[PATCH v3] socket: evaluate buffer size in recv / recvfrom

Alexander Turenko alexander.turenko at tarantool.org
Tue Aug 28 21:21:45 MSK 2018 

When size parameter is not passed to socket:recv() or socket:recvfrom()
it will call a) or b) on the socket to evaluate size of the buffer to
store the receiving datagram. Before this commit a datagram will be
truncated to 512 bytes in the case.

a) Linux: recv(fd, NULL, 0 , MSG_TRUNC | MSG_PEEK)
b) Mac OS: getsockopt(fd, SOL_SOCKET, SO_NREAD, &val, &len)

It is recommended to set 'size' parameter (size of the input buffer)
explicitly based on known message format and known network conditions
(say, set it less then MTU to prevent IP fragmentation, which can be
inefficient) or pass it from a configuration option of a library / an
application. The reason is that explicit buffer size provided allows to
avoid extra syscall to evaluate necessary buffer size.

When 'size' parameter is set explicitly for recv / recvfrom on a UDP
socket and the next datagram length is larger then the size, the
returned message will be truncated to the size provided and the rest of
the datagram will be discarded. Of course, the tail will not be
discarded in case of a TCP socket and will be available to read by the
next recv / recvfrom call.

Fixes #3619.
---

https://github.com/tarantool/tarantool/tree/Totktonada/gh-3619-socket-recvfrom-detect-a-cropped-packet
https://github.com/tarantool/tarantool/issues/3619

Changes from v2:

- code style fixes according to Vladimir's review;
- fixed zero-length datagrams receiving;
- removed zero-length datagram tests (were incorrect, see the previous
  email in the thread).

 src/lua/socket.c         |   3 +
 src/lua/socket.lua       |  62 +++++-
 test/app/socket.result   | 407 +++++++++++++++++++++++++++++++++++++++
 test/app/socket.test.lua | 140 ++++++++++++++
 4 files changed, 606 insertions(+), 6 deletions(-)

diff --git a/src/lua/socket.c b/src/lua/socket.c
index 1458d3ba4..84711b441 100644
--- a/src/lua/socket.c
+++ b/src/lua/socket.c
@@ -268,6 +268,9 @@ static const struct lbox_sockopt_reg so_opts[] = {
 #ifdef SO_PROTOCOL
 	{"SO_PROTOCOL",		SO_PROTOCOL,		1,	0, },
 #endif
+#ifdef SO_NREAD /* available on Darwin */
+	{"SO_NREAD",		SO_NREAD,		1,	0, },
+#endif
 
 	{"SO_TYPE",		SO_TYPE,		1,	0, },
 
diff --git a/src/lua/socket.lua b/src/lua/socket.lua
index 06306eae2..04e99a668 100644
--- a/src/lua/socket.lua
+++ b/src/lua/socket.lua
@@ -86,8 +86,12 @@ local function check_socket(socket)
     end
 end
 
-local function make_socket(fd)
-    local socket = { _gc_socket = ffi.new(gc_socket_t, { fd = fd }) }
+local function make_socket(fd, itype)
+    assert(itype ~= nil)
+    local socket = {
+        _gc_socket = ffi.new(gc_socket_t, { fd = fd }),
+        itype = itype,
+    }
     return setmetatable(socket, socket_mt)
 end
 
@@ -590,7 +594,7 @@ local function socket_accept(self)
         self._errno = boxerrno()
         return nil
     end
-    local client = make_socket(client_fd)
+    local client = make_socket(client_fd, self.itype)
     if not client:nonblock(true) then
         client:close()
         return
@@ -770,6 +774,44 @@ local function socket_send(self, octets, flags)
     return tonumber(res)
 end
 
+-- Returns nil and sets EAGAIN when tries to determine the next datagram length
+-- and there are no datagrams in the receive queue.
+local function get_recv_size(self, size)
+    local fd = check_socket(self)
+
+    if size ~= nil then
+        return size
+    end
+
+    if self.itype ~= get_ivalue(internal.SO_TYPE, 'SOCK_DGRAM') then
+        return 512
+    end
+
+    -- Determine the next datagram length.
+    self._errno = nil
+    if jit.os == 'OSX' then
+        size = self:getsockopt('SOL_SOCKET', 'SO_NREAD')
+        -- recv() with zero length buffer is always successful on Mac OS (at
+        -- least for valid UDP sockets). The assignment of size to 1 below
+        -- allows to handle 'no datagram' situation consistently: returning nil
+        -- and setting EAGAIN.
+        if size == 0 then
+            size = 1
+        end
+    else
+        local iflags = get_iflags(internal.SEND_FLAGS, {'MSG_TRUNC', 'MSG_PEEK'})
+        assert(iflags ~= nil)
+        size = tonumber(ffi.C.recv(fd, nil, 0, iflags))
+    end
+
+    if size == -1 then
+        self._errno = boxerrno()
+        return nil
+    end
+
+    return size
+end
+
 local function socket_recv(self, size, flags)
     local fd = check_socket(self)
     local iflags = get_iflags(internal.SEND_FLAGS, flags)
@@ -778,7 +820,11 @@ local function socket_recv(self, size, flags)
         return nil
     end
 
-    size = size or 512
+    size = get_recv_size(self, size)
+    if size == nil then
+        return nil
+    end
+
     self._errno = nil
     local buf = ffi.new("char[?]", size)
 
@@ -799,7 +845,11 @@ local function socket_recvfrom(self, size, flags)
         return nil
     end
 
-    size = size or 512
+    size = get_recv_size(self, size)
+    if size == nil then
+        return nil
+    end
+
     self._errno = nil
     local res, from = internal.recvfrom(fd, size, iflags)
     if res == nil then
@@ -860,7 +910,7 @@ local function socket_new(domain, stype, proto)
 
     local fd = ffi.C.socket(idomain, itype, iproto)
     if fd >= 0 then
-        local socket = make_socket(fd)
+        local socket = make_socket(fd, itype)
         if not socket:nonblock(true) then
             socket:close()
         else
diff --git a/test/app/socket.result b/test/app/socket.result
index b4d4a9a78..f2c73b305 100644
--- a/test/app/socket.result
+++ b/test/app/socket.result
@@ -2230,6 +2230,413 @@ s:close()
 ---
 - 1
 ...
+--------------------------------------------------------------------------------
+-- gh-3619: socket.recvfrom crops UDP packets
+--------------------------------------------------------------------------------
+receiving_socket = socket('AF_INET', 'SOCK_DGRAM', 'udp')
+---
+...
+receiving_socket:bind('127.0.0.1', 0)
+---
+- true
+...
+receiving_socket_port = receiving_socket:name().port
+---
+...
+sending_socket = socket('AF_INET', 'SOCK_DGRAM', 'udp')
+---
+...
+-- case: recv, no datagram
+received_message = receiving_socket:recv()
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == nil -- expected true
+---
+- true
+...
+received_message
+---
+- null
+...
+e == errno.EAGAIN -- expected true
+---
+- true
+...
+-- case: recvfrom, no datagram
+received_message, from = receiving_socket:recvfrom()
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == nil -- expected true
+---
+- true
+...
+received_message
+---
+- null
+...
+from == nil -- expected true
+---
+- true
+...
+from
+---
+- null
+...
+e == errno.EAGAIN -- expected true
+---
+- true
+...
+-- case: recv, no datagram, explicit size
+received_message = receiving_socket:recv(512)
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == nil -- expected true
+---
+- true
+...
+received_message
+---
+- null
+...
+e == errno.EAGAIN -- expected true
+---
+- true
+...
+-- case: recvfrom, no datagram, explicit size
+received_message, from = receiving_socket:recvfrom(512)
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == nil -- expected true
+---
+- true
+...
+received_message
+---
+- null
+...
+from == nil -- expected true
+---
+- true
+...
+from
+---
+- null
+...
+e == errno.EAGAIN -- expected true
+---
+- true
+...
+message_len = 1025
+---
+...
+message = string.rep('x', message_len)
+---
+...
+-- case: recv, non-zero length datagram, the buffer size should be evaluated
+sending_socket:sendto('127.0.0.1', receiving_socket_port, message)
+---
+- 1025
+...
+received_message = receiving_socket:recv()
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == message -- expected true
+---
+- true
+...
+received_message:len()
+---
+- 1025
+...
+received_message
+---
+- xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+...
+e == 0 -- expected true
+---
+- true
+...
+e
+---
+- 0
+...
+-- case: recvfrom, non-zero length datagram, the buffer size should be
+-- evaluated
+sending_socket:sendto('127.0.0.1', receiving_socket_port, message)
+---
+- 1025
+...
+received_message, from = receiving_socket:recvfrom()
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == message -- expected true
+---
+- true
+...
+received_message:len()
+---
+- 1025
+...
+received_message
+---
+- xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+...
+from ~= nil -- expected true
+---
+- true
+...
+from.host == '127.0.0.1' -- expected true
+---
+- true
+...
+e == 0 -- expected true
+---
+- true
+...
+e
+---
+- 0
+...
+-- case: recv truncates a datagram larger then the buffer of an explicit size
+sending_socket:sendto('127.0.0.1', receiving_socket_port, message)
+---
+- 1025
+...
+received_message = receiving_socket:recv(512)
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == message:sub(1, 512) -- expected true
+---
+- true
+...
+received_message:len() == 512 -- expected true
+---
+- true
+...
+received_message
+---
+- xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+...
+received_message:len()
+---
+- 512
+...
+e == 0 -- expected true
+---
+- true
+...
+e
+---
+- 0
+...
+-- we set the different message to ensure the tail of the previous datagram was
+-- discarded
+message_len = 1025
+---
+...
+message = string.rep('y', message_len)
+---
+...
+-- case: recvfrom truncates a datagram larger then the buffer of an explicit size
+sending_socket:sendto('127.0.0.1', receiving_socket_port, message)
+---
+- 1025
+...
+received_message, from = receiving_socket:recvfrom(512)
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == message:sub(1, 512) -- expected true
+---
+- true
+...
+received_message:len() == 512 -- expected true
+---
+- true
+...
+received_message
+---
+- yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy
+...
+received_message:len()
+---
+- 512
+...
+from ~= nil -- expected true
+---
+- true
+...
+from.host == '127.0.0.1' -- expected true
+---
+- true
+...
+e == 0 -- expected true
+---
+- true
+...
+e
+---
+- 0
+...
+receiving_socket:close()
+---
+- true
+...
+sending_socket:close()
+---
+- true
+...
+message_len = 513
+---
+...
+message = string.rep('x', message_len)
+---
+...
+listening_socket = socket('AF_INET', 'SOCK_STREAM', 'tcp')
+---
+...
+listening_socket:setsockopt('SOL_SOCKET', 'SO_REUSEADDR', true)
+---
+- true
+...
+listening_socket:bind('127.0.0.1', 0)
+---
+- true
+...
+listening_socket:listen()
+---
+- true
+...
+listening_socket_port = listening_socket:name().port
+---
+...
+sending_socket = socket('AF_INET', 'SOCK_STREAM', 'tcp')
+---
+...
+sending_socket:sysconnect('127.0.0.1', listening_socket_port) or errno() == errno.EINPROGRESS
+---
+- true
+...
+receiving_socket = listening_socket:accept()
+---
+...
+sending_socket:write(message)
+---
+- 513
+...
+-- case: recvfrom reads first 512 bytes from the message with tcp
+received_message, from = receiving_socket:recvfrom()
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == message:sub(1, 512) -- expected true
+---
+- true
+...
+received_message:len() == 512 -- expected true
+---
+- true
+...
+received_message
+---
+- xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+...
+received_message:len()
+---
+- 512
+...
+from == nil -- expected true
+---
+- true
+...
+from
+---
+- null
+...
+e == 0 -- expected true
+---
+- true
+...
+e
+---
+- 0
+...
+-- case: recvfrom does not discard the message tail with tcp
+received_message, from = receiving_socket:recvfrom()
+---
+...
+e = receiving_socket:errno()
+---
+...
+received_message == message:sub(513, 513) -- expected true
+---
+- true
+...
+received_message:len() == 1 -- expected true
+---
+- true
+...
+received_message
+---
+- x
+...
+received_message:len()
+---
+- 1
+...
+from == nil -- expected true
+---
+- true
+...
+from
+---
+- null
+...
+e == 0 -- expected true
+---
+- true
+...
+e
+---
+- 0
+...
+receiving_socket:close()
+---
+- true
+...
+sending_socket:close()
+---
+- true
+...
+listening_socket:close()
+---
+- true
+...
 test_run:cmd("clear filter")
 ---
 - true
diff --git a/test/app/socket.test.lua b/test/app/socket.test.lua
index f6539438b..840f042e4 100644
--- a/test/app/socket.test.lua
+++ b/test/app/socket.test.lua
@@ -736,4 +736,144 @@ sc:connect(host, port)
 sc:close()
 s:close()
 
+--------------------------------------------------------------------------------
+-- gh-3619: socket.recvfrom crops UDP packets
+--------------------------------------------------------------------------------
+
+receiving_socket = socket('AF_INET', 'SOCK_DGRAM', 'udp')
+receiving_socket:bind('127.0.0.1', 0)
+receiving_socket_port = receiving_socket:name().port
+sending_socket = socket('AF_INET', 'SOCK_DGRAM', 'udp')
+
+-- case: recv, no datagram
+received_message = receiving_socket:recv()
+e = receiving_socket:errno()
+received_message == nil -- expected true
+received_message
+e == errno.EAGAIN -- expected true
+
+-- case: recvfrom, no datagram
+received_message, from = receiving_socket:recvfrom()
+e = receiving_socket:errno()
+received_message == nil -- expected true
+received_message
+from == nil -- expected true
+from
+e == errno.EAGAIN -- expected true
+
+-- case: recv, no datagram, explicit size
+received_message = receiving_socket:recv(512)
+e = receiving_socket:errno()
+received_message == nil -- expected true
+received_message
+e == errno.EAGAIN -- expected true
+
+-- case: recvfrom, no datagram, explicit size
+received_message, from = receiving_socket:recvfrom(512)
+e = receiving_socket:errno()
+received_message == nil -- expected true
+received_message
+from == nil -- expected true
+from
+e == errno.EAGAIN -- expected true
+
+message_len = 1025
+message = string.rep('x', message_len)
+
+-- case: recv, non-zero length datagram, the buffer size should be evaluated
+sending_socket:sendto('127.0.0.1', receiving_socket_port, message)
+received_message = receiving_socket:recv()
+e = receiving_socket:errno()
+received_message == message -- expected true
+received_message:len()
+received_message
+e == 0 -- expected true
+e
+
+-- case: recvfrom, non-zero length datagram, the buffer size should be
+-- evaluated
+sending_socket:sendto('127.0.0.1', receiving_socket_port, message)
+received_message, from = receiving_socket:recvfrom()
+e = receiving_socket:errno()
+received_message == message -- expected true
+received_message:len()
+received_message
+from ~= nil -- expected true
+from.host == '127.0.0.1' -- expected true
+e == 0 -- expected true
+e
+
+-- case: recv truncates a datagram larger then the buffer of an explicit size
+sending_socket:sendto('127.0.0.1', receiving_socket_port, message)
+received_message = receiving_socket:recv(512)
+e = receiving_socket:errno()
+received_message == message:sub(1, 512) -- expected true
+received_message:len() == 512 -- expected true
+received_message
+received_message:len()
+e == 0 -- expected true
+e
+
+-- we set the different message to ensure the tail of the previous datagram was
+-- discarded
+message_len = 1025
+message = string.rep('y', message_len)
+
+-- case: recvfrom truncates a datagram larger then the buffer of an explicit size
+sending_socket:sendto('127.0.0.1', receiving_socket_port, message)
+received_message, from = receiving_socket:recvfrom(512)
+e = receiving_socket:errno()
+received_message == message:sub(1, 512) -- expected true
+received_message:len() == 512 -- expected true
+received_message
+received_message:len()
+from ~= nil -- expected true
+from.host == '127.0.0.1' -- expected true
+e == 0 -- expected true
+e
+
+receiving_socket:close()
+sending_socket:close()
+
+message_len = 513
+message = string.rep('x', message_len)
+
+listening_socket = socket('AF_INET', 'SOCK_STREAM', 'tcp')
+listening_socket:setsockopt('SOL_SOCKET', 'SO_REUSEADDR', true)
+listening_socket:bind('127.0.0.1', 0)
+listening_socket:listen()
+listening_socket_port = listening_socket:name().port
+sending_socket = socket('AF_INET', 'SOCK_STREAM', 'tcp')
+sending_socket:sysconnect('127.0.0.1', listening_socket_port) or errno() == errno.EINPROGRESS
+receiving_socket = listening_socket:accept()
+sending_socket:write(message)
+
+-- case: recvfrom reads first 512 bytes from the message with tcp
+received_message, from = receiving_socket:recvfrom()
+e = receiving_socket:errno()
+received_message == message:sub(1, 512) -- expected true
+received_message:len() == 512 -- expected true
+received_message
+received_message:len()
+from == nil -- expected true
+from
+e == 0 -- expected true
+e
+
+-- case: recvfrom does not discard the message tail with tcp
+received_message, from = receiving_socket:recvfrom()
+e = receiving_socket:errno()
+received_message == message:sub(513, 513) -- expected true
+received_message:len() == 1 -- expected true
+received_message
+received_message:len()
+from == nil -- expected true
+from
+e == 0 -- expected true
+e
+
+receiving_socket:close()
+sending_socket:close()
+listening_socket:close()
+
 test_run:cmd("clear filter")
-- 
2.17.1

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