I am writing a Postgres extension, and thought that I had memory corruption, (thanks for the --enable-cassert lead). I might, but It now looks like I need to understand the use of shared memory and locking in Postgres. So I have two questions.
1) I am now guessing that my original problem is caused by relying on static memory in my extension (i.e., in the source declaring PG_MODULE_MAGIC). This static memory is almost but not quite constant -- it is initialized from _PG_init, and then never modified. I suspect that this cannot work in general (since Postgres is multi-process), but I thought it would be adequate for early development. However, I am seeing this static memory get corrupted even when there is only a single process executing the extension code (verified by examining getpid()). So the question is this: Is the use of non-constant static memory ill-advised, even assuming there is just one process relying on it? Or is it more likely that I still have run-of-the-mill memory corruption. (--enable-cassert hasn't notified me of any problems).
2) Assuming that I should be using shared memory instead of static, I am reading https://www.postgresql.org/docs/12/xfunc-c.html#id-1.8.3.13.14, and examining contrib/pg_prewarm. The xfunc-c documentation mentions RequestNamedLWLockTranche to get an array of LWLocks. But the sample code that follows calls GetNamedLWLockTranche. And the pg_prewarm code doesn't rely on an array of locks, it initializes a single lock, (which I think would be adequate for my needs). I understand the purpose of locks for obtaining and manipulating shared memory but I am confused about the correct way to proceed. I'm guessing it is safe to assume that pg_prewarm works and is a good model to follow, and that the doc may be buggy. Can someone clarify my confusion, and perhaps point me at a tutorial on correct usage of the interfaces for LWLocks and shared memory, (I haven't been able to find one).
Thank you.
Jack Orenstein
1) I am now guessing that my original problem is caused by relying on static memory in my extension (i.e., in the source declaring PG_MODULE_MAGIC). This static memory is almost but not quite constant -- it is initialized from _PG_init, and then never modified. I suspect that this cannot work in general (since Postgres is multi-process), but I thought it would be adequate for early development. However, I am seeing this static memory get corrupted even when there is only a single process executing the extension code (verified by examining getpid()). So the question is this: Is the use of non-constant static memory ill-advised, even assuming there is just one process relying on it? Or is it more likely that I still have run-of-the-mill memory corruption. (--enable-cassert hasn't notified me of any problems).
2) Assuming that I should be using shared memory instead of static, I am reading https://www.postgresql.org/docs/12/xfunc-c.html#id-1.8.3.13.14, and examining contrib/pg_prewarm. The xfunc-c documentation mentions RequestNamedLWLockTranche to get an array of LWLocks. But the sample code that follows calls GetNamedLWLockTranche. And the pg_prewarm code doesn't rely on an array of locks, it initializes a single lock, (which I think would be adequate for my needs). I understand the purpose of locks for obtaining and manipulating shared memory but I am confused about the correct way to proceed. I'm guessing it is safe to assume that pg_prewarm works and is a good model to follow, and that the doc may be buggy. Can someone clarify my confusion, and perhaps point me at a tutorial on correct usage of the interfaces for LWLocks and shared memory, (I haven't been able to find one).
Thank you.
Jack Orenstein
