diff --git a/contrib/python3-whitelist b/contrib/python3-whitelist
--- a/contrib/python3-whitelist
+++ b/contrib/python3-whitelist
@@ -450,6 +450,7 @@
 test-sparse.t
 test-split.t
 test-ssh-clone-r.t
+test-ssh-proto-unbundle.t
 test-ssh-proto.t
 test-sshserver.py
 test-stack.t
diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -782,6 +782,13 @@
         if res is None:
             res = ''
 
+        if size == -1 and res == '':
+            # Suppress pointless read(-1) calls that return
+            # nothing. These happen _a lot_ on Python 3, and there
+            # doesn't seem to be a better workaround to have matching
+            # Python 2 and 3 behavior. :(
+            return
+
         if self.logdataapis:
             self.fh.write('%s> read(%d) -> %d' % (self.name, size, len(res)))
 
diff --git a/tests/test-ssh-proto-unbundle.t b/tests/test-ssh-proto-unbundle.t
--- a/tests/test-ssh-proto-unbundle.t
+++ b/tests/test-ssh-proto-unbundle.t
@@ -93,7 +93,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 115:
   e>     abort: incompatible Mercurial client; bundle2 required\n
   e>     (see https://www.mercurial-scm.org/wiki/IncompatibleClient)\n
@@ -144,7 +143,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 115:
   e>     abort: incompatible Mercurial client; bundle2 required\n
   e>     (see https://www.mercurial-scm.org/wiki/IncompatibleClient)\n
@@ -274,7 +272,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 196:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -331,7 +328,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 196:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -402,7 +398,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 218:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -460,7 +455,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 218:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -532,7 +526,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 202:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -589,7 +582,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 202:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -660,7 +652,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 206:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -718,7 +709,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 206:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -790,7 +780,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 232:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -850,7 +839,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 232:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -924,7 +912,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 193:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -981,7 +968,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 193:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1052,7 +1038,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 218:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1112,7 +1097,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 218:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1186,7 +1170,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 216:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1246,7 +1229,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 216:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1326,7 +1308,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 212:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1384,7 +1365,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 212:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1457,7 +1437,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 212:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1515,7 +1494,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 212:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1590,7 +1568,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 230:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1650,7 +1627,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 230:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1733,7 +1709,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 273:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1797,7 +1772,6 @@
   o> read(1) -> 1: 0
   result: 0
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 273:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1875,7 +1849,6 @@
   o> read(1) -> 1: 1
   result: 1
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 100:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -1928,7 +1901,6 @@
   o> read(1) -> 1: 1
   result: 1
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 100:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -2007,7 +1979,6 @@
   o> read(1) -> 1: 1
   result: 1
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 152:
   e>     adding changesets\n
   e>     adding manifests\n
@@ -2064,7 +2035,6 @@
   o> read(1) -> 1: 1
   result: 1
   remote output: 
-  o> read(-1) -> 0: 
   e> read(-1) -> 152:
   e>     adding changesets\n
   e>     adding manifests\n
diff --git a/tests/test-ssh-proto.t b/tests/test-ssh-proto.t
--- a/tests/test-ssh-proto.t
+++ b/tests/test-ssh-proto.t
@@ -1154,7 +1154,6 @@
   i>     hello\n
   o> readline() -> 1:
   o>     \n
-  o> read(-1) -> 0: 
   e> read(-1) -> 42:
   e>     cannot upgrade protocols multiple times\n
   e>     -\n
@@ -1246,7 +1245,6 @@
   i>     invalid\n
   o> readline() -> 1:
   o>     \n
-  o> read(-1) -> 0: 
   e> read(-1) -> 46:
   e>     malformed handshake protocol: missing hello\n
   e>     -\n
@@ -1266,7 +1264,6 @@
   i>     invalid\n
   o> readline() -> 1:
   o>     \n
-  o> read(-1) -> 0: 
   e> read(-1) -> 48:
   e>     malformed handshake protocol: missing between\n
   e>     -\n
@@ -1288,7 +1285,6 @@
   i>     invalid\n
   o> readline() -> 1:
   o>     \n
-  o> read(-1) -> 0: 
   e> read(-1) -> 49:
   e>     malformed handshake protocol: missing pairs 81\n
   e>     -\n