Strange behaviour when using glob in if condition

[yulivee07]

Hi fellow perlmonks,
I noticed a strange behaviour when using glob to expand filenames. I try to identify directories which contain nothing besides one special sort of file. I noticed that the return condition of my script failed to execute, even if glob returned something.

#!/usr/bin/perl -w

use strict;
use warnings;
use File::Basename;

sub remove_files_in_empty_directories($) {
  my ($directory) = @_;

  # do nothing if the directory is not empty
  # This is the problematic part 
  return if glob("$directory/*");

  # directory is empty, remove the .Ecurep-Lenovo* files
  foreach my $file ( glob("$directory/.EcuRep-Lenovo_*") ) {
     print $file,"\n";
  }
}


my $list_file = shift @ARGV;
die "list_file is missing - Usage: $0 list_file\n" unless $list_file;
open(F,"<",$list_file) || die "can not open $list_file: $!\n";

while (my $line = <F> ) {
  chomp $line;
  my $dir = dirname $line;

  remove_files_in_empty_directories($dir);
}

I pass this script a file containing a list of directories (filename is test):

/ecurep/hw/7/7042/21/EC/21EC3EC/.EcuRep-Lenovo_outofscope
/ecurep/hw/7/7042/21/EC/21EC41C/.EcuRep-Lenovo_outofscope
/ecurep/hw/7/7042/21/EC/21EC4DC/.EcuRep-Lenovo_outofscope

I call the script like this: perl test_perl test

Output:

/ecurep/hw/7/7042/21/EC/21EC4DC/.EcuRep-Lenovo_outofscope

This is kinda unexpected, as all three directories contain other directories. When I step through with the debugger, I can evaluate the glob with x, and it returns a list of directories every single time. If I evaluate x scalar glob("$directory/*") it also evaluates to the first entry from the list every time. But in case if the last pathname it steps further into the function anyway instead of executing my return statement. I cannot get my head wrapped around this - what is happening here?

Further testing of the problem:
If I delete the first line of the testfile, thereby removing the first directory from the list of directories to be one shorter, it works.

test2:

/ecurep/hw/7/7042/21/EC/21EC3EC/.EcuRep-Lenovo_outofscope
/ecurep/hw/7/7042/21/EC/21EC41C/.EcuRep-Lenovo_outofscope

perl test_perl test2 No Output (as expected)

File Permissions to the directories:

drwxr-s---    4 root     swsupt          512 Apr 08 04:53 21EC3EC
drwxr-s---    4 root     swsupt          512 Apr 08 05:24 21EC41C
drwxr-s---    5 root     swsupt          512 Apr 11 12:12 21EC4DC
[1:root@itcaix23:]/ecurep/tmp/perl_test # ls -l /ecurep/hw/7/7042/21/E
+C/21EC41C
total 0
-rw-r--r--    1 root     swsupt           25 Jun 24 2016  .EcuRep-Leno
+vo_outofscope
drwxr-s---    2 root     swsupt          512 Mar 31 09:06 2017-03-31
drwxr-s---    2 root     swsupt          512 Apr 07 09:05 2017-04-07
[1:root@itcaix23:]/ecurep/tmp/perl_test # ls -l /ecurep/hw/7/7042/21/E
+C/21EC4DC
total 0
-rw-------    1 root     swsupt           29 Apr 11 12:12 .EcuRep-Leno
+vo_outofscope
drwxr-s---    2 root     swsupt          512 Mar 28 09:48 2017-03-28
drwxr-s---    2 root     swsupt          512 Apr 04 09:49 2017-04-04
drwxr-s---    2 root     swsupt          512 Apr 11 09:48 2017-04-11
[1:root@itcaix23:]/ecurep/tmp/perl_test # ls -l /ecurep/hw/7/7042/21/E
+C/21EC3EC
total 0
-rw-r--r--    1 root     swsupt           25 Nov 04 05:26 .EcuRep-Leno
+vo_outofscope
drwxr-s---    2 root     swsupt          512 Mar 31 06:26 2017-03-31
drwxr-s---    2 root     swsupt          512 Apr 07 06:26 2017-04-07

I am working with perl on AIX here. I have tested with perl 5.20 on AIX7.2 and perl 5.8.8 on AIX6.1. The behaviour is the same.
I tried the following variations on the code (just substituting the problematic part):

# makes no difference
return if  glob "'${directory}/*'";

# with this construct it works!
my @files = glob("$directory/*");
return if scalar @files;

# doesn't work either - prints "hi hi <directory-name>
if ( glob("$directory/*") ) {
print "hi\n";
return;
}

I am not shure if I am misusing glob here of if there is a problem with my perl-version. Can someone help me understanding what is going on?

Kind regards, Yulivee

[haukex]

I looked into this further and was able to reproduce the issue using your code, and find an explanation and workaround. glob in scalar context acts like an iterator. However, the iterator state is attached to the glob call site, even when the argument to glob changes. Here's a simple way to reproduce the issue:

$ touch foo bar
$ perl -MData::Dump -e '
  sub myglob {scalar glob($_[0])}
  dd myglob($_) for qw/foo bar/   '
"foo"
undef

One might expect the output here to be "foo" and "bar". There is an excellent discussion in RT#123404, and it doesn't sound like this behavior is going to change, as potentially confusing as it is.

A simple workaround is to force list context:

return if ()=glob("$directory/*");

[LanX]

Brilliant analysis!

Sounds like we need a warning if glob is used in scalar context with a pattern holding interpolated variables. :/

I for my part will now only use for glob() constructs for looping over a static list.

[vrk]

Is it just a difference between scalar and list context? See the glob help: In list context (like my @files = glob(...)), you get a list of filenames, or the empty list if nothing matches. In scalar context, you get an iterator. When you evaluate the iterator, it yields the next matching filename, or undef when it reaches the end. Since your code works correctly when glob is in list context, maybe some of the glob calls in scalar context actually share an iterator?

[haukex]

# do nothing if the directory is not empty
return if glob("$directory/*");

glob skips filenames beginning with a dot, try return if glob("$directory/{*,.*}");

Update: Actually, upon re-reading the question I think I misunderstood what you are trying to do. Update 2: Yep, see my other reply for the solution.