Re: Avoid unncessary always true test (src/backend/storage/buffer/bufmgr.c)

Hi,

Good catch. Indeed, eb.rel shouldn't be NULL there and the tests should be
unnecessary.

However, it doesn't follow from the code of these functions.
From what I can see eb.rel can be NULL in both of these functions. There is
the following Assert in the beginning of the ExtendBufferedRelTo() function:

Assert((eb.rel != NULL) != (eb.smgr != NULL));

And ExtendBufferedRelShared() is only called from ExtendBufferedRelCommon()
which can be called from ExtendBufferedRelTo() or ExtendBufferedRelBy() that
also has the same Assert(). And none of these functions assigns eb.rel, so
it can be NULL from the very beginning and it stays the same.


And there is the following call in xlogutils.c, which is exactly the case when
eb.rel is NULL:

buffer = ExtendBufferedRelTo(EB_SMGR(smgr, RELPERSISTENCE_PERMANENT),
                             forknum,
                             NULL,
                             EB_PERFORMING_RECOVERY |
                             EB_SKIP_EXTENSION_LOCK,
                             blkno + 1,
                             mode);

So as for me calling LockRelationForExtension() and UnlockRelationForExtension()
without testing eb.rel first looks more like a bug here. However, they are never
actually called with eb.rel=NULL because of the EB_* flags, so there is no bug
here. I believe we should add Assert checking that when eb.rel is NULL, flags
are such that we won't use eb.rel. And yes, we can remove unnecessary checks
where the flags value guaranty us that eb.rel is not NULL.

And another minor observation. It seems to me that we don't need a "could have
been closed while waiting for lock" in ExtendBufferedRelShared(), because I
believe the comment above already explains why updating eb.smgr:

 * Note that another backend might have extended the relation by the time
 * we get the lock.