Re: [Patch] ALTER SYSTEM READ ONLY

> On Sep 10, 2021, at 8:42 AM, Mark Dilger <mark.dilger@enterprisedb.com> wrote:
>
> Take for example a code stanza from heapam.c:
>
>    if (needwal)
>        CheckWALPermitted();
>
>    /* NO EREPORT(ERROR) from here till changes are logged */
>    START_CRIT_SECTION();
>
> Now, I know that interrupts won't be processed after starting the critical section, but I can see plain as day that
aninterrupt might get processed *during* CheckWALPermitted, since that function isn't atomic.  

A better example may be found in ginmetapage.c:

        needwal = RelationNeedsWAL(indexrel);
        if (needwal)
        {
            CheckWALPermitted();
            computeLeafRecompressWALData(leaf);
        }

        /* Apply changes to page */
        START_CRIT_SECTION();

Even if CheckWALPermitted is assumed to be close enough to atomic to not be a problem (I don't agree), that argument
can'tbe made here, as computeLeafRecompressWALData is not trivial and signals could easily be processed while it is
running.

—
Mark Dilger
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