[SOLVED] Indy: check if client is effectively connected
lazarus / 38-Networking and Web Programming

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torbente · 2021-02-28 19:22

Hi everyone
Using Lazarus 2.0.10 on windows 8.1

I use the following procedure to periodically check if the client
receives a line from the server:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Procedure
ReadClientLines();var  linea : string;Beginif not
ClientChannel.Connected then exit;try   if
ClientChannel.IOHandler.InputBufferIsEmpty then      begin     
ClientChannel.IOHandler.CheckForDataOnSource(10);      if
ClientChannel.IOHandler.InputBufferIsEmpty then Exit;      end; 
 While not ClientChannel.IOHandler.InputBufferIsEmpty do      begin  
  Linea := ClientChannel.IOHandler.ReadLn(IndyTextEncoding_UTF8);    
  ....      end;Except on E:Exception do 
 ReportTheError(E.Message);End;
But if the internet connection just ends (i turn off the wifi
connection to test), the application hangs in this procedure.

How i could prevent this?

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krolikbest · 2021-03-01 07:03

e.g. use ping in separate thread

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engkin · 2021-03-01 16:21

I don't know where it hangs in your code, but I'll assume inside the
while loop. Simply use the other form of ReadLn that has a timeout
parameter.

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Remy Lebeau · 2021-03-01 19:02

Quote from: engkin on March 01, 2021, 05:21:26 pmSimply use the other
form of ReadLn that has a timeout parameter.

Or, you can set the IOHandler's ReadTimeout property beforehand. And
check the IOHandler's ReadLnTimedOut property after calling ReadLn().

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Remy Lebeau · 2021-03-01 19:11

Quote from: torbente on February 28, 2021, 08:22:41 pmCode: Pascal
 [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i if not
ClientChannel.Connected then exit;try   if
ClientChannel.IOHandler.InputBufferIsEmpty then      begin     
ClientChannel.IOHandler.CheckForDataOnSource(10);
Note that Connected() performs a read operation internally (with a
0-second timeout), so calling CheckForDataOnSource() immediately
afterwards is a bit redundant.  The InputBuffer will either be empty
or not after Connected() returns.

Quote from: torbente on February 28, 2021, 08:22:41 pmBut if the
internet connection just ends (i turn off the wifi connection to
test), the application hangs in this procedure.

Makes sense, since an abnormal loss of connection is not reported by
the OS in a timely manner (it can take minutes/hours!), so until the
OS eventually times out internally and invalidates the connection,
Connected() will happily continue to return True, and ReadLn() will
happily wait for new data up to the specified timeout (via its
optional ATimeout parameter, or the IOHandler's ReadTimeout property -
neither of which you appear to be using).

Quote from: torbente on February 28, 2021, 08:22:41 pmHow i could
prevent this?

Just because Connected()/CheckForDataOnSource() report that data has
arrived does not guarantee that a complete line is available in the
InputBuffer.  You should assign a non-infinite timeout to the ReadLn()
operation itself.

You might also consider enabling TCP-level keepalives (via
ClientChannel.Socket.Binding.SetKeepAliveValues(), for instance) so
that the OS can timeout internally sooner rather than later if the
connection is lost unexpectedly.

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torbente · 2021-03-15 23:38

Ok, trying to improve the connection.

I now included a variable:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
ReadTimeOutTime : integer; //set to 50
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Procedure
ReadClientLines();var  linea : string;Beginif not
ClientChannel.Connected then exit;try   if
ClientChannel.IOHandler.InputBufferIsEmpty then      begin     
ClientChannel.IOHandler.CheckForDataOnSource(10);      if
ClientChannel.IOHandler.InputBufferIsEmpty then Exit;      end; 
 While not ClientChannel.IOHandler.InputBufferIsEmpty do      begin  
  ClientChannel.ReadTimeout:=ReadTimeOutTime ;      Linea :=
ClientChannel.IOHandler.ReadLn(IndyTextEncoding_UTF8);            
....      end;Except on E:Exception do 
 ReportTheError(E.Message);End;
This helps to keep connection with slow peers, but how i could detect
if the readtimeout expired? How is...

QuoteOr, you can set the IOHandler's ReadTimeout property beforehand.
And check the IOHandler's ReadLnTimedOut property after calling
ReadLn().
... this implemented?

QuoteYou might also consider enabling TCP-level keepalives (via
ClientChannel.Socket.Binding.SetKeepAliveValues(), for instance) so
that the OS can timeout internally sooner rather than later if the
connection is lost unexpectedly.
The system includes a ping system every 5 seconds to keep connections
data updated.

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Remy Lebeau · 2021-03-16 02:43

Quote from: torbente on March 16, 2021, 12:38:50 amI now included a
variable:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
ReadTimeOutTime : integer; //set to 50
FYI, you can pass that variable directly to ReadLn(), you don't have
to reset the ReadTimeout property every time:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Procedure
ReadClientLines();var  linea : string;Begin  if not
ClientChannel.Connected then Exit;  try    if
ClientChannel.IOHandler.InputBufferIsEmpty then    begin     
ClientChannel.IOHandler.CheckForDataOnSource(10);      if
ClientChannel.IOHandler.InputBufferIsEmpty then Exit;    end;   
repeat      // consider setting the IOHandler.DefStringEncoding
beforehand, such as      // at connection open.  No need to pass it in
to ReadLn() every time...      //      // ClientChannel.ReadTimeout :=
ReadTimeOutTime;      Linea := ClientChannel.IOHandler.ReadLn('',
ReadTimeOutTime, -1, IndyTextEncoding_UTF8);      if
ClientChannel.IOHandler.ReadLnTimedOut then Exit;      // use Linea as
needed...    until ClientChannel.IOHandler.InputBufferIsEmpty; 
except    on E: Exception do      ReportTheError(E.Message);  end;end;
Quote from: torbente on March 16, 2021, 12:38:50 amThis helps to keep
connection with slow peers, but how i could detect if the readtimeout
expired?

By checking the IOHandler.ReadLnTimedOut property, like I said
earlier, and demonstrated above.

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torbente · 2021-04-10 21:22

I fixed all issues and now the app is running for days without hang 

I will open a new thread for a different answer.