p5pRT
commented
7 years ago From @mauke
Created by @mauke
PL_keyword_plugin (see perldoc perlapi) is global to the entire process and shared across threads. Its documentation says:
| [...] take a copy of PL_keyword_plugin before assigning your own function | pointer to it. Your handler function should look for keywords that it is | interested in and handle those. Where it is not interested\, it should call | the saved plugin function\, passing on the arguments it received.
But it does not show any example code of how this "taking a copy" is supposed to be done.
The XS::APItest module in the core test suite does it the following way (excerpt):
| static int (*next_keyword_plugin)(pTHX_ char *\, STRLEN\, OP **); |
|---|
| static int my_keyword_plugin(pTHX_ |
| char *keyword_ptr\, STRLEN keyword_len\, OP **op_ptr) |
| { |
| if (...) { |
| ... |
| } else { |
| return next_keyword_plugin(aTHX_ keyword_ptr\, keyword_len\, op_ptr); |
| } |
| } |
| BOOT: |
| { |
| next_keyword_plugin = PL_keyword_plugin; |
| PL_keyword_plugin = my_keyword_plugin; |
| } |
This code has found its way into various CPAN modules (e.g. Function::Parameters\, Syntax::Keyword::Try\, etc).
It is also not thread-safe:
% perl -Mthreads -e 'threads->create(sub { require Function::Parameters })->join for 1 .. 2' Segmentation fault
This is because each thread runs BOOT separately\, but PL_keyword_plugin is shared. This means the second thread BOOTing will set next_keyword_plugin to the value the first thread left in PL_keyword_plugin\, which is my_keyword_plugin.
Then the next call to my_keyword_plugin that enters the 'else' block will attempt to run the next handler in the chain (via 'return next_keyword_plugin(...)')\, but now next_keyword_plugin == my_keyword_plugin.
In other words\, the list of linked handlers contains a loop\, leading to a stack overflow.
PL_check (the other global variable documented in perlapi) has this paragraph in its documentation:
| For thread safety\, modules should not write directly to this array. Instead\, | use the function wrap_op_checker.
This equally applies to PL_keyword_plugin\, but a corresponding wrap_keyword_plugin function is missing. It should be added.
Migrated from rt.perl.org#132413 (status was 'open')
Searchable as RT132413$