Re: SerializedSnapshotData alignment

On Mon, Feb 27, 2017 at 12:11:54PM +0530, Robert Haas wrote:
> On Mon, Feb 27, 2017 at 5:13 AM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> > Dear 7b4ac19 authors,
> >
> > Field ps_snapshot_data usually receives four-byte alignment within
> > ParallelIndexScanDescData, but it contains the eight-byte whenTaken field.
> > The select_parallel test dies with SIGBUS on "Oracle Solaris 10 1/13
> > s10s_u11wos_24a SPARC", building with gcc 4.9.2. Some credible fixes:
> >
> > 1. Move the SerializedSnapshotData declaration from snapmgr.c to snapmgr.h and
> > declare the ps_snapshot_data field to be of type SerializedSnapshotData.
> > Probably also add a field "TransactionId xids[FLEXIBLE_ARRAY_MEMBER]" to
> > SerializedSnapshotData, to assert the variable-length nature.
> >
> > 2. Change "char ps_snapshot_data[FLEXIBLE_ARRAY_MEMBER]" to "int64 ...". I
> > have attached this in SerializedSnapshot-int64-v1.patch.
> >
> > 3. Change no declarations, and make snapmgr.c memcpy() the
> > SerializedSnapshotData through a local buffer. I have attached this as
> > SerializedSnapshot-memcpy-v1.patch.
> >
> > I like (2) well enough, but I don't see that technique used elsewhere in the
> > tree. (1) is more typical of PostgreSQL, though I personally like it when
> > structs can stay private to a file. (3) is also well-attested, particularly
> > in xlog replay code. I am leaning toward (2). Other opinions?
>
> In my imagination, we were already doing #3, so I'd probably favor
> that approach.

Pushed that way.