AttachCurrentThread hangs after a few calls in a Apache2 PHP5 Extension
850072Mar 24 2011 — edited Mar 28 2011
I have created a small C Library/API that calls with JNI a simple Java class with a few methods to work with a underlying Java Framework (Carrot2). That JVM/JNI code functions some kind like a bridge between C and Java. In a single-threaded environment I experienced no problems, the complete runtime flow works well, I get no crashes or other strange behaviour.
Now I have built an PHP5 extension that makes use of that JNI/C code, the extension is built as a class with constructor/destructor and has a few methods that call functions of my C library. The JNI/C side I have 2 data structs, one is for the JVM and its handle (JavaVM) as well as the Java class name and its path, the jclass object of that class, jvm options, classpath, etc. A second data struct is for the Java environment containing the JNIEnv pointer, the jobject of the Java class and a few more application related variables. The C code consists of JNI functions to work with the Java class to call its methods, a few JNI helper functions and some internal application code and data handling.
At module initialization (Apache startup) I use the PHP_MINIT and PHP_MSHUTDOWN functions to load and unload (on apache stop) the JVM. The JVM gets created and destroyed at these points.
I use the PHP constructor (inside the extension) to attach to the JVM, that is AttachCurrentThread with the class constructor, I create and object of the Java class and preload a few Java primitives that I need for my methods later on and after all this I can work with the Java class through JNI. On PHP5 class destruction (also in the extension), I detach from the JVM using 'DetachCurrentThread', free some resources and "delete" the JNIEnv pointer (set it to NULL).
The JVM data struct (containing the JVM object) is a static global in the PHP5 module since I think there is no need for thread safety. This data is only modified/used in module startup/shutdown. The JNI worker struct (data struct with the JNIEnv pointer and jobject, etc is safely handled by the Zend object store throughout the complete class, so this should be thread-safe. So I do not suspect any problem here.
Testing the PHP 5 extension with a small PHP5 script using that PHP class and a few methods in Apache debug mode (that is the -X switch for just one worker process) works like a charm, no problems, crashes or misbehaving situations, not up to now with my testing though. But running Apache in regular mode with multiple processes, I run in to problems. One major problem is, after a few reloads of the php script using the extension class (lets say 10-15 reloads; The request is handled by the same apache child process) the `AttachCurrentThread' call hangs and the child process remains in that state. Continuing reloading the script a new child process answers and also here, after 10-15 reloads of the script, I get the same problem. So I am not sure what is really happening here nor I do not know how to solve that issue. Is that a signal problem (I read, that there can be signal mask issues and the JVM hangs when a new thread tries to attach to it, for example, the Jakarta Tomcat JK JNI worker uses a signal hack because of this problem. But that does not help me.
Or is there some problem with the multi-threading itself (since the JVM is multithreaded too)? Another problem is, besides that hanging thread attach, that the processing sometimes stops in middle of a java call (I have one method that insert data (calling a add method in a for loop) with JNI into a Java list on the Java class side, there it can stop in middle of the insertion process.
So, for now, I have no idea how to continue. Like said, in a single threaded environment, as a process, the application runs fine without a flaw.
Some details of my development machines: Ubuntu Lucic 10.04 LTS (also another Ubuntu 8 LTS), Apache2 MPM Worker (multithreaded model) and also on another machine the prefork version, Java JRE 6.0.22, PHP 5.2.16 (compiled from source).
So any hints that can help me proceeding the development of this module for PHP would be great. Any help is appreciated. If there are questions or some unclear points, please let me know.