--- a/mercurial/bundle2.py	Wed May 27 00:10:30 2015 -0700
+++ b/mercurial/bundle2.py	Wed May 27 00:19:16 2015 -0700
@@ -736,6 +736,29 @@
         if self._generated is not None:
             raise RuntimeError('part can only be consumed once')
         self._generated = False
+
+        if ui.debugflag:
+            msg = ['bundle2-output-part: "%s"' % self.type]
+            if not self.mandatory:
+                msg.append(' (advisory)')
+            nbmp = len(self.mandatoryparams)
+            nbap = len(self.advisoryparams)
+            if nbmp or nbap:
+                msg.append(' (params:')
+                if nbmp:
+                    msg.append(' %i mandatory' % nbmp)
+                if nbap:
+                    msg.append(' %i advisory' % nbmp)
+                msg.append(')')
+            if not self.data:
+                msg.append(' empty payload')
+            elif util.safehasattr(self.data, 'next'):
+                msg.append(' streamed payload')
+            else:
+                msg.append(' %i bytes payload' % len(self.data))
+            msg.append('\n')
+            ui.debug(''.join(msg))
+
         #### header
         if self.mandatory:
             parttype = self.type.upper()

--- a/tests/test-bundle2-format.t	Wed May 27 00:10:30 2015 -0700
+++ b/tests/test-bundle2-format.t	Wed May 27 00:19:16 2015 -0700
@@ -390,32 +390,39 @@
   bundle2-output: bundle parameter: 
   bundle2-output: start of parts
   bundle2-output: bundle part: "test:empty"
+  bundle2-output-part: "test:empty" (advisory) empty payload
   bundle2-output: part 0: "test:empty"
   bundle2-output: header chunk size: 17
   bundle2-output: closing payload chunk
   bundle2-output: bundle part: "test:empty"
+  bundle2-output-part: "test:empty" (advisory) empty payload
   bundle2-output: part 1: "test:empty"
   bundle2-output: header chunk size: 17
   bundle2-output: closing payload chunk
   bundle2-output: bundle part: "test:song"
+  bundle2-output-part: "test:song" (advisory) 178 bytes payload
   bundle2-output: part 2: "test:song"
   bundle2-output: header chunk size: 16
   bundle2-output: payload chunk size: 178
   bundle2-output: closing payload chunk
   bundle2-output: bundle part: "test:debugreply"
+  bundle2-output-part: "test:debugreply" (advisory) empty payload
   bundle2-output: part 3: "test:debugreply"
   bundle2-output: header chunk size: 22
   bundle2-output: closing payload chunk
   bundle2-output: bundle part: "test:math"
+  bundle2-output-part: "test:math" (advisory) (params: 2 mandatory 2 advisory) 2 bytes payload
   bundle2-output: part 4: "test:math"
   bundle2-output: header chunk size: 43
   bundle2-output: payload chunk size: 2
   bundle2-output: closing payload chunk
   bundle2-output: bundle part: "test:song"
+  bundle2-output-part: "test:song" (advisory) (params: 1 mandatory) empty payload
   bundle2-output: part 5: "test:song"
   bundle2-output: header chunk size: 29
   bundle2-output: closing payload chunk
   bundle2-output: bundle part: "test:ping"
+  bundle2-output-part: "test:ping" (advisory) empty payload
   bundle2-output: part 6: "test:ping"
   bundle2-output: header chunk size: 16
   bundle2-output: closing payload chunk
@@ -741,6 +748,7 @@
   bundle2-output: bundle parameter: 
   bundle2-output: start of parts
   bundle2-output: bundle part: "changegroup"
+  bundle2-output-part: "changegroup" (advisory) streamed payload
   bundle2-output: part 0: "changegroup"
   bundle2-output: header chunk size: 18
   bundling: 1/4 changesets (25.00%)