PG17beta2: SMGR: inconsistent type for nblocks

On Tue, Jul 30, 2024 at 11:24 PM Matthias van de Meent
<boekewurm@gmail.com> wrote:

While working on rebasing the patches of Neon's fork onto the
REL_17_STABLE branch, I noticed that the nblocks arguments of various
smgr functions have inconsistent types: smgrzeroextend accepts
`nblocks` as signed integer, as does the new signature for
smgrprefetch, but the new vectorized operations of *readv and *writev,
and the older *writeback all use an unsigned BlockNumber as indicator
for number of blocks.

Can we update the definition to be consistent across this (new, or
also older) API? As far as I can see, in none of these cases are
negative numbers allowed or expected, so updating this all to be
consistently BlockNumber across the API seems like a straigthforward
patch.

cc-ed Thomas as committer of the PG17 smgr API changes.

Hi Matthias,

Yeah, right, I noticed that once myself[1]/messages/by-id/CA+hUKGLx5bLwezZKAYB2O_qHj=ov10RpgRVY7e8TSJVE74oVjg@mail.gmail.com. For the cases from my
keyboard, I guess I was trying to be consistent with nearby existing
stuff in each case, which was already inconsistent... Do you have a
patch?

[1]: /messages/by-id/CA+hUKGLx5bLwezZKAYB2O_qHj=ov10RpgRVY7e8TSJVE74oVjg@mail.gmail.com

On Tue, 30 Jul 2024 at 14:32, Thomas Munro <thomas.munro@gmail.com> wrote:

On Tue, Jul 30, 2024 at 11:24 PM Matthias van de Meent
<boekewurm@gmail.com> wrote:

While working on rebasing the patches of Neon's fork onto the
REL_17_STABLE branch, I noticed that the nblocks arguments of various
smgr functions have inconsistent types: smgrzeroextend accepts
`nblocks` as signed integer, as does the new signature for
smgrprefetch, but the new vectorized operations of *readv and *writev,
and the older *writeback all use an unsigned BlockNumber as indicator
for number of blocks.

Can we update the definition to be consistent across this (new, or
also older) API? As far as I can see, in none of these cases are
negative numbers allowed or expected, so updating this all to be
consistently BlockNumber across the API seems like a straigthforward
patch.

cc-ed Thomas as committer of the PG17 smgr API changes.

Yeah, right, I noticed that once myself[1]. For the cases from my
keyboard, I guess I was trying to be consistent with nearby existing
stuff in each case, which was already inconsistent... Do you have a
patch?

Here's one that covers both master and the v17 backbranch.

Kind regards,

Matthias van de Meent

Hi,

On 2024-08-01 12:45:16 +0200, Matthias van de Meent wrote:

On Tue, 30 Jul 2024 at 14:32, Thomas Munro <thomas.munro@gmail.com> wrote:

On Tue, Jul 30, 2024 at 11:24 PM Matthias van de Meent
<boekewurm@gmail.com> wrote:

While working on rebasing the patches of Neon's fork onto the
REL_17_STABLE branch, I noticed that the nblocks arguments of various
smgr functions have inconsistent types: smgrzeroextend accepts
`nblocks` as signed integer, as does the new signature for
smgrprefetch, but the new vectorized operations of *readv and *writev,
and the older *writeback all use an unsigned BlockNumber as indicator
for number of blocks.

Can we update the definition to be consistent across this (new, or
also older) API? As far as I can see, in none of these cases are
negative numbers allowed or expected, so updating this all to be
consistently BlockNumber across the API seems like a straigthforward
patch.

cc-ed Thomas as committer of the PG17 smgr API changes.

Yeah, right, I noticed that once myself[1]. For the cases from my
keyboard, I guess I was trying to be consistent with nearby existing
stuff in each case, which was already inconsistent... Do you have a
patch?

Here's one that covers both master and the v17 backbranch.

FWIW, I find it quite ugly to use BlockNumber to indicate the number of blocks
to be written. It's just further increasing the type confusion by conflating
"the first block to be targeted" and "number of blocks".

diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 6796756358..1d02766978 100644
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -523,11 +523,11 @@ mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
*/
void
mdzeroextend(SMgrRelation reln, ForkNumber forknum,
-			 BlockNumber blocknum, int nblocks, bool skipFsync)
+			 BlockNumber blocknum, BlockNumber nblocks, bool skipFsync)
{
MdfdVec    *v;
BlockNumber curblocknum = blocknum;
-	int			remblocks = nblocks;
+	int64		remblocks = nblocks;

Assert(nblocks > 0);

Isn't this particularly bogus? What's the point of using a 64bit remblocks
here?

Greetings,

Andres Freund

Hi,

On Thu, 1 Aug 2024 at 18:44, Andres Freund <andres@anarazel.de> wrote:

On 2024-08-01 12:45:16 +0200, Matthias van de Meent wrote:

Here's one that covers both master and the v17 backbranch.

FWIW, I find it quite ugly to use BlockNumber to indicate the number of blocks
to be written. It's just further increasing the type confusion by conflating
"the first block to be targeted" and "number of blocks".

IIf BlockNumber doesn't do it for you, then between plain uint32 and
int64, which would you prefer? int itself doesn't allow syncing of all
blocks of a relation's fork, so that's out for me.

diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 6796756358..1d02766978 100644
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -523,11 +523,11 @@ mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
*/
void
mdzeroextend(SMgrRelation reln, ForkNumber forknum,
-                      BlockNumber blocknum, int nblocks, bool skipFsync)
+                      BlockNumber blocknum, BlockNumber nblocks, bool skipFsync)
{
MdfdVec    *v;
BlockNumber curblocknum = blocknum;
-     int                     remblocks = nblocks;
+     int64           remblocks = nblocks;

Assert(nblocks > 0);

Isn't this particularly bogus? What's the point of using a 64bit remblocks
here?

To prevent underflows in the loop below, if any would happen to exist.
Could've been BlockNumber too, but I went with a slightly more
defensive approach.

Kind regards,

Matthias van de Meent