I'm observing an odd interaction of my module with Digest::MD5 that I haven't been able to figure out.

I'm implementing a read/write interface to a gzip variant using tied filehandles, with full seek/read/readline/tell support. I'm in the testing phase and nearly all of the tests I'm throwing at it seem to be working (basically performing exactly the same combinations of seek, read, <>, etc, on both my tied filehandle object (with the compressed file loaded) and a regular Perl filehandle opened on the uncompressed version, and comparing the output).

The one exception at this point is when I try to provide my tied filehandle to Digest::MD5's addfile() method. This doesn't work:

my $fh = B2B::BGZF::Reader->new_filehandle( $fn_bgzf );
my $hex = Digest::MD5->new()->addfile($fh)->hexdigest;
print $hex, "\n"; # prints d41d8cd98f00b204e9800998ecf8427e

[download]

The test returns almost immediately and it appears the hash returned is that of an empty string, so clearly the file is not actually being read. However, this works as expected:

my $fh = B2B::BGZF::Reader->new_filehandle( $fn_bgzf );
my $d = Digest::MD5->new();
my $buf = '';
$d->add($buf) while ( read $fh, $buf, 4096 );
my $hex = $d->hexdigest;
print $hex, "\n"; # prints the expected sum

[download]

It also works fine if I run the original code but substitute the pure-Perl module (although painfully slowly):

my $fh = B2B::BGZF::Reader->new_filehandle( $fn_bgzf );
my $hex = Digest::Perl::MD5->new()->addfile($fh)->hexdigest;
print $hex, "\n"; # prints the expected sum

[download]

I additionally tried it with Digest::SHA, and that works fine too:

my $fh = B2B::BGZF::Reader->new_filehandle( $fn_bgzf );
my $hex = Digest::SHA->new(1)->addfile($fh)->hexdigest;
print $hex, "\n"; # prints the expected sum

[download]

Basically, I have only been able to observe the issue when using my module with the XS implementation of Digest::MD5. Debugging is difficult because I'm not sure what code is actually being called (apparently not the addfile() method of Digest::base or any other actual perl code I can find on my system). I have no problem just using the explicit read()/add() form with Digest::MD5, but if this is an indication of a subtle bug in my code I'd like to work it out - I'm just not sure how to do so.

Any help with understanding what Digest::MD5::addfile() is actually calling under the hood or what might be going on here would be greatly appreciated.

In reply to Digest::MD5 addfile() w/ tied filehandle by jdv

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.