调试Perl模块时，为何Perl调试器执行R(重载)命令会挂起？

在SLES12 SP5的Perl 5.18.2环境中，调试模块时执行R命令后Perl调试器会出现挂起问题，复现步骤如下：

% perl -Ilib -mSimple -d -e1

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.

Simple::CODE(0x215fa60)(lib/Simple.pm:19):
19:         die "bad\n"
20:             if (something);
  DB<1> n
Simple::CODE(0x1e6fde0)(lib/Simple.pm:4):
4:      require 5.018_000;
  DB<1> R
Warning: some settings and command-line options may be lost!

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.

^C^C^C^C^Z
[2]+  Stopped                 perl -Ilib -mSimple -d -e1

该问题可复现于任意模块，以下为用于演示的Simple.pm代码：

% cat lib/Simple.pm
#!/usr/bin/perl
package Simple;

require 5.018_000;

use strict;
use warnings;

sub something
{
    use integer;
    my $result = 0;

    return $result;
}

UNITCHECK {
    $DB::single = 1;
    die "bad\n"
        if (something);
}

1;

问题分析 #

通过strace追踪发现，Perl实际从STDIN（文件描述符0）读取行。在执行R命令时用gdb附加到进程（测试环境为openSUSE Leap 15.4），得到以下输出：

Continuing.
process 2427 is executing new program: /usr/bin/perl
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
^C
Program received signal SIGINT, Interrupt.
0x00007ff83747071e in read () from /lib64/libpthread.so.0
Missing separate debuginfos, use: zypper install perl-base-debuginfo-5.26.1-150300.17.14.1.x86_64
(gdb) bt
#0  0x00007ff83747071e in read () from /lib64/libpthread.so.0
#1  0x000055933f42576d in PerlIOUnix_read ()
#2  0x000055933f428ba8 in PerlIOBuf_fill ()
#3  0x000055933f4272a8 in Perl_PerlIO_fill ()
#4  0x000055933f428aa0 in PerlIOBase_read ()
#5  0x000055933f42aa78 in PerlIO_getc ()
#6  0x000055933f3c0595 in Perl_sv_gets ()
#7  0x000055933f326d37 in Perl_lex_next_chunk ()
#8  0x000055933f330bd3 in Perl_yylex ()
#9  0x000055933f34489d in Perl_yyparse ()
#10 0x000055933f318617 in perl_parse ()
#11 0x000055933f2f0e5c in main ()

后续测试 #

Perl虽在读取STDIN但不显示调试器提示符，在Linux系统中按下Ctrl+D后，得到以下输出：

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.

此时本应可以输入Perl调试器命令，但实际Perl未处理任何输入，说明调试器处于挂起状态。

疑点排查 #

进一步测试发现，Perl调试器确实在读取输入，按下Ctrl+D可终止该无响应状态，且从重启后到按下Ctrl+D期间输入的命令未被实时处理，而是在输入EOF后才被回显：

DB<0> R
Warning: some settings and command-line options may be lost!

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.

l
ghgh
<<<<<<<<< ^D (EOF) entered here
main::(-:1):    l
main::(-:2):    ghgh

从strace输出中看到open("/lib64/libreadline.so.6", O_RDONLY|O_CLOEXEC) = 3，因此怀疑libreadline库可能与该问题有关。

内容的提问来源于stack exchange，提问作者U. Windl