Issue #5539 has been reported by Christopher Huff.

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Bug #5539: Readline.readline() blocks all threads
http://redmine.ruby-lang.org/issues/5539

Author: Christopher Huff
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-darwin10.8.0]


The standard library Readline.readline() function blocks the thread it's called from without unlocking the VM, thus blocking all threads. This code demonstrates the problem:

require 'pp'
t = Thread.new {10.times {|x| puts x; sleep 1}}
while(buf = Readline.readline)
    p buf
    if(buf == 'q')
        break
    end
end

Thread t will be blocked by the readline call, numbers being printed occasionally when lines are entered and readline() returns, rather than once per second. Reproduced on ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.6.0] and ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-darwin10.8.0].

The RubyInline gem allows a workaround:

require 'inline'
class BetterReadline
    inline :C do |builder|
        builder.include '<ruby.h>'
        builder.include '<readline/readline.h>'
src = <<END
    VALUE readline_intern(void * data) {
        char ** rstr = (char **)data;
        *rstr = readline(NULL);
        return Qnil;
    }
END
        builder.prefix(src)
src = <<END
    VALUE rb_readline(void) {
        char * str = NULL;
        rb_thread_blocking_region(readline_intern, &str, NULL, NULL);
        return rb_str_new2(str);
    }
END
        builder.c_singleton(src, method_name: 'readline')
    end
end # class BetterReadline


This then illustrates the desired behavior, with the thread running in the background while readline() waits for input:

require 'pp'
t = Thread.new {10.times {|x| puts x; sleep 1}}
while(buf = BetterReadline.readline)
    p buf
    if(buf == 'q')
        break
    end
end


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