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From "Bill Mitchell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (AXIS2C-884) Seg fault in libxml when svc client torn down in a multithreaded client
Date Mon, 07 Jan 2008 14:41:34 GMT

    [ https://issues.apache.org/jira/browse/AXIS2C-884?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12556572#action_12556572
] 

Bill Mitchell commented on AXIS2C-884:
--------------------------------------

To handle the multithreaded case, clearly the libxml2 documentation means what it says and
there must be one initialize and one cleanup for the entire application.  This means creating
new entry points, say axis2_initialize() and axis2_cleanup, that the application client calls
at the appropriate points.  But demanding that all clients call these as appropriate means
changing all the test cases, all the samples, all the documentation....

So, my suggestion is:

(1) Add new entry points, axis2_initialize() and axis2_cleanup() that the client may call
to manage the initialization and tear down itself.  This would be required for correct multi-thread
operation with libxml.  

(2) Add internal entry points, axis2_implicit_initialize, and axis2_implicit_cleanup, that
op_client.c could use to accomplish the axiom_xml_reader_init() and cleanup.  These routines
would keep a use-count of active users, and, when there was no explicit initialize, would
perform the underlying initialization only on the first use and the cleanup only when the
last use was complete.  

Change (2) will fix the single-thread crash above by ensuring that no xmlCleanupParser is
issued while any structures are still in use.  And libxml2 seems to tolerate serial reuse
as long as it is in a single thread.  Change (1) gives the multithreaded client the chance
to take control of the initialization and termination.  

> Seg fault in libxml when svc client torn down in a multithreaded client
> -----------------------------------------------------------------------
>
>                 Key: AXIS2C-884
>                 URL: https://issues.apache.org/jira/browse/AXIS2C-884
>             Project: Axis2-C
>          Issue Type: Bug
>          Components: core/deployment
>    Affects Versions: Current (Nightly)
>         Environment: Windows XP, Visual Studio 2005, libxml 2.6.25 and libxml 2.6.30,
libcurl
>            Reporter: Bill Mitchell
>         Attachments: axis2.trace, desc_builder_diff.txt
>
>
> In a multithreaded application, if the stub/svc client is freed in a thread different
from that used when the svc client was built, libxml crashes.  The trace below shows the information
available from a release build with debug information embedded.  
> I have verified this is not an effect of combining debug and release C runtimes, or different
versions of the C runtime.  Rebuilding all the libxml related dlls with the same runtime as
is used for Axis and the client app does not solve the problem.  
> Interestingly, rebuilding libxml with threads disabled does make the crash go away. 
But the default build of libxml commonly available has native threads enabled, and building
without thread support may make the library not thread safe.  
> By adding debug trace statements in the axis2.trace file, I have verified that the xml_reader
being torn down when the crash happens is the one used to read the axis2.xml file when the
configuration was first read.  (axis2.trace file attached.)
> Looking at the code in libxml, it appears that libxml decides to close the reader using
an internal close routine intended for closing compressed channels through zlib.  Apparently
the C runtime library returns a -1 EOF status when closing a file opened for read.  The close
routine, gzio.c in zlib, treats this as an error, and when libxml attempts to report the error
and determines that it is in a different thread, things really go downhill fast.  I have not
isolated why the EnterCriticalSection call crashes in the system, but it does.  
> One way to avoid the problem would be to guarantee that the stub/svc client is freed
in the same thread as created it.  In my multithreaded client application, though, I work
hard to share the stub across threads deliberately to reduce the number of distinct service
clients and the associated demand on the server.  
> Windows call traceback at time of crash:
>  	ntdll.dll!7c918fea() 	
>  	[Frames below may be incorrect and/or missing, no symbols loaded for ntdll.dll]	
>  	msvcr80.dll!78134d09() 	
>  	ntdll.dll!7c910e91() 	
>  	ntdll.dll!7c9106eb() 	
>  	msvcr80.dll!78134d83() 	
>  	ntdll.dll!7c90104b() 	
> >	libxml2.dll!xmlGetGlobalState()  Line 570	C
>  	libxml2.dll!__xmlLastError()  Line 709 + 0x5 bytes	C
>  	libxml2.dll!__xmlRaiseError(void (void *, _xmlError *)* schannel=0x00000000, void (void
*, const char *, <no type>)* channel=0x00000000, void * data=0x00000000, void * ctx=0x00000000,
void * nod=0x00000000, int domain=8, int code=0, xmlErrorLevel level=XML_ERR_ERROR, const
char * file=0x00000000, int line=0, const char * str1=0x00c5d420, const char * str2=0x00000000,
const char * str3=0x00000000, int int1=0, int col=0, const char * msg=0x00c5d360, ...)  Line
452 + 0x5 bytes	C
>  	libxml2.dll!__xmlSimpleError(int domain=8, int code=0, _xmlNode * node=0x00000000,
const char * msg=0x00c5d360, const char * extra=0x00c5d420)  Line 657 + 0x2d bytes	C
>  	libxml2.dll!__xmlIOErr(int domain=8, int code=0, const char * extra=0x00c5d420)  Line
417 + 0x1a bytes	C
>  	libxml2.dll!xmlGzfileClose(void * context=0x03301f80)  Line 1155 + 0x10 bytes	C
>  	libxml2.dll!xmlFreeParserInputBuffer(_xmlParserInputBuffer * in=0x03304578)  Line 2207
+ 0x5 bytes	C
>  	libxml2.dll!xmlTextReaderClose(_xmlTextReader * reader=0x03304760)  Line 2244 + 0x6
bytes	C
>  	axis2_parser.dll!axis2_libxml2_reader_wrapper_free(axiom_xml_reader * parser=0x03301e00,
const axutil_env * env=0x031aa150)  Line 510	C
>  	axiom.dll!axiom_stax_builder_free(axiom_stax_builder * om_builder=0x03271aa8, const
axutil_env * env=0x031aa150)  Line 886	C
>  	axis2_engine.dll!axis2_desc_builder_free(axis2_desc_builder * desc_builder=0x03301d58,
const axutil_env * env=0x031aa150)  Line 141	C
>  	axis2_engine.dll!axis2_conf_builder_free(axis2_conf_builder * conf_builder=0x03301d70,
const axutil_env * env=0x031aa150)  Line 128	C
>  	axis2_engine.dll!axis2_dep_engine_free(axis2_dep_engine * dep_engine=0x031aa2e8, const
axutil_env * env=0x031aa150)  Line 380	C
>  	axis2_engine.dll!axis2_conf_free(axis2_conf * conf=0x0330cad8, const axutil_env * env=0x031aa150)
 Line 328	C
>  	axis2_engine.dll!axis2_conf_ctx_free(axis2_conf_ctx * conf_ctx=0x00000000, const axutil_env
* env=0x031aa150)  Line 439	C
>  	axis2_engine.dll!axis2_svc_client_free(axis2_svc_client * svc_client=0x031aa1a8, const
axutil_env * env=0x031aa150)  Line 1303	C
>  	axis2_engine.dll!axis2_stub_free(axis2_stub * stub=0x031aa198, const axutil_env * env=0x031aa150)
 Line 131	C

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