WebReference.com - Chapter 4 from The mod_perl Developer's Cookbook, from Sams Publishing (5/6)

Technique

Use the aborted() method from the Apache::Connection class to detect the broken connection.

while (my $line = ) {
 # No need to waste cycles printing to nobody.
 last if $r->connection->aborted;
 print $line;
}

Comments

If you have your LogLevel set to info or below, every once in a while, you may see an information message similar to the following in your error log:

Broken pipe: client stopped connection before rflush completed

This represents the typical "user pressed stop" case—the end user got tired of waiting around and clicked the big red Stop button on the browser. Handling this case specifically used to be an issue with older versions of Apache, but recent versions handle it internally. For mod_perl users, the result of a broken connection is that all print() operations (and similar methods that write data to the client) become no-ops—no data is written to the socket once Apache detects that the connection has been terminated.

The aborted() method is provided by the Apache::Connection class as a way of letting you know that there is no longer an open connection to the client so you can take appropriate action. One problem with this approach is that it is dependent on how much data you send to the client—aborted() will only return true after Apache flushes its print buffers. If you have a long running process that only sends out a bit of data at a time, or you aren't printing at all, it may be a while before you can detect a client disconnect using aborted().

For more granular control, you can take advantage of the fileno() method, also provided by the Apache::Connection class, as in the following package:

package Cookbook::CheckConnection;
use IO::Select;
use strict;
sub client_connected {
 my $c = Apache->request->connection;
 # First, check to see whether Apache tripped the aborted flag.
 return if $c->aborted;
 # Now for the real test.
 # Check to see if we can read from the output file descriptor.
 my $s = IO::Select->new($c->fileno);
 return if $s->can_read(0);
 # Looks like the client is still there...
 return 1;
}
1;

The Cookbook::CheckConnection package offers the client_connected() utility function, which returns true if the client is still on the wire. After checking the status of the connection using aborted() (which has less overhead), the function checks the status of the output file descriptor using $r->connection->fileno() and the IO::Select module.

When called with no arguments, fileno() will return the output file descriptor, which can be used to check the status of the connection. Although calling $s->can_read() to check for a broken connection may seem counterintuitive, when the client aborts the connection Apache will populate the output file descriptor with a zero-byte packet available for reading.

You might use this new utility function from within a long-running or expensive process, such as when iterating through a fairly time-consuming database query, where the overhead of checking the client connection is worth the effort.

# Find shops in the vicinity of a given zipcode. We already
# have the latitude and longitude of the starting zip,
# as well as the search radius.
my $sql = qq(
 select shop.name, shops.address1, shops.city, shops.state,
     69*SQRT( POWER( (? - zips.latitude),2 ) +
     POWER( (? - zips.longitude),2 ) ) as distance
   from zipcodes zips, shops shops
   where
    69*SQRT( POWER( (? - zips.latitude),2 ) +
    POWER( (? - zips.longitude),2 ) )
    <= ?
   and substr(shops.zip,0,5)=zips.zipcode
   order by distance
);
$sth = $dbh->prepare($sql);
# No sense executing this SQL nightmare if the client aborted.
return OK unless Cookbook::CheckConnection::client_connected();
$sth->execute($lat, $long, $lat, $long, $radius);
my ($name, $address, $city, $state, $distance);
$sth->bind_columns(\($name, $address, $city, $state, $distance));
while ($sth->fetch) {
 # Do something with the results...
}

[previous] [next]

Copyright © Pearson Education and
Created: March 18, 2002
Revised: March 18, 2002

URL: https://webreference.com/programming/perl/cookbook/chap4/5.html