Similar to the way AUTOLOAD can be used to define subroutines on demand, I am wondering if there is a way to tie a package's stash so that I can intercept access to variables in that package.
I've tried various permutations of the following idea, but none seem to work:
{package Tie::Stash;
use Tie::Hash;
BEGIN {our #ISA = 'Tie::StdHash'}
sub FETCH {
print "calling fetch\n";
}
}
{package Target}
BEGIN {tie %Target::, 'Tie::Stash'}
say $Target::x;
This dies with Bad symbol for scalar ... on the last line, without ever printing "calling fetch". If the say $Target::x; line is removed, the program runs and exits properly.
My guess is that the failure has to do with stashes being like, but not the same as hashes, so the standard tie mechanism is not working right (or it might just be that stash lookup never invokes tie magic).
Does anyone know if this is possible? Pure Perl would be best, but XS solutions are ok.
You're hitting a compile time internal error ("Bad symbol for scalar"), this happens while Perl is trying to work out what '$Target::x' should be, which you can verify by running a debugging Perl with:
perl -DT foo.pl
...
### 14:LEX_NORMAL/XOPERATOR ";\n"
### Pending identifier '$Target::x'
Bad symbol for scalar at foo.pl line 14.
I think the GV for '::Target' is replaced by something else when you tie() it, so that whatever eventually tries to get to its internal hash cannot. Given that tie() is a little bit of a mess, I suspect what you're trying to do won't work, which is also suggested by this (old) set of exchanges on p5p:
https://groups.google.com/group/perl.perl5.porters/browse_thread/thread/f93da6bde02a91c0/ba43854e3c59a744?hl=en&ie=UTF-8&q=perl+tie+stash#ba43854e3c59a744
A little late to the question, but although it's not possible to use tie to do this, Variable::Magic allows you to attach magic to a stash and thereby achieve something similar.
Related
I've been learning about filehandles in Perl, and I was curious to see if there's a way to alter the source code of a program as it's running. For example, I created a script named "dynamic.pl" which contained the following:
use strict;
use warnings;
open(my $append, ">>", "dynamic.pl");
print $append "print \"It works!!\\n\";\n";
This program adds the line
print "It works!!\n";
to the end of it's own source file, and I hoped that once that line was added, it would then execute and output "It works!!"
Well, it does correctly append the line to the source file, but it doesn't execute it then and there.
So I assume therefore that when perl executes a program that it loads it to memory and runs it from there, but my question is, is there a way to access this loaded version of the program so you can have a program that can alter itself as you run it?
The missing piece you need is eval EXPR. This compiles, "evaluates", any string as code.
my $string = q[print "Hello, world!";];
eval $string;
This string can come from any source, including a filehandle.
It also doesn't have to be a single statement. If you want to modify how a program runs, you can replace its subroutines.
use strict;
use warnings;
use v5.10;
sub speak { return "Woof!"; }
say speak();
eval q[sub speak { return "Meow!"; }];
say speak();
You'll get a Subroutine speak redefined warning from that. It can be supressed with no warnings "redefine".
{
# The block is so this "no warnings" only affects
# the eval and not the entire program.
no warnings "redefine";
eval q[sub speak { return "Shazoo!"; }];
}
say speak();
Obviously this is a major security hole. There is many, many, many things to consider here, too long for an answer, and I strongly recommend you not do this and find a better solution to whatever problem you're trying to solve this way.
One way to mitigate the potential for damage is to use the Safe module. This is like eval but limits what built in functions are available. It is by no means a panacea for the security issues.
With a warning about all kinds of issues, you can reload modules.
There are packages for that, for example, Module::Reload. Then you can write code that you intend to change in a module, change the source at runtime, and have it reloaded.
By hand you would delete that from %INC and then require, like
# ... change source code in the module ...
delete $INC{'ModuleWithCodeThatChages.pm'};
require ModuleWithCodeThatChanges;
The only reason I can think of for doing this is experimentation and play. Otherwise, there are all kinds of concerns with doing something like this, and whatever your goal may be there are other ways to accomplish it.
Note The question does specify a filehandle. However, I don't see that to be really related to what I see to be the heart of the question, of modifying code at runtime.
The source file isn't used after it's been compiled.
You could just eval it.
use strict;
use warnings;
my $code = <<'__EOS__'
print "It works!!\n";
__EOS__
open(my $append_fh, ">>", "dynamic.pl")
or die($!);
print($append_fh $code);
eval("$code; 1")
or die($#);
There's almost definitely a better way to achieve your end goal here. BUT, you could recursively make exec() or system() calls -- latter if you need a return value. Be sure to setup some condition or the dominoes will keep falling. Again, you should rethink this, unless it's just practice of some sort, or maybe I don't get it!
Each call should execute the latest state of the file; also be sure to close the file before each call.
i.e.,
exec("dynamic.pl"); or
my retval;
retval = system("perl dynamic.pl");
Don't use eval ever.
What I want to achieve:
###############CODE########
old_procedure(arg1, arg2);
#############CODE_END######
I have a huge code which has a old procedure in it. I want that the call to that old_procedure go to a call to a new procedure (new_procedure(arg1, arg2)) with the same arguments.
Now I know, the question seems pretty stupid but the trick is I am not allowed to change the code or the bad_function. So the only thing I can do it create a procedure externally which reads the code flow or something and then whenever it finds the bad_function, it replaces it with the new_function. They have a void type, so don't have to worry about the return values.
I am usng perl. If someone knows how to atleast start in this direction...please comment or answer. It would be nice if the new code can be done in perl or C, but other known languages are good too. C++, java.
EDIT: The code is written in shell script and perl. I cannot edit the code and I don't have location of the old_function, I mean I can find it...but its really tough. So I can use the package thing pointed out but if there is a way around it...so that I could parse the thread with that function and replace function calls. Please don't remove tags as I need suggestions from java, C++ experts also.
EDIT: #mirod
So I tried it out and your answer made a new subroutine and now there is no way of accessing the old one. I had created an variable which checks the value to decide which way to go( old_sub or new_sub)...is there a way to add the variable in the new code...which sends the control back to old_function if it is not set...
like:
use BadPackage; # sub is defined there
BEGIN
{ package BapPackage;
no warnings; # to avoid the "Subroutine bad_sub redefined" message
# check for the variable and send to old_sub if the var is not set
sub bad_sub
{ # good code
}
}
# Thanks #mirod
This is easier to do in Perl than in a lot of other languages, but that doesn't mean it's easy, and I don't know if it's what you want to hear. Here's a proof-of-concept:
Let's take some broken code:
# file name: Some/Package.pm
package Some::Package;
use base 'Exporter';
our #EXPORT = qw(forty_two nineteen);
sub forty_two { 19 }
sub nineteen { 19 }
1;
# file name: main.pl
use Some::Package;
print "forty-two plus nineteen is ", forty_two() + nineteen();
Running the program perl main.pl produces the output:
forty-two plus nineteen is 38
It is given that the files Some/Package.pm and main.pl are broken and immutable. How can we fix their behavior?
One way we can insert arbitrary code to a perl command is with the -M command-line switch. Let's make a repair module:
# file: MyRepairs.pm
CHECK {
no warnings 'redefine';
*forty_two = *Some::Package::forty_two = sub { 42 };
};
1;
Now running the program perl -MMyRepairs main.pl produces:
forty-two plus nineteen is 61
Our repair module uses a CHECK block to execute code in between the compile-time and run-time phase. We want our code to be the last code run at compile-time so it will overwrite some functions that have already been loaded. The -M command-line switch will run our code first, so the CHECK block delays execution of our repairs until all the other compile time code is run. See perlmod for more details.
This solution is fragile. It can't do much about modules loaded at run-time (with require ... or eval "use ..." (these are common) or subroutines defined in other CHECK blocks (these are rare).
If we assume the shell script that runs main.pl is also immutable (i.e., we're not allowed to change perl main.pl to perl -MMyRepairs main.pl), then we move up one level and pass the -MMyRepairs in the PERL5OPT environment variable:
PERL5OPT="-I/path/to/MyRepairs -MMyRepairs" bash the_immutable_script_that_calls_main_pl.sh
These are called automated refactoring tools and are common for other languages. For Perl though you may well be in a really bad way because parsing Perl to find all the references is going to be virtually impossible.
Where is the old procedure defined?
If it is defined in a package, you can switch to the package, after it has been used, and redefine the sub:
use BadPackage; # sub is defined there
BEGIN
{ package BapPackage;
no warnings; # to avoid the "Subroutine bad_sub redefined" message
sub bad_sub
{ # good code
}
}
If the code is in the same package but in a different file (loaded through a require), you can do the same thing without having to switch package.
if all the code is in the same file, then change it.
sed -i 's/old_procedure/new_procedure/g codefile
Is this what you mean?
I serialized my data to string in Perl using Data::Dumper. Now in another program I'm trying to deserialize it by using eval and I'm getting:
Global symbol "$VAR1" requires explicit package name
I'm using use warnings; use strict; in my program.
Here is how I'm evaling the code:
my $wiki_categories = eval($db_row->{categories});
die $# if $#;
/* use $wiki_categories */
How can I disable my program dying because of "$VAR1" not being declared as my?
Should I append "my " before the $db_row->{categories} in the eval? Like this:
my $wiki_categories = eval("my ".$db_row->{categories});
I didn't test this yet, but I think it would work.
Any other ways to do this? Perhaps wrap it in some block, and turn off strict for that block? I haven't ever done it but I've seen it mentioned.
Any help appreciated!
This is normal. By default, when Data::Dumper serializes data, it outputs something like:
$VAR1 = ...your data...
To use Data::Dumper for serialization, you need to configure it a little. Terse being the most important option to set, it turns off the $VAR thing.
use Data::Dumper;
my $data = {
foo => 23,
bar => [qw(1 2 3)]
};
my $dumper = Data::Dumper->new([]);
$dumper->Terse(1);
$dumper->Values([$data]);
print $dumper->Dump;
Then the result can be evaled straight into a variable.
my $data = eval $your_dump;
You can do various tricks to shrink the size of Data::Dumper, but on the whole it's fast and space efficient. The major down sides are that it's Perl only and wildly insecure. If anyone can modify your dump file, they own your program.
There are modules on CPAN which take care of this for you, and a whole lot more, such as Data::Serializer.
Your question has a number of implications, I'll try to address as many as I can.
First, read the perldoc for Data::Dumper. Setting $Data::Dumper::Terse = 1 may suffice for your needs. There are many options here in global variables, so be sure to localise them. But this changes the producer, not the consumer, of the data. I don't know how much control you have over that. Your question implies you're working on the consumer, but makes no mention of any control over the producer. Maybe the data already exists, and you have to use it as is.
The next implication is that you're tied to Data::Dumper. Again, the data may already exist, so too bad, use it. If this is not the case, I would recommend switching to another storable format. A fairly common one nowadays is JSON. While JSON isn't part of core perl, it's pretty trivial to install. It also makes this much easier. One advantage is that the data is useful in other languages, too. Another is that you avoid eval STRING which, should the data be compromised, could easily compromise your consumer.
The next item is just how to solve it as is. If the data exists, for example. A simple solution is to just add the my as you did. This works fine. Another one is to strip the $VAR1: (my $dumped = $db_row->{categories}) =~ s/^\s*\$\w+\s*=\s*//;. Another one is to put the "no warnings" right into the eval: eval ("no warnings; no strict; " . $db_row->{categories});.
Personally, I go with JSON whenever possible.
Your code would work as it stood except that the eval fails because $VAR1 is undeclared in the scope of the eval and use strict 'vars' is in effect.
Get around this by disabling strictures within as tight a block as possible. A do block does the trick, like this
my $wiki_categories = do {
no strict 'vars';
eval $db_row->{categories};
};
Is there a way to load entire modules during runtime in Perl? I had thought I found a good solution with autouse but the following bit of code fails compilation:
package tryAutouse2;
use autouse 'tryAutouse';
my $obj = tryAutouse->new();
I imagine this is because autouse is specifically meant to be used with exported functions, am I correct? Since this fails compilation, is it impossible to have a packaged solution? Am I forced to require before each new module invocation if I want dynamic loading?
The reasoning behind this is that my team loads many modules, but we're afraid this is eating up memory.
You want Class::Autouse or ClassLoader.
Due to too much magic, I use ClassLoader only in my REPL for convenience. For serious code, I always load classes explicitely. Jack Maney points out in a comment that Module::Load and Module::Load::Conditional are suitable for delayed loading.
There's nothing wrong with require IMO. Skip the export of the function and just call the fully qualified name:
require Some::Module;
Some::Module::some_function(#some_arguments);
eval 'use tryAutouse; 1;' or die $#;
Will work. But you might want to hide the ugliness.
When you say:
use Foo::Bar;
You're loading module Foo::Bar in at compile time. Thus, if you want to load your module in at run time, you'd use require:
require Foo::Bar;
They are sort of equivalent, but there are differences. See the Perldoc on use to understand the complete difference. For example, require used in this way won't automatically load in imported functions. That might be important to you.
If you want to test whether a module is there or not, wrap up your require statement in an eval and test whether or not eval is successful.
I use a similar technique to see if a particular Perl module is available:
eval { require Mail::Sendmail; };
if ($#) {
$watch->_Send_Email_Net_SMTP($watcher);
return;
}
In the above, I'll attempt to use Mail::Sendmail which is an optional module if it's available. If not, I'll run another routine that uses Net::SMTP:
sub _Send_Email_Net_SMTP {
my $self = shift;
my $watcher = shift;
require Net::SMTP; #Standard module: It should be available
WORD O'WARNING: You need to use curly braces around your eval statement and not parentheses. Otherwise, if the require doesn't work, your program will exit which is probably not what you want to do.
Instruction 'use' is performed at compile time, so check the path to the module also takes place at compile time. This may cause incorrect behavior, which are difficult to understand until you consider the contents of the #INC array.
One solution is to add block 'BEGIN', but the solution shown below is inelegant.
BEGIN { unshift #INC, '/path/to/module/'; }
use My::Module;
You can replace the whole mess a simple directive:
use lib '/path/to/module';
use My::Module;
This works because it is performed at compile time. So everything is ready to execute 'use' instruction.
Instead of the 'BEGIN' block, you can also decide to different instruction executed at compile time ie declaring a constant.
use constant LIB_DIR => '/path/to/module';
use lib LIB_DIR;
use My::Module;
I'm hitting a bug in the SVN perl module when using git:
Bizarre copy of UNKNOWN in subroutine entry at
/usr/lib/perl5/vendor_perl/SVN/Base.pm line 80.
And I'm not quite sure if this is a perl bug or a subversion bug. This is the relevant part:
# insert the accessor
if (m/(.*)_get$/) {
my $member = $1;
*{"${caller}::$1"} = sub {
&{"SVN::_${pkg}::${prefix}${member}_". # <<<< line 80
(#_ > 1 ? 'set' : 'get')} (#_)
}
}
(full source)
What is a "Bizarre copy"? And whose fault is it?
Edit: software versions
subversion 1.6.15-1
perl 5.14.0-1
Resolution: This happens when you compile with incompatible flags:
https://groups.google.com/d/msg/subversion_users/EOru50ml6sk/5xrbu3luPk4J
That perldoc gives you the short answer, but a brief STFW session yields a little more detail. This is basically evidence of a smashed stack in Perl.
Trivial example:
#!/usr/bin/perl
my #A = 1..5;
sub blowUp {
undef #A;
my $throwAway = {};
print for #_; # <== line 6
}
blowUp(#A);
__END__
bash$ ./blowitup
Bizarre copy of HASH in print at ./blowitup line 6.
And to make it that much more entertaining, without the $throwAway assignment, it's an invisible error (though under 'use warnings' it will at least still tell you that you're trying to access an uninitialized value). It's just when you make a new assignment that you see the strange behavior.
Since #_ is essentially lexically scoped to the subroutine, and arguments are passed by reference, that little subroutine basically pulls the rug out from under itself by undef'ing the thing that #_ was pointing to (you get the same behavior if you change the undef to an assignment, fwiw). I've found a number of postings on perl5-porters that mention this as an artifact of the fact that items on the stack are not reference counted and therefore not cleanly freed.
So while I haven't looked through all of the code in your full source in depth, I'll go ahead and guess that something in there is messing with something that was passed in on #_ ; then when #_ is referenced again, Perl is telling you that something's rotten in Denmark.
The immediate problem is a bug in the script/module, iow. The deeper issue of Perl not reference counting these items is also there, but I suspect you'll have better luck fixing the module in the short term. :-)
HTH-
Brian
A "Bizarre copy" occurs when Perl's stack is corrupted or contains non-scalars. It occurs as the result of bugs in Perl itself or in XS modules. (Brian Gerard's example exercises one of a long list of known bugs related to the stack not being ref-counted.)
You could isolate the problem by adding the following to the anon sub:
warn("Calling SVN::_${pkg}::${prefix}${member}_".(#_ > 1 ? 'set' : 'get')."...");
You might even want to emit a stack trace, but you might have to build it yourself using caller to avoid triggering the panic when building the stack trace.
Probably a perl bug. SVN::Base has XS components, but the error is occurring in pure-perl code and it's my opinion that perl should never allow it to happen. However, it's possible that there's some weird XS in SVN::Base that's tweaking it.
Best idea: file it against Subversion subcomponent bindings_swig_perl and perlbug both.