Perl调试器-出错时中断(异常)

Question

考虑这个小perl程序，test.pl:

#!/usr/bin/env perl
use warnings;
use strict;
use Number::Format qw(:subs); # sudo perl -MCPAN -e 'install Number::Format'
my $tstr = "";
my $numFormatter = new Number::Format();
for (my $ix=0; $ix<20; $ix++) {
  $tstr = $tstr . int(rand(10));
  my $ftstr = $numFormatter->format_number($tstr, 2, 1);
  print "ix: $ix ; in: $tstr ; out: $ftstrn";
}

如果我运行它，它会失败并出现一个错误。如果我在Perl调试器中使用perl -d运行它，它也会失败，并出现一个错误:

$ perl -d test.pl
Loading DB routines from perl5db.pl version 1.39_10
Editor support available.
Enter h or 'h h' for help, or 'man perldebug' for more help.
main::(test.pl:6):  my $tstr = "";
  DB<1> c
ix: 0 ; in: 6 ; out: 6.00
ix: 1 ; in: 63 ; out: 63.00
ix: 2 ; in: 637 ; out: 637.00
ix: 3 ; in: 6379 ; out: 6,379.00
ix: 4 ; in: 63790 ; out: 63,790.00
ix: 5 ; in: 637906 ; out: 637,906.00
ix: 6 ; in: 6379062 ; out: 6,379,062.00
ix: 7 ; in: 63790624 ; out: 63,790,624.00
ix: 8 ; in: 637906246 ; out: 637,906,246.00
ix: 9 ; in: 6379062467 ; out: 6,379,062,467.00
ix: 10 ; in: 63790624671 ; out: 63,790,624,671.00
ix: 11 ; in: 637906246715 ; out: 637,906,246,715.00
ix: 12 ; in: 6379062467152 ; out: 6,379,062,467,152.00
ix: 13 ; in: 63790624671522 ; out: 63,790,624,671,522.00
round() overflow. Try smaller precision or use Math::BigFloat at test.pl line 11.
 at /usr/local/share/perl/5.18.2/Number/Format.pm line 535.
    Number::Format::round('Number::Format=HASH(0x9d0b6cc)', 637906246715226, 2) called at /usr/local/share/perl/5.18.2/Number/Format.pm line 601
    Number::Format::format_number('Number::Format=HASH(0x9d0b6cc)', 637906246715226, 2, 1) called at test.pl line 11
Debugged program terminated.  Use q to quit or R to restart,
use o inhibit_exit to avoid stopping after program termination,
h q, h R or h o to get additional info.
  DB<1> p $ix         
  DB<2> 

…但是当它失败时，它不会在失败行"停止"，就像C程序中的gdb可能会做的那样-程序再次终止，因此我不再需要检查上下文变量了。

当然，像这样的循环可能会运行数千次，这就是为什么在有问题的行设置一个断点并手动执行c继续在这里没有多大帮助…

是否有一种方法可以让Perl调试器在错误/异常时中断程序，从而保留局部变量上下文，以便检查那里的变量?

Answer 1

将违规行换行为eval，并在设置$@时设置$DB::single:

#!/usr/bin/env perl
use warnings;
use strict;
use Number::Format qw(:subs); # sudo perl -MCPAN -e 'install Number::Format'
my $tstr = "";
my $numFormatter = new Number::Format();
for (my $ix=0; $ix<20; $ix++) {
  $tstr = $tstr . int(rand(10));
  my $ftstr = eval { $numFormatter->format_number($tstr, 2, 1); };
  $DB::single = 1 if $@;
  print "ix: $ix ; in: $tstr ; out: $ftstrn";
}

,

% perl -d test.pl
Loading DB routines from perl5db.pl version 1.49
Editor support available.
Enter h or 'h h' for help, or 'man perldebug' for more help.
main::(test.pl:6):  my $tstr = "";
  DB<1> r
ix: 0 ; in: 7 ; out: 7.00
ix: 1 ; in: 71 ; out: 71.00
ix: 2 ; in: 715 ; out: 715.00
ix: 3 ; in: 7153 ; out: 7,153.00
ix: 4 ; in: 71537 ; out: 71,537.00
ix: 5 ; in: 715379 ; out: 715,379.00
ix: 6 ; in: 7153794 ; out: 7,153,794.00
ix: 7 ; in: 71537941 ; out: 71,537,941.00
ix: 8 ; in: 715379417 ; out: 715,379,417.00
ix: 9 ; in: 7153794174 ; out: 7,153,794,174.00
ix: 10 ; in: 71537941740 ; out: 71,537,941,740.00
ix: 11 ; in: 715379417408 ; out: 715,379,417,408.00
ix: 12 ; in: 7153794174086 ; out: 7,153,794,174,086.00
ix: 13 ; in: 71537941740864 ; out: 71,537,941,740,864.00
main::(test.pl:13):   print "ix: $ix ; in: $tstr ; out: $tstrn";
  DB<1> print $tstr
715379417408646
  DB<2> 

What the Heck??

魔术背后有两个原则:

1. 防止致命的异常(例如捕获异常)
2. 在代码的某一点停止调试器而不重复步骤

要捕获异常，请使用eval BLOCK构造。这将把异常存储在$@变量中。如果$@不是空字符串，则抛出异常。请注意，上面的代码虽然是惯用的，但并不完全正确;如果抛出的异常是字符串0，它将被忽略(因为if 0将为false)。Perl中的异常处理是复杂的。Try::Tiny有很好的讨论。

现在异常不再是致命的，如何停止调试器?有很多方法可以做到这一点。本例使用$DB::single变量，当它为true时，向调试器发出停止的信号。缺点是您必须编辑代码来实现这种行为。另一个选择是用条件设置断点:

% perl -d test.pl
Loading DB routines from perl5db.pl version 1.49
Editor support available.
Enter h or 'h h' for help, or 'man perldebug' for more help.
main::(test.pl:6):  my $tstr = "";
  DB<1> b 11 $@ ne ''
  DB<2> r
[... output as above ...]
main::(test.pl:11):   my $ftstr = eval { $numFormatter->format_number($tstr, 2, 1); };
  DB<2> p $tstr
3247014520717436

Answer 2

用eval{}和$DB::single = 1 if $@;捕捉骰子是一个很好的技巧，但是如果你想在库中找到问题的根源，你需要在抛出错误的地方设置一个断点。

这里用

  DB<1> b Carp::croak
  DB<2> c
Carp::croak(/opt/local/lib/perl5/5.22/Carp.pm:166):
166:    sub croak   { die shortmess @_ }
  DB<3> T
@ = DB::DB called from file '/opt/local/lib/perl5/5.22/Carp.pm' line 166
. = Carp::croak('round() overflow. Try smaller precision or use Math::BigFloat') 
    called from file '/opt/local/lib/perl5/site_perl/5.22/Number/Format.pm' line 535
$ = Number::Format::round(ref(Number::Format), 637906246715227, 2)
    called from file '/opt/local/lib/perl5/site_perl/5.22/Number/Format.pm' line 601
$ = Number::Format::format_number(ref(Number::Format), 637906246715227, 2, 1)
    called from file 'mytest.pl' line 12