While doing some work on Postgres 10, I have noticed that in a lot of code paths some system calls related to memory allocation, like malloc(), realloc() or strdup() simply missed to check for NULL values, causing a crash. Note that crashes are highly unlikely to happen, still the result of this investigation to make the code of Postgres cleaner, with problems detected as well by Heikki Linnakangas and Aleksander Alekseev, has resulted on a set of commits, like that, that and finally that.
One extra thing, discovered by Aleksander, was related to the incorrect use of ShmemAlloc(), which is a routine aimed at allocating shared memory for the whole system. And the topic of this post is about specifically that, that resulted in the following commit:
commit: 6c03d981a6b64ed8caaed4e94b54ef926202c9f3 author: Tom Lane <firstname.lastname@example.org> date: Thu, 1 Sep 2016 10:13:55 -0400 Change API of ShmemAlloc() so it throws error rather than returning NULL. A majority of callers seem to have believed that this was the API spec already, because they omitted any check for a NULL result, and hence would crash on an out-of-shared-memory failure. The original proposal was to just add such error checks everywhere, but that does nothing to prevent similar omissions in future. Instead, let's make ShmemAlloc() throw the error (so we can remove the caller-side checks that do exist), and introduce a new function ShmemAllocNoError() that has the previous behavior of returning NULL, for the small number of callers that need that and are prepared to do the right thing. This also lets us remove the rather wishy-washy behavior of printing a WARNING for out-of-shmem, which never made much sense: either the caller has a strategy for dealing with that, or it doesn't. It's not ShmemAlloc's business to decide whether a warning is appropriate. The v10 release notes will need to call this out as a significant source-code change. It's likely that it will be a bug fix for extension callers too, but if not, they'll need to change to using ShmemAllocNoError().
First, as mentioned by the commit log, ShmemAlloc is a routine that originally does not fail if an allocation error happens, logging a WARNING in case of failure to let him know for which size it failed. That’s not a problem in itself, as long as the callers check for a NULL result and issue an error message correctly. However, after looking at the call sites of this routine, it happened that only few code paths actually do that. So after discussion it has been decided to change how this behaves and make it issue an ERROR by default in case of failure. An additional routine, called ShmemAllocNoError(), has been created to provide the original behavior.
As there are a couple of code paths that need to take some actions when an allocation failure happens, take for example ShmemInitStruct() that needs to remove a shared memory segment reference from the existing index before failing, extension and plugin developers should fallback to ShmemAllocNoError() where ShmemAlloc() was previously used if there were NULL-checks done.
If your extension code has been using ShmemAlloc() directly, like for example to allocate some shared memory using the system hook shmem_startup_hook, be sure that what you are doing is correct. As ShmemAlloc() is a rather widely-used routine in plugin and extension code, be sure to check how your existing code behaves, and then fix it. The release notes of Postgres 10 will likely mention this behavior change, and this post is here to inform people a bit earlier than that, so be aware of the change. One need to make sure as well that any existing code compiled on versions older than 10 are doing the correct job.