**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).

**Use cases**

***Background connect and notification about connect***

The typical use-case for background connect is to install a writable event to get notified about the connect.
If there is an additional connect timeout, this is canceled by the writable connect.

Typical code:
======tcl
proc Connected {aid h fromip toip} {
    # cancel timeout
    after cancel aid
    # check connect succes or fail
    set error [fconfigure $h -error]
    if {$error ne ""} {
        catch {close $h}
        return
    }
    # disable writable event as it will come again and again if nothing written here
    fileevent $h writable ""
    # do something with the socket
    puts $h "HELO"
    # install readable event to process reply
    fileevent $h readable Receive
}
proc Timeout {h} {
    # Connect timeout
    catch {close $h}
}
# Receive function is not shown here and may be derived from the example below
set h [socket -async $host $port]
set aid [after 10000 [namespace code [list Timeout $h]]
fileevent $h writable [namespace code [list Connected $aid]
vwait forever
======

***Background connect and only readevent***

If there is no need to get notified on a succesfull connect and no connect timeout needed, one may use a readable connect only.

Attention, this did not work in Windows before 8.5.16 due to bugs:

======tcl
proc Receive {h fromip toip} {
    # check connect succes or fail
    set error [fconfigure $h -error]
    if {$error ne ""} {
        catch {close $h}
        return
    }
    # get read data and process it
    if {[catch {gets $h} data]} {
        # read error
        catch {close $h}
        return
    }
    if {[eof $h]} {
        # other side disconnected
        catch {close $h}
        return
    }
    # now do something with the data...
}
set h [socket -async $host $port]
fileevent $h readable Read
# if a message is needed by the server after the connect, send it now non-blocking
# It will be automatically sent when the connect succeeds
fconfigure $h -blocking 0
puts $h "HELO"
flush $h
vwait forever
======

***async connect and blocking operation***

A use case is to start multiple connect, do something else and then process the connect state, all in a linear program without event queue.
An example is a test if multiple servers are alive.

Example program:
======tcl
set h [socket -async $host $port]
# do something else which needs time
# check if failed. Start also next try of multiple IPs of $host
set error [fconfigure $h -error]
if {$error ne""} {
    # connect failed
    catch {close $h}
    return
}
# do something else which needs time
# check if failed. Start also next try of multiple IPs of $host
set error [fconfigure $h -error]
if {$error ne""} {
    # connect failed
    catch {close $h}
    return
}
# nothing to do, so do the rest syncroneously
# this blocks !
if {[catch {
    puts $h "HELO"
    set Data [gets $h]
    close $h
} error] {
    # connect failed
    catch {close $h}
}
======

***async connect and no event queue***

This example requires the command 'fconfigure -connecting' which is contained in the proposed TIP.
It gives the state if the connection process is still running.
This allows to do the upper example without blocking commands.

Example program:
======tcl
set h [socket -async $host $port]
while {[fconfigure $h -connecting]} {
    # do something else which needs time
}
# connection process terminated - check if failed
set error [fconfigure $h -error]
if {$error ne ""} {
    # connect failed
    catch {close $h}
    return
}
# do something with the connected socket
======

**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.
It is not absolutely necessary, as all other socket commands also advance the connect process.

The event queue may also initiate a pending background flush when the socket is succesfully opened.

***close on error***

As a start point for all other commands:
if a failed async connect socket is not closed after the first reported error, bad things like unreported errors etc. may happen.

''Please'' close an async socket connect after the first reported error.

***fileevent writable***

Fires when async connect terminates with success or error.

'fconfigure -error' may be used in the event procedure to check if the connect was succesful.

%|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.

On a succesful connect, it fires only, if there is data received.

'fconfigure -error' may be used in the event procedure to check if the connect was succesful.

%|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,flush***

Remark: a puts may be delayed to a following flush.

The async connect is terminated syncroneously.

On success, the operation is performed.

On connect failure, the error "socket is not connected" is returned.
The reason for the connect failure may be investigated using ''fconfigure -error''.

%|Version|Status|%
&|8.5.15 unix|ok. Instead of "socket is not connected", "broken pipe" may be reported.|&
&|8.5.15 win|ok|&
&|8.5.16+ unix|ok|&
&|8.5.16+|ok|&
&|8.6.1 win|only first IP tested (broken).|&
&|8.6.1 unix|ok. Instead of "socket is not connected", "broken pipe" may be reported.|&
&|8.6.2+|ok|&


***non blocking gets,read,puts,flush***

Remark: a puts may be delayed to a following flush.

The async connect state is checked or continued (next IP) in a non-blocking way.

Eventual pending flush is executed in the background automatically when the connection is established and the event queue is running.

Possible results:

%|Number|Condition|Action|%
&|NB1|async connect still in progress|write operation is buffered and sheduled for background flush.<<br>>Read operation returns empty string|&
&|NB2|async connect succeeded|operation is directly executed|&
&|NB3|async connect failed|Error "socket is not connected" is returned|&

Implementation status:

%|Version|Status|%
&|8.5.15 unix|ok. Instead of "socket is not connected", "broken pipe" may be reported.|&
&|8.5.15 win|ok|&
&|8.5.16+ unix|ok. Instead of "socket is not connected", "broken pipe" may be reported.|&
&|8.5.16+ win|ok|&
&|8.6.1 win|only first IP (broken)|&
&|8.6.1 unix|ok. Instead of "socket is not connected", "broken pipe" may be reported.|&
&|8.6.2+ win|ok|&
&|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 succeeds.

If a background flush is pending (or already resulted in an error), an error may be shown.

%|Version|Status|%
&|8.5.15|ok. Empty error message may appear.|&
&|8.5.16+ unix|ok|&
&|8.5.16+ win|ok|&
&|8.6.1|ok. Empty error message may appear.|&
&|8.6.2+|ok|&


***eof***

eof should be active:
   *   After a read on a socket closed from the other side.
   *   never active with async sockets and may not be used to detect the connection status

%|Version|Status|%
&|8.5.15|ok|&
&|8.5.16+|ok|&
&|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 unix|ok.|&
&|8.5.15 win|ok. Small bug: Failed socket connect error is reported indefinitely|&
&|8.5.16+ unix|ok.|&
&|8.5.16+ win|ok. Small bug: Failed socket connect error is reported indefinitely|&
&|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|&

To fix the small bug, that a connect error is repeated indefinitively may introduce compatibility issues of programs which rely on that.

***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 (see TIP):
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 (see TIP):
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, specially 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|&

<<discussion>> 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
======

<<discussion>>

***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%|%]|&

**ToDo's**

***Clarify demanded functionality***

   * What returns fconfigure -peername,-sockname ?

***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.

***TclWinGetSockOpt() stubs entry may return wrong state***

The Win TCL stubs table contains an entry for '''TclWinGetSockOpt()''' which returns the info from '''getsockopt()'''.

In TCL8.5, the result of '''fconfigure -error''' was always the return value of the system call '''getsockopt()'''.
In TCL8.6, a connect failure is cached in a variable and returned by '''fconfigure -error'''.
Eventually, this should also be done by the routine called by the '''TclWinGetSockOpt''' stubs entry.

The purpose of this stub entry seams to be from the times of Windows 98 where a WinSock2.dll may not be present.
There are no known usage of this.
Thus it was decided to leave it as depreciated and to remove it for Tcl9.0.

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