Subject: Re: Starting build-system work (Windows/Mac users please speak up)
To: Kartik Ohri <kartikohri13@gmail.com>
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Cc: pljava-dev@lists.postgresql.org
From: Chapman Flack <chap@anastigmatix.net>
Message-ID: <5ED3D488.1010401@anastigmatix.net>
Date: Sun, 31 May 2020 12:00:08 -0400
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On 05/30/20 10:42, Chapman Flack wrote:
> Diagnostic messages from Java code generally follow one of these paths:
> 
> - they get explicitly sent to PostgreSQL's elog system, and go wherever
>   the PostgreSQL logging is pointed, or suppressed depending on the
>   enabled log level
> 
> - Java exceptions caught and turned into PostgreSQL errors go into the
>   PostgreSQL logs at level ERROR.
> 
> - Separately, the stack traces of those exceptions are output if the
>   PostgreSQL log level is set to DEBUG1 or finer. The code adjusts
>   the vfprintf-hook log level (see below) to DEBUG1 before printing
>   the stack trace, but I'm not positive it isn't just going to System.err
>   and not through the hook. Should be easy enough to confirm, I just
>   haven't had to before.

I've just confirmed this with gdb; an exception stack trace emitted
with printStackTrace [0] goes straight to System.err and the vfprintf hook
is not involved, so the surrounding calls to setJavaLogLevel aren't
really accomplishing anything.

> - Messages from the JVM itself that are emitted through a 'vfprintf hook'
>   are caught by PL/Java's hook [1] and injected into PostgreSQL's
>   elog system at a selectable level, usually INFO. (That is probably
>   the path being followed by the -Xcheck:jni messages, explaining
>   why they have INFO: in front.)

Confirmed also: -Xcheck:jni messages go through the vfprintf hook, so
they are always funneled into the PostgreSQL server log, regardless of
the setting of logging_collector.

> - Messages that are simply written to Java's System.err *can be* caught
>   and included in PostgreSQL's logs, depending on PostgreSQL's
>   logging_collector setting. [2] Otherwise, they just go wherever stderr
>   was pointing when the PostgreSQL server was started (or wherever
>   the server startup scripts point it). That could be, for example,
>   the systemd journal.

-Chap


[0]
https://github.com/tada/pljava/blob/efbbc1d/pljava-so/src/main/c/JNICalls.c#L82

> [1]
> https://github.com/tada/pljava/blob/efbbc1d/pljava-so/src/main/c/Backend.c#L200
> 
> [2]
> https://www.postgresql.org/docs/9.5/runtime-config-logging.html#GUC-LOGGING-COLLECTOR