While reviewing Jeff's notice_lock_waits patch, I came across his comment about having a general facility for promoting selected LOG messages. So I thought I should post it here, even though the patch is probably far from being accepted in Postgres.
I recently wrote a patch for Postgres-XL to do exactly this and I found it very useful, especially while debugging race conditions and problems with ongoing sessions. I'm attaching a patch rebased on PG-master. It works, but TBH I don't think it's anywhere close for being acceptable in PG. But I hope this is good enough to show how this can be done with minimal changes and spark ideas.
The patch uses some preprocessing and scripting magic to assign distinct identifiers to each module (a subdir in the source code), to each file and to each elog message. It then provides a set of functions by which an user can increase/decrease/set log levels for either individual messages or all messages within a source file or source module. The log levels can be changed only for specific backends or all current or future backends. If you configure with --enable-genmsgids switch, a MSGMODULES and MSGIDS file is created in $srcdir, which can later be used to know ids assigned to various modules/messages.
Now there are several problems with the patch:
- I don't know if it will work for anything other than clang and gcc (I've tested those two, but only specific versions)
- The shared memory representation for msgs is not at all optimal and with a large value of max_connections, it can quickly go out of control. But I believe we can find a better representation without losing runtime performance.
- The APIs themselves can be significantly improved.
- MSGMODULES and MSGIDS should probably be some sort of a view that user can readily access without requiring access to the environment where it was built.
- Right now it only supports changing levels for DEBUG[1-5] and LOG messages. But that could be easily extended to cover INFO or NOTICE.
- The patch should probably have many more comments
- Finally, I don't know if better and more elegant ways exist to automatically assigning module/file/msgids. I couldn't think of any without making excessive changes to all source files and hence did what I did. That does not mean better ways don't exists.
Thanks,
Pavan
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