode = repository.CG_DELTAMODE_PREV
+        elif not self._storedeltachains:
+            deltamode = repository.CG_DELTAMODE_FULL
+
         return storageutil.emitrevisions(
             self, nodes, nodesorder, revlogrevisiondelta,
             deltaparentfn=self.deltaparent,
@@ -2220,10 +2226,9 @@
             rawsizefn=self.rawsize,
             revdifffn=self.revdiff,
             flagsfn=self.flags,
-            sendfulltext=not self._storedeltachains,
+            deltamode=deltamode,
             revisiondata=revisiondata,
-            assumehaveparentrevisions=assumehaveparentrevisions,
-            deltaprevious=deltaprevious)
+            assumehaveparentrevisions=assumehaveparentrevisions)
 
     DELTAREUSEALWAYS = 'always'
     DELTAREUSESAMEREVS = 'samerevs'

--- a/mercurial/utils/storageutil.py	Tue Oct 23 21:11:13 2018 +0900
+++ b/mercurial/utils/storageutil.py	Tue Oct 09 22:02:01 2018 +0200
@@ -22,6 +22,7 @@
     error,
     mdiff,
     pycompat,
+    repository,
 )
 
 _nullhash = hashlib.sha1(nullid)
@@ -269,9 +270,8 @@
 
 def emitrevisions(store, nodes, nodesorder, resultcls, deltaparentfn=None,
                   candeltafn=None, rawsizefn=None, revdifffn=None, flagsfn=None,
-                  sendfulltext=False,
-                  revisiondata=False, assumehaveparentrevisions=False,
-                  deltaprevious=False):
+                  deltamode=repository.CG_DELTAMODE_STD,
+                  revisiondata=False, assumehaveparentrevisions=False):
     """Generic implementation of ifiledata.emitrevisions().
 
     Emitting revision data is subtly complex. This function attempts to
@@ -322,14 +322,17 @@
        Callable receiving a revision number and returns the integer flags
        value for it. If not defined, flags value will be 0.
 
-    ``sendfulltext``
+    ``deltamode``
+       constaint on delta to be sent:
+       * CG_DELTAMODE_STD  - normal mode, try to reuse storage deltas,
+       * CG_DELTAMODE_PREV - only delta against "prev",
+       * CG_DELTAMODE_FULL - only issue full snapshot.
+
        Whether to send fulltext revisions instead of deltas, if allowed.
 
     ``nodesorder``
     ``revisiondata``
     ``assumehaveparentrevisions``
-    ``deltaprevious``
-       See ``ifiledata.emitrevisions()`` interface documentation.
     """
 
     fnode = store.node
@@ -345,7 +348,7 @@
 
     prevrev = None
 
-    if deltaprevious or assumehaveparentrevisions:
+    if deltamode == repository.CG_DELTAMODE_PREV or assumehaveparentrevisions:
         prevrev = store.parentrevs(revs[0])[0]
 
     # Set of revs available to delta against.
@@ -364,11 +367,11 @@
             deltaparentrev = nullrev
 
         # Forced delta against previous mode.
-        if deltaprevious:
+        if deltamode == repository.CG_DELTAMODE_PREV:
             baserev = prevrev
 
         # We're instructed to send fulltext. Honor that.
-        elif sendfulltext:
+        elif deltamode == repository.CG_DELTAMODE_FULL:
             baserev = nullrev
 
         # There is a delta in storage. We try to use that because it
@@ -427,7 +430,8 @@
                         baserevisionsize = len(store.revision(baserev,
                                                               raw=True))
 
-            elif baserev == nullrev and not deltaprevious:
+            elif (baserev == nullrev
+                    and deltamode != repository.CG_DELTAMODE_PREV):
                 revision = store.revision(node, raw=True)
                 available.add(rev)
             else: