--- a/mercurial/debugcommands.py	Fri Mar 09 15:57:16 2018 +0100
+++ b/mercurial/debugcommands.py	Mon Mar 12 22:47:33 2018 +0900
@@ -2690,7 +2690,8 @@
     readavailable
     -------------
 
-    Read all available data from the server.
+    Close the write end of the connection and read all available data from
+    the server.
 
     If the connection to the server encompasses multiple pipes, we poll both
     pipes and read available data.
@@ -2835,17 +2836,9 @@
         elif action == 'close':
             peer.close()
         elif action == 'readavailable':
-            fds = [stdout.fileno(), stderr.fileno()]
-            try:
-                act = util.poll(fds)
-            except NotImplementedError:
-                # non supported yet case, assume all have data.
-                act = fds
-
-            if stdout.fileno() in act:
-                util.readpipe(stdout)
-            if stderr.fileno() in act:
-                util.readpipe(stderr)
+            stdin.close()
+            stdout.read()
+            stderr.read()
         elif action == 'readline':
             stdout.readline()
         else:

--- a/tests/test-ssh-proto-unbundle.t	Fri Mar 09 15:57:16 2018 +0100
+++ b/tests/test-ssh-proto-unbundle.t	Mon Mar 12 22:47:33 2018 +0900
@@ -93,6 +93,7 @@
   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
@@ -143,6 +144,7 @@
   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
@@ -260,6 +262,7 @@
   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
@@ -316,6 +319,7 @@
   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
@@ -386,6 +390,7 @@
   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
@@ -443,6 +448,7 @@
   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
@@ -514,6 +520,7 @@
   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
@@ -570,6 +577,7 @@
   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
@@ -640,6 +648,7 @@
   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
@@ -697,6 +706,7 @@
   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
@@ -768,6 +778,7 @@
   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
@@ -827,6 +838,7 @@
   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
@@ -900,6 +912,7 @@
   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
@@ -956,6 +969,7 @@
   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
@@ -1026,6 +1040,7 @@
   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
@@ -1085,6 +1100,7 @@
   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
@@ -1158,6 +1174,7 @@
   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
@@ -1217,6 +1234,7 @@
   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
@@ -1296,6 +1314,7 @@
   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
@@ -1353,6 +1372,7 @@
   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
@@ -1425,6 +1445,7 @@
   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
@@ -1482,6 +1503,7 @@
   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
@@ -1556,6 +1578,7 @@
   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
@@ -1615,6 +1638,7 @@
   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
@@ -1697,6 +1721,7 @@
   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
@@ -1760,6 +1785,7 @@
   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
@@ -1837,6 +1863,7 @@
   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
@@ -1889,6 +1916,7 @@
   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
@@ -1967,6 +1995,7 @@
   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
@@ -2023,6 +2052,7 @@
   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

--- a/tests/test-ssh-proto.t	Fri Mar 09 15:57:16 2018 +0100
+++ b/tests/test-ssh-proto.t	Mon Mar 12 22:47:33 2018 +0900
@@ -1138,6 +1138,7 @@
   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
@@ -1229,6 +1230,7 @@
   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
@@ -1248,6 +1250,7 @@
   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
@@ -1269,6 +1272,7 @@
   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