[Pljava-dev] Is it safe to use multi-threaded PL/Java in single-threaded postgres process?

From: maumau307 at gmail(dot)com (MauMau)
To:
Subject: [Pljava-dev] Is it safe to use multi-threaded PL/Java in single-threaded postgres process?
Date: 2014-03-16 01:56:13
Message-ID: A832684586A84610AD7173AB42138F38@maumau
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I got the reply in pgsql-hackers about this. According to Tom's comment,
I'm worried about PL/Java's design.

http://www.postgresql.org/message-id/11347.1394545919 at sss.pgh.pa.us

Is there any rationale that it's safe for PL/Java to:

* load multi-threaded Java VM in single-threaded postgres process (which is
what Tom is concerned about)

* make multi-threaded Java VM load single-threaded pljava.so (which is what
JNI specification says should not be done).

----- Original Message -----
From: "MauMau" <maumau307 at gmail.com>
To: <pljava-dev at lists.pgfoundry.org>
Sent: Saturday, March 08, 2014 7:21 PM
Subject: Is it safe to use multi-threaded PL/Java in single-threaded
postgres process?

> Hello,
>
> Is PL/Java safe to use in terms of its threading design? I've just asked
> pgsql-hackers about this and am waiting for response, adding the sentence:
>
> "To put the question in other words, is it safe to load a multi-threaded
> PL library in the single-threaded backend process, if the PL only calls
> SPI in the main thread?"
>
> My understanding is:
>
> * PL/Java (pljava.so) is linked with the JNI (Java Native Interface)
> library, libjvm.so, in JRE. libjvm.so is linked with libpthread.so,
> because Java VM is multi-threaded. SO, "ldd pljava.so" shows libjvm.so
> and libpthread.so. pljava.so doesn't seem to be built for
> multh-threading --- none of -mt, -D_REENTRANT or -D_POSIX_C_SOURCE is
> specified when building it.
>
> * When the application calls Java stored function, pljava.so calls a
> function in libjvm.so to create a JVM in the backend process, then invokes
> the user-defined Java method in the main thread. The user-defined Java
> method calls JDBC methods to access database. The JDBC method calls are
> translated to backend SPI function calls through JNI.
>
> * The main thread can create Java threads using Java Thread API, and those
> threads can call JDBC methods. However, PL/Java intercepts JDBC method
> calls and serializes SPI calls. So, only one thread calls SPI functions
> at a time. I'm wondering if this is the reason why PL/Java is safe for
> use.
>
>
> What I'm concerned about is whether multi-threaded code (Java VM) can run
> safely in a single-threaded code (postgres). I cannot point out what can
> be a particular problem with PL/Java, but in general, the mixture of
> single-threaded code and multi-threaded one seems to cause trouble around
> handling errno, memory and file handles/pointers.
>
> FYI, JNI specification says that the code called from Java VM should be
> built for multi-threading as follows. But postgres is not.
>
> http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/design.html#wp9502
>
> [Excerpt]
> Compiling, Loading and Linking Native Methods
> Since the Java VM is multithreaded, native libraries should also be
> compiled and linked with multithread aware native compilers. For example,
> the -mt flag should be used for C++ code compiled with the Sun Studio
> compiler. For code complied with the GNU gcc compiler, the
> flags -D_REENTRANT or -D_POSIX_C_SOURCE should be used. For more
> information please refer to the native compiler documentation.

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