Re: DBI Mysql Driver Handle Mysteriously Changes!

Hello Duncan.

On Tue, Mar 25, 2014 at 04:42:56PM +1300, Duncan McEwan wrote:
> 
> First, a brief recap.  We have a web-based application that runs as a FosWiki 
> plugin under Apache/fcgid.

Could you post the fcgid configuration details for us?
https://httpd.apache.org/mod_fcgid/mod/mod_fcgid.html

> The database connection used by this application should be persistent
> due to the application calling DBI->connect_cached() on each invocation.

Persistent within a single backend process, yes.

> The new piece of information that I have discovered recently is that on a
> call to connect_cached() which *doesn't* return the cached database handle,
> the database driver handle reference passed into connect_cached() has changed.
> Since the dbh cache is obtained from "$drh->{CachedKids} ||= {}" I can now
> understand why the cached handled is not found!
> 
> What I can't understand is why the driver handle passed into connect_cached()
> has changed!

Neither can I.

> I've got 1000's of lines of debugging showing this happening.  Some of it
> is my own, inserted as print statements or cluck calls directly into DBI.pm
> and some of it the standard DBI debugging set to level 9.  There is way too
> much to include in this message, but I've included some small extracts below
> to illustrate what I am seeing.

What you've included doesn't show the drh changing.

I suggest you focus on that. Specifically the code path taken by the
request that notices that the drh has changed, _and_ the code path taken
by the _previous_ request _in the same process_.

> One thing I did just notice is that our application calls connect_cached()
> in the way shown in the DBI pod - that is:
> 
>       my $dbh = DBI->connect_cached(...)
> 
> I don't know enough about perl internals to know exactly what this does.
> But I did wonder if something like the following might be better given the
> persistent nature of our application provided by fcgid.
> 
>       my $dbi = new DBI;
>       my $dbh = $dbi->connect_cached(...)

No. Using new DBI (or DBI->new) isn't a valid way to use the DBI.
Just DBI->connect_cached is fine.

Tim.