Re: [HACKERS] 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@leadboat.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.