Segmentation fault at XmlXpathContext:evaluate (EXC_BAD_ACCESS)

[poshaughnessy]

Hi,

Please could anyone help me out with this issue? (I'm a bit of a newbie).

I have a little Node script that makes some HTTP calls to grab some XML, parses it and generates some HTML from the results. It was working fine in Node v0.6. But since I upgraded to Node v0.8, it's failing with Segmentation Fault 11. (Clearing out the node_modules didn't help).

I installed Node from source and ran it using gdb. Here's the error I'm getting:

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
0x0000000100cd8f6e in libxmljs::XmlXpathContext::evaluate (this=<value temporarily unavailable, due to optimizations>, 
    xpath=<value temporarily unavailable, due to optimizations>) at xml_xpath_context.h:35
35  xml_xpath_context.h: No such file or directory.
    in xml_xpath_context.h
(gdb) backtrace
#0  0x0000000100cd8f6e in libxmljs::XmlXpathContext::evaluate (this=<value temporarily unavailable, due to optimizations>, 
    xpath=<value temporarily unavailable, due to optimizations>) at xml_xpath_context.h:35
#1  0x0000000100cd4cf6 in libxmljs::XmlElement::Find (args=<value temporarily unavailable, due to optimizations>) at ../src/xml_element.cc:170
#2  0x000000010017a662 in Builtin_Impl_HandleApiCall [inlined] () at ../deps/v8/src/builtins.cc:1145
#3  0x000000010017a662 in HandleApiCallHelper [inlined] () at :1161
#4  0x000000010017a662 in Builtin_HandleApiCall (args=
        {<v8::internal::Arguments> = {<v8::internal::Embedded> = {<No data fields>}, length_ = 4, arguments_ = 0x7fff5fbfe8b0}, <No data fields>}, 
    isolate=0x10101e200) at ../deps/v8/src/builtins.cc:1162
#5  0x0000373bbdd0618e in ?? ()
#6  0x0000373bbdddc5bd in ?? ()
#7  0x0000373bbdd098ee in ?? ()
#8  0x0000373bbdd8f08e in ?? ()
#9  0x0000373bbdd27fde in ?? ()
#10 0x0000373bbdd33f1f in ?? ()
#11 0x0000373bbdd098ee in ?? ()
#12 0x0000373bbddc4631 in ?? ()
#13 0x0000373bbddbdfeb in ?? ()
#14 0x0000373bbdd24ae7 in ?? ()
#15 0x0000373bbdd11477 in ?? ()
#16 0x00000001001c95e5 in Invoke (is_construct=<value temporarily unavailable, due to optimizations>, argc=0, args=0x0, 
    has_pending_exception=0x7fff5fbfec87) at ../deps/v8/src/execution.cc:118
#17 0x000000010013500c in v8::Function::Call (this=0x102001390, argc=0, argv=0x0) at ../deps/v8/src/api.cc:3644
#18 0x0000000100013bed in node::Parser::on_message_complete_ (this=0x101061e00) at ../src/node_http_parser.cc:333
#19 0x0000000100037fe4 in http_parser_execute (parser=0x101061e18, settings=0x10082af98, 
    data=0x1010dee00 "HTTP/1.1 200 OK\r\nServer: Apache-Coyote/1.1\r\nX-JAL: 2453\r\nContent-Type: text/xml;charset=UTF-8\r\nX-JSL: D=2453821 t=1346163674961319\r\nX-Cnection: close\r\nDate: Tue, 28 Aug 2012 14:16:57 GMT\r\nContent-Leng"..., len=2001) at ../deps/http_parser/http_parser.c:1565

The libxmljs method I'm using is parseXmlString.

Thanks in advance for any pointers or advice!

*Edit: Should have said, I'm on a Mac, v10.7.4 and I'm now running Node v0.8.8

[defunctzombie]

Can you provide a small XML sample and some js code that causes the segfault?

[poshaughnessy]

Hi shtylman,

Sorry it turns out it didn't break because I upgraded Node. It broke because the XML I was downloading and parsing coincidentally changed during the same period.

Here's a simple script and data file that demonstrates the problem:

http://dl.dropbox.com/u/45897012/libxmljs/test.js
http://dl.dropbox.com/u/45897012/libxmljs/data.xml

It seems that the trouble is with the XPath. I'm trying to pull out a 'value' node within a 'return' node that doesn't exist.

...find('//return[fieldID=8]/value')

With this XML, there is no 'return' node with a fieldID of 8.

I can simply get around this by doing it in two steps. First, pulling out //return[fieldID=8] and then if I get a result back, looking inside it for value.

I was expecting that the find method would just silently return me an empty array or null back.

In the meantime I can easily rewrite it. But in case it happens for anyone else, it would be good if it didn't kill the app?

Thank you.

[polotek]

I agree. It should probably be a little more graceful than a segfault :)

[jmandel]

+1

Simple failing test (segfaults):

module.exports.get_missing = function(assert) {
    var doc = libxml.Document();
    var root = doc.node('root');

    var missing = doc.get('missing/text()');
    assert.equal(missing, undefined);
    assert.done();
};

[defunctzombie]

version 0.6.1 released which contains this patch, thank you for the contribution :)