Re: README.tuplock and SHARE lock

On 2024-Nov-19, Alvaro Herrera wrote:

> Hmm, yeah, it seems you're correct about this being an oversight -- we
> don't necessarily use a multixact if all we want to do is to store a FOR
> SHARE lock.

The "Infomask Bits" section explains correctly. I propose the following
amendment,

diff --git a/src/backend/access/heap/README.tuplock b/src/backend/access/heap/README.tuplock
index 31c52ad28f9..843c2e58f92 100644
--- a/src/backend/access/heap/README.tuplock
+++ b/src/backend/access/heap/README.tuplock
@@ -70,13 +70,8 @@ KEY SHARE conflict

When there is a single locker in a tuple, we can just store the locking info
in the tuple itself. We do this by storing the locker's Xid in XMAX, and
-setting infomask bits specifying the locking strength. There is one exception
-here: since infomask space is limited, we do not provide a separate bit
-for SELECT FOR SHARE, so we have to use the extended info in a MultiXact in
-that case. (The other cases, SELECT FOR UPDATE and SELECT FOR KEY SHARE, are
-presumably more commonly used due to being the standards-mandated locking
-mechanism, or heavily used by the RI code, so we want to provide fast paths
-for those.)
+setting infomask bits specifying the locking strength. See "Infomask Bits"
+below for details on the bit patterns we use.

MultiXacts
----------

--
Álvaro Herrera Breisgau, Deutschland — https://www.EnterpriseDB.com/
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