Check abnormal connection drop before writing - tcl

Question

I am new to TCL scripting and writing a production code to open a socket to our server and write command and then read its output. Below is my code:
set chan [socket 123.345.45.33 23]
fconfigure $chan -buffering line
foreach item [dict keys $command] {
set cmd [dict get $command $item]
set res [Data_write $chan "get data $cmd"]
}
Where Data_write procedure is mentioned below:
proc Data_write { channel data } {
if {[eof $channel]} {
close $channel
ST_puts "Data_write: Error while writing to chanel"
return -1
} else {
puts $channel $data
return 0
}
}
I am not sure that how can we achive the validations below:
set chan [socket 123.345.45.33 23] - socket connection open is success
fconfigure on the channel is success
How to know before any write that any abnormal connection drop has happen on channel?

Answer 1

set chan [socket 123.345.45.33 23] - socket connection open is success
fconfigure on the channel is success
These are simple enough: if there's a failure, you get a Tcl error, which is a sort of exception. You can use catch or try to trap the error if you want:
try {
set chan [socket 123.345.45.33 23]
fconfigure $chan -buffering line
} on error msg {
puts "a serious problem happened: $msg"
# Maybe also exit here...
}
How to know before any write that any abnormal connection drop has happen on channel?
The bad news is that you can't know this. The OS itself won't really know this until you do one of two things: write to the channel, or read from the channel. (There are sometimes hints available, such as fileevent events firing, but they're not certain at all.) Instead, you need to trap the error when actually you do the write. See above for the general pattern.
Remember: Tcl operations throw errors when they fail, and EOF is not an error when reading, but is an error when writing.

Answer 2

Use socket -async and the readable and writeable fileevents to make the whole connection process event oriented.
In the writable event you can check the status of the connection using fconfigure $channel -error. If something failed in the connection, the socket is made writable and the error condition presented on the error property. If this is empty then you can configure the readable event and start processing data from the socket. In any readable event handler you should check for eof after reading and disable the readable event handler or close the socket once eof is seen as a socket in eof state becomes constantly readable.
This roughly works out to be:
proc OnWritable {chan} {
set err [fconfigure $chan -error]
if {$err ne "" } {
puts stderr "ERROR: $err"
exit 1
}
fconfigure $chan -blocking 0 -buffering none -encoding binary -translation lf
fileevent $chan writable {}
fileevent $chan readable [list OnReadable $chan]
}
proc OnReadable {chan} {
set data [read $chan]
puts "[string length $data]: $data"
if {[eof $chan]} {
fileevent $chan readable {}
puts "closed $chan"
set ::forever "closed"
}
}
set sock [socket -async $address $port]
fileevent $sock writable [list OnWriteable $sock]
vwait ::forever