Asp Forum - HOWTO: "catching" a segfault in a ruby/dl C library

Ken Bloom

7/26/2006 10:09:00 PM

For my research, I've written bindings for the link-grammar[1] library
in ruby, and I am using them to parse all of the sentences in a corpus
of text and insert the parses into a database. We begin with code
which works roughly as follows:

$dbh=DBI.connect(...)

#for simplicity, it doesn't matter what the Something is.
sentences=Something.new($dbh)

d=Dictionary.new #Dictionary is an object wrapping the C library

def putindatabase link
#for our purposes, it doesn't really matter what this does
#except to say it uses $dbh which we opened before
end

def parse sentencetext
sentence=d.parse(sentencetext) #sentence also wraps the C library
sentence.linkage[0].links.each do |link|
putindatabase link
end
end

#sentences.each fetches every sentence from the database and yields each
#one to the block. it keeps an open connection between yields
sentences.each do |sentence|
parse sentence
end

Now, the link-grammar library that I'm using has a bug. It
segfaults[2] while freeing resources under some conditions that I
haven't quite figured out enough to fix in the C library itself.
Now, arguably it would be nice to catch the segfault as though it's an
exception, so that we could move on to the next sentence and get on
with our lives. But ruby/dl doesn't let us do that, and even if
ruby/dl did let us do that, it could leave the link-grammar library in
an inconsistent state. So we'll do the next best thing. We'll fork a
subprocess to handle each sentence. This will solve a few problems:

* If a sentence fails, we'll be able to move on to the next one.
* A sentence won't fail before being put in the database, since the
problem occurs when freeing the resources used to parse the sentence.
* We probably won't segfault at all because this only occurs under
complicated circumstances which seem to involve the fact that you've
parsed more than one sentence with the same dictionary.
* We don't have to clean up properly at all, since the termination of the
child process after each sentence automatically takes care of that
for us. (The link-grammar library doesn't allocate any resources
that the OS doesn't know how to dispose of.) This may make
subprocess termination faster.

So we would like to change our code to say:

sentences.each do |sentence|
Process.waitpid fork {parse sentence}
end

(for that last bullet point, we'd also need to edit the link-grammar
bindings, but I won't worry you with the details of that. I haven't
actually implemented it yet.)

That's all, right?
Oy, vey! Testing this, we quickly see that DBI can't put anything in
the database, except for the first sentence. Why? Because when the
child process exits, it closes the database connection, which affects
the parent too. (i.e. DBI isn't fork-safe)

But it turns out DRb is fork-safe, so I create another process of
"middleware" and have that be responsible for the database connection:

serverpid = fork do
dbh=DBI.connect(...)
Signal.trap("INT"){exit}
acl=ACL.new(%w{deny all allow 127.0.0.1})
DRb.install_acl(acl)
DRb.start_service('druby://localhost:9001',dbh)
DRb.thread.join
end
sleep 1 #wait for the server to be setup before continuing
DRb.start_service
$dbh=DRbObject.new(nil,'druby://localhost:9001')

## all of the previous code like before
## and then at the end, we kill the DRb server thread:

Process.kill("INT",serverpid)

Now, we have a painless way to make DBI fork-safe. (Note that DBI
still isn't *thread safe*, and this only works because I'm keeping all
of the child processes serially ordered, but it's not a bad
modification technique to handle this kind of error.)

Footnotes:
[1] http://www.link.cs.cmu...
http://www.abisource.com/downloads/li...
[2] http://bugzilla.abisource.com/show_bug.cg...

--
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu...

7 Answers

Ernie

11/21/2011 4:11:00 AM

On Nov 20, 7:14 pm, Nomen Nescio <nomennes...@mail.com> wrote:
> On Nov 18, 8:11 pm, "%" <pers...@gmail.com> wrote:
>
> > the teacher kept him everyday until 4:30 ,
> > and all the other kids got to leave at 3:30
>
> That's because his teacher caught him fingering himself.

Is anybody going to believe anything posted by the crazy old pussy and
asshole farrel after seeing her lie about a mole being a wart? Now let
the lying stupid whining cowardly vile HUNK of shit and convicted
child molester go finger herself, again. Ernie

Marcus

11/21/2011 8:03:00 AM

On Nov 20, 8:10 pm, Ernie <eprimea...@gmail.com> wrote:
> On Nov 20, 7:14 pm, Nomen Nescio <nomennes...@mail.com> wrote:
>
> > On Nov 18, 8:11 pm, "%" <pers...@gmail.com> wrote:
>
> > > the teacher kept him everyday until 4:30 ,
> > > and all the other kids got to leave at 3:30
>
> > That's because his teacher caught him fingering himself.
>
> Is anybody going to believe anything posted by the crazy old pussy and
> asshole farrel after seeing her lie about a mole being a wart? Now let
> the lying stupid whining cowardly vile HUNK of shit and convicted
> child molester go finger herself, again. Ernie

I can believe you were caught fingering yourself in class.

Marcus

Ernie

11/21/2011 1:29:00 PM

On Nov 21, 3:02 am, Marcus <marcus_cox2...@yahoo.com> wrote:
> On Nov 20, 8:10 pm, Ernie <eprimea...@gmail.com> wrote:
>
> > On Nov 20, 7:14 pm, Nomen Nescio <nomennes...@mail.com> wrote:
>
> > > On Nov 18, 8:11 pm, "%" <pers...@gmail.com> wrote:
>
> > > > the teacher kept him everyday until 4:30 ,
> > > > and all the other kids got to leave at 3:30
>
> > > That's because his teacher caught him fingering himself.
>
> > Is anybody going to believe anything posted by the crazy old pussy and
> > asshole farrel after seeing her lie about a mole being a wart? Now let
> > the lying stupid whining cowardly vile HUNK of shit and convicted
> > child molester go finger herself, again. Ernie
>
> I can believe you were caught fingering yourself in class.
>
> Marcus

Is anybody going to believe anything posted by the crazy old pussy and
asshole farrel after seeing her lie about a mole being a wart? Now let
the lying stupid whining cowardly vile HUNK of shit and convicted
child molester go finger herself, again. Ernie

Marcus

11/21/2011 2:27:00 PM

On Nov 21, 5:29 am, Ernie <eprimea...@gmail.com> wrote:
> On Nov 21, 3:02 am, Marcus <marcus_cox2...@yahoo.com> wrote:
>
>
>
>
>
>
>
>
>
> > On Nov 20, 8:10 pm, Ernie <eprimea...@gmail.com> wrote:
>
> > > On Nov 20, 7:14 pm, Nomen Nescio <nomennes...@mail.com> wrote:
>
> > > > On Nov 18, 8:11 pm, "%" <pers...@gmail.com> wrote:
>
> > > > > the teacher kept him everyday until 4:30 ,
> > > > > and all the other kids got to leave at 3:30
>
> > > > That's because his teacher caught him fingering himself.
>
> > > Is anybody going to believe anything posted by the crazy old pussy and
> > > asshole farrel after seeing her lie about a mole being a wart? Now let
> > > the lying stupid whining cowardly vile HUNK of shit and convicted
> > > child molester go finger herself, again. Ernie
>
> > I can believe you were caught fingering yourself in class.
>
> > Marcus
>
> Is anybody going to believe anything posted by the crazy old pussy and
> asshole farrel after seeing her lie about a mole being a wart? Now let
> the lying stupid whining cowardly vile HUNK of shit and convicted
> child molester go finger herself, again. Ernie

I am Marcus, but we still believe you were caught fingering yourself
in school.

Marcus

Ernie

11/21/2011 7:28:00 PM

On Nov 21, 9:26 am, Marcus <marcus_cox2...@yahoo.com> wrote:
> On Nov 21, 5:29 am, Ernie <eprimea...@gmail.com> wrote:
>
>
>
>
>
>
>
>
>
> > On Nov 21, 3:02 am, Marcus <marcus_cox2...@yahoo.com> wrote:
>
> > > On Nov 20, 8:10 pm, Ernie <eprimea...@gmail.com> wrote:
>
> > > > On Nov 20, 7:14 pm, Nomen Nescio <nomennes...@mail.com> wrote:
>
> > > > > On Nov 18, 8:11 pm, "%" <pers...@gmail.com> wrote:
>
> > > > > > the teacher kept him everyday until 4:30 ,
> > > > > > and all the other kids got to leave at 3:30
>
> > > > > That's because his teacher caught him fingering himself.
>
> > > > Is anybody going to believe anything posted by the crazy old pussy and
> > > > asshole farrel after seeing her lie about a mole being a wart? Now let
> > > > the lying stupid whining cowardly vile HUNK of shit and convicted
> > > > child molester go finger herself, again. Ernie
>
> > > I can believe you were caught fingering yourself in class.
>
> > > Marcus
>
> > Is anybody going to believe anything posted by the crazy old pussy and
> > asshole farrel after seeing her lie about a mole being a wart? Now let
> > the lying stupid whining cowardly vile HUNK of shit and convicted
> > child molester go finger herself, again. Ernie
>
> I am Marcus, but we still believe you were caught fingering yourself
> in school.
>
> Marcus

Is anybody going to believe anything posted by the crazy old pussy and
asshole farrel after seeing her lie about a mole being a wart? Now let
the lying stupid whining cowardly vile HUNK of shit and convicted
child molester go finger herself, again. Ernie

Nomen Nescio

11/22/2011 1:44:00 AM

On Nov 20, 9:10 pm, Ernie <eprimea...@gmail.com> wrote:
> On Nov 20, 7:14 pm, Nomen Nescio <nomennes...@mail.com> wrote:
>
> > On Nov 18, 8:11 pm, "%" <pers...@gmail.com> wrote:
>
> > > the teacher kept him everyday until 4:30 ,
> > > and all the other kids got to leave at 3:30
>
> > That's because his teacher caught him fingering himself.
>
> Is anybody going to believe anything posted by the crazy old pussy and
> asshole farrel after seeing her lie about a mole being a wart? Now let
> the lying stupid whining cowardly vile HUNK of shit and convicted
> child molester go finger herself, again. Ernie

I'm not Farrel, and you were the one caught fingering himself.

Ernie

11/22/2011 12:45:00 PM

On Nov 21, 8:44 pm, Nomen Nescio <nomennes...@mail.com> wrote:
> On Nov 20, 9:10 pm, Ernie <eprimea...@gmail.com> wrote:
>
> > On Nov 20, 7:14 pm, Nomen Nescio <nomennes...@mail.com> wrote:
>
> > > On Nov 18, 8:11 pm, "%" <pers...@gmail.com> wrote:
>
> > > > the teacher kept him everyday until 4:30 ,
> > > > and all the other kids got to leave at 3:30
>
> > > That's because his teacher caught him fingering himself.
>
> > Is anybody going to believe anything posted by the crazy old pussy and
> > asshole farrel after seeing her lie about a mole being a wart? Now let
> > the lying stupid whining cowardly vile HUNK of shit and convicted
> > child molester go finger herself, again. Ernie
>
> I'm not Farrel, and you were the one caught fingering himself.

Is anybody going to believe anything posted by the crazy old pussy and
asshole farrel after seeing her lie about a mole being a wart? Now let
the lying stupid whining cowardly vile HUNK of shit and convicted
child molester go finger herself, again. Ernie

comp.lang.ruby

HOWTO: "catching" a segfault in a ruby/dl C library

Ken Bloom

Ernie

Marcus

Ernie

Marcus

Ernie

Nomen Nescio

Ernie

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