ConnectionPool: monitor idle connections (#236)

* ConnectionPool: monitor idle connections

When a connection is returned to the (read) pool,
add a monitor to it for receiving unexpected data (or EOF),
and kill / close the Connection object if any activity occurs
before the next write (when it should have simply been waiting idle in the pool)

per JuliaLang/MbedTLS.jl#145 (comment)

closes #214
closes #199
closes #220
closes JuliaWeb/GitHub.jl#106

*  - Encapsulate read|writebusy/sequence/count logic in new isbusy function.
 - Move close() on eof() || !isbusy() to new monitor_idle_connection function.
 - Make monitor_idle_connection() a noop for ::Connection{SSLContext}

* require Julia 0.6.3 #236 (comment)

REQUIRE

@@ -1,3 +1,3 @@
-julia 0.6.2
-MbedTLS 0.5.7
+julia 0.6.3
+MbedTLS 0.5.9
 IniFile

src/ConnectionPool.jl

@@ -138,6 +138,12 @@ Base.isopen(t::Transaction) = isopen(t.c) &&
                               t.c.readcount <= t.sequence &&
                               t.c.writecount <= t.sequence
 
+"""
+Is `c` currently in use or expecting a response to request already sent?
+"""
+isbusy(c::Connection) = isopen(c) && (c.writebusy || c.readbusy ||
+                                      c.writecount > c.readcount)
+
 function Base.eof(t::Transaction)
     @require isreadable(t) || !isopen(t)
     if bytesavailable(t) > 0
@@ -264,10 +270,32 @@ function IOExtras.closeread(t::Transaction)
     notify(t.c.readdone)                          ;@debug 2 "✉️  Read done:  $t"
     notify(poolcondition)
 
+    if !isbusy(t.c)
+        @schedule monitor_idle_connection(t.c)
+    end
+
     @ensure !isreadable(t)
     return
 end
 
+"""
+Wait for `c` to receive data or reach EOF.
+Close `c` on EOF or if response data arrives when no request was sent.
+"""
+function monitor_idle_connection(c::Connection)
+    if eof(c.io)                                  ;@debug 2 "💀  Closed:     $c"
+        close(c.io)
+    elseif !isbusy(c)                             ;@debug 1 "😈  Idle RX!!:  $c"
+        close(c.io)
+    end
+end
+
+function monitor_idle_connection(c::Connection{SSLContext})
+    # MbedTLS.jl monitors idle connections for TLS close_notify messages.
+    # https://github.com/JuliaWeb/MbedTLS.jl/pull/145
+end
+
+
 function Base.close(t::Transaction)
     close(t.c)
     if iswritable(t)