**Purpose**
This page is dedicated to the asyncroneous socket connect, started by 'socket -async'.
See the [socket] page for a general description.
It also serves as communication page for development and compares current TCL 8.5.15, TCL 8.6.1 and future versions.
Async connect got more complicated in TCL 8.6, as multiple destination IPs are internally supported (due to IPV6 or DNS lookup resulting in multiple IPs).
**Command behaviour**
***socket -async***
'socket -async' ''host'' first does a syncroneous DNS lookup.
Then the connect is started as background process.
* In TCL8.5, this terminates without any interaction by background processes.
* In TCL8.6, the event loop or command invocation is required to check multiple IPs.
%|Version|Status|%
&|8.5.15|ok|&
&|8.5.16+|ok|&
&|8.6.1 unix|ok, requires event loop|&
&|8.6.1 win|only first IP (broken)|&
&|8.6.2+|ok, requires event loop|&
&|ideas|may be moved in own thread to not require event loop and not to pause between connect tries|&
***update,vwait***
Starting the event loop allows in TCL8.6 to continue with the next try or to fail finally
***fileevent writable***
Fires when async connect terminates with success or error.
%|Version|Status|%
&|8.5.15|ok, see bugs|&
&|8.5.16+|ok|&
&|8.6.1 win|only first IP (broken)|&
&|8.6.1 unix|ok|&
&|8.6.2+|ok|&
***fileevent readable***
Fires when async connect terminates with error.
%|Version|Status|%
&|8.5.15 unix|ok|&
&|8.5.15 win|only works when also writable event installed, see bugs|&
&|8.5.16+|ok|&
&|8.6.1 win|only first IP and only with writable event (broken)|&
&|8.6.1 unix|ok|&
&|8.6.2+|ok|&
***blocking gets,read,puts***
The async connect is terminated syncroneously.
On success, the operation is performed.
''On connect failure, the connect error is returned by the command. (still to discuss)''
%|Version|Status|%
&|8.5.15|ok|&
&|8.5.16+|ok|&
&|8.6.1 win|only first IP (broken)|&
&|8.6.1 unix|ok|&
&|8.6.2+|ok|&
***non blocking gets,read,puts***
The async connect state is checked or continued (next IP) in a non-blocking way.
Possible results:
%|Number|Condition|Action|%
&|NB1|async connect still in progress|write operation is buffered and sheduled for background flush.<
>Read operation returns empty string|&
&|NB2|async connect succeeded|operation is directly executed|&
&|NB3|async connect failed|connect error or other error is returned|&
Implementation status:
%|Version|Status|%
&|8.5.15 unix|?|&
&|8.5.15 win|NB3 buggy: operation is buffered, no error|&
&|8.5.16+ unix|?|&
&|8.5.16+ win|Bug not attacked jet|&
&|8.6.1 win|NB3 buggy: operation is buffered, no error|&
&|8.6.1 unix|?|&
&|8.6.2+ win|Ok (see TIP)|&
&|8.6.2+ unix|?|&
***close***
A close while connection is in progress or after a succesful connection should succeed.
A close after a failed connection should return the connect error if not jet reported.
%|Version|Status|%
&|8.5.15|ok. Small bug: returned error message might be the one of a pending flush. |&
&|8.5.16+|ok Small bug still present, which is acceptable|&
&|8.6.1|ok|&
&|8.6.2+|ok|&
***fconfigure***
Any fconfigure command on the socket continues the connect process.
%|Version|Status|%
&|8.6.1 win|no|&
&|8.6.1 unix|no|&
&|8.6.2+|ok|&
***fconfigure -error***
A final connect error should be returned by 'fconfigure -error'.
No error should be flagged while connection is running.
Implementation status:
%|Version|Status|%
&|8.5.15|ok|&
&|8.5.16+|ok|&
&|8.6.1 win|result of first tested IP (broken)|&
&|8.6.1 unix|The errors of all tested IPs show temporarely up|&
&|8.6.2+|ok|&
***fconfigure -sockname***
My own IP of the socket connection.
Returns list of IP, Name, Port.
Since IPV6, this value may change within connect tries from "127.0.0.1" to "::1".
Also the port may change.
Implementation status:
%|Version|Status|%
&|8.5.15 win|returns "0.0.0.0 0.0.0.0 51063"|&
&|8.5.16+|?|&
&|8.6.1 win|returns ":: :: 51070" if first try is IPV6, otherwise an IP V4 address|&
&|8.6.1 unix|?|&
&|8.6.2+|?|&
Idea:
The connection process should not be reflected.
A compatibility value or an error should returned while connecting.
If we reflect the connection process, we would have issues later, if we would push the connect process to an own thread.
***fconfigure -peername***
The destination IP.
Returns list of IP, Name, Port.
Since IPV6, this value may change within connect tries.
Also the port may change (if auto).
Implementation status:
%|Version|Status|%
&|8.5.15 win|returns information of tried IP while connecting. Error if connection failed|&
&|8.5.16+|?|&
&|8.6.1 win|returns information of first tried IP. Error if first connect try failed|&
&|8.6.1 unix|reflects connection process|&
&|8.6.2+|?|&
Idea:
The connection process should not be reflected.
A compatibility value or an error should returned while connecting.
**TIPs**
***Behaviour of fconfigure -peername, -sockname, -error, -connecting***
A discussion tip on the behaviour of those commands while connecting was drafted.
A new option '-connecting' should return 1 if connection is still in process.
Reason for that: there are use cases for socket -async without event loop.
Example: Within Web-Server verify multiple servers for connectivity.
Open many sockets in the background, do some other calculations, check if connect terminated by "-connecting".
***fconfigure -errorno***
A possibility to return the full POSIX information of a background error was drafted.
Two possible solutions:
* fconfigure -errordict: returns a dict as 'catch {} e d'
* fconfigure -invokeerror: throws the stored background error or does nothing if no error present
**Bugs**
***Win TCL8.6.1 only tries first IP***
[https://core.tcl.tk/tcl/tktview?name=13d3af3ad5%|%Bug 13d3af3ad5%|%]
TCL8.6.1 only tries the first of eventual multiple IP addresses to connect.
This may cause serious connect issues with IPV6.
This is fixed in branch
[https://core.tcl.tk/tcl/timeline?r=bug-13d3af3ad5&unhide%|%bug-13d3af3ad5%|%]
which also serves as main branch to fix all bugs in TCL8.6.1 and to test enhancements too.
***Win connect ignored***
[https://core.tcl.tk/tcl/info/336441ed59%|%Bug 336441ed59%|%]
When a connect terminates to quick so the notifier is not ready yet, the connect is ignored and thus it waits forever for it.
%|Version|Status|%
&|8.5.15|bug present|&
&|8.5.16+|fixed|&
&|8.6.1|bug present|&
&|8.6.2+|fixed in branch [https://core.tcl.tk/tcl/timeline?r=bug-13d3af3ad5&unhide%|%bug-13d3af3ad5%|%]|&
Test is timing dependent and may ignore issue on some machines.
***Empty error message on close on failed background flush***
[https://core.tcl.tk/tcl/info/97069ea11a%|%Bug 97069ea11a%|%]
%|Version|Status|%
&|8.5.15|bug present|&
&|8.5.16+|fixed|&
&|8.6.1|bug present|&
&|8.6.2+|fixed|&
<> Test proposal
The test is difficult, as an async connect must fail after a puts is issued on the channel.
Idea: write a dummy channel driver, which may be set to an error state by fconfigure -seterror and where the readbale/writable state may be set.
So one could:
======tcl
set h [open dummy]
fconfigure $h -seterror EWOULDBLOCK
fileevent $h writable {set x writable}
fconfigure $h -blocking 0
puts $h abc
fconfigure $h -setwritable 1
vwait x
catch {close $h} e d
======
<>
***No readable event on async socket connect failure***
[https://core.tcl.tk/tcl/info/581937ab1e%|%Bug 581937ab1e%|%]
%|Version|Status|%
&|8.5.15 win|bug present|&
&|8.5.16+ win|Fixed|&
&|8.6.1 win|bug present|&
&|8.6.2+ win|Fixed in [https://core.tcl.tk/tcl/timeline?r=bug-13d3af3ad5&unhide%|%bug-13d3af3ad5%|%], test in core-8-5-branch|&
**non-blocking operations do not fail after failed connect**
This is present on windows 8.5 and is not jet attacked.
**ToDo's**
***Clarify demanded functionality***
* When to throw which error?
* When does a non-blocking function fail ?
* What returns fconfigure -peername,-sockname ?
***Process test failures***
The branch of
[https://core.tcl.tk/tcl/timeline?r=bug-13d3af3ad5&unhide%|%bug-13d3af3ad5%|%]
requires fine tuning.
<> Current windows test failures
======none
==== socket-14.7.2 pending [socket -async] and blocking [gets], no listener FAILED
==== Contents of test case:
set sock [socket -async localhost [randport]]
catch {gets $sock} x
list $x [fconfigure $sock -error]
---- Result was:
{error reading "sock01B57690": connection refused} {}
---- Result should have been (glob matching):
{error reading "sock*": socket is not connected} {connection refused}
==== socket-14.7.2 FAILED
==== socket-14.11.0 pending [socket -async] and blocking [puts], no listener, no flush FAILED
==== Contents of test case:
set sock [socket -async localhost [randport]]
fconfigure $sock -blocking 0
puts $sock ok
fileevent $sock writable {set x 1}
vwait x
close $sock
---- Result was:
socket is not connected
---- Result should have been (exact matching):
broken pipe
==== socket-14.11.0 FAILED
==== socket-14.11.1 pending [socket -async] and blocking [puts], no listener, flush FAILED
==== Contents of test case:
set sock [socket -async localhost [randport]]
fconfigure $sock -blocking 0
puts $sock ok
flush $sock
fileevent $sock writable {set x 1}
vwait x
close $sock
---- Result was:
socket is not connected
---- Result should have been (exact matching):
broken pipe
==== socket-14.11.1 FAILED
======
<>
***return connection failure of non-blocking gets/puts on (win) 8.5***
In tcl 8.5 (win), read or puts after a connection failure do not fail.
This might be corrected or not by another bug fix.
No priority at the moment.
***prioritize connect errors and return most appropriate***
If a socket connect fails, the error in the latest connect stage should be returned.
This would prioritize "access denied" (e.g. socket in use) before "network unreachable" (no route).
Project stage for Win and Unix.
This is already implemented for Unix server sockets.
<>Tcl syntax | Arts and crafts of Tcl-Tk programming | Command | Networking | Channel