Reserve zero as an invalid DSM handle.

Previously, the handle for the control segment could not be zero, but
some other DSM segment could potentially have a handle value of zero.
However, that means that if someone wanted to store a dsm_handle that
might or might not be valid, they would need a separate boolean to
keep track of whether the associated value is legal.  That's annoying,
so change things so that no DSM segment can ever have a handle of 0 -
or as we call it here, DSM_HANDLE_INVALID.

Thomas Munro.  This was submitted as part of a much larger patch to
add an malloc-like allocator for dynamic shared memory, but this part
seems like a good idea independently of the rest of the patch.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/backend/storage/ipc/dsm.c b/src/backend/storage/ipc/dsm.c
index d8066647a07..8c6abe3b45b 100644
--- a/src/backend/storage/ipc/dsm.c
+++ b/src/backend/storage/ipc/dsm.c
@@ -182,7 +182,7 @@ dsm_postmaster_startup(PGShmemHeader *shim)
 		Assert(dsm_control_address == NULL);
 		Assert(dsm_control_mapped_size == 0);
 		dsm_control_handle = random();
-		if (dsm_control_handle == 0)
+		if (dsm_control_handle == DSM_HANDLE_INVALID)
 			continue;
 		if (dsm_impl_op(DSM_OP_CREATE, dsm_control_handle, segsize,
 						&dsm_control_impl_private, &dsm_control_address,
@@ -476,6 +476,8 @@ dsm_create(Size size, int flags)
 	{
 		Assert(seg->mapped_address == NULL && seg->mapped_size == 0);
 		seg->handle = random();
+		if (seg->handle == DSM_HANDLE_INVALID)	/* Reserve sentinel */
+			continue;
 		if (dsm_impl_op(DSM_OP_CREATE, seg->handle, size, &seg->impl_private,
 						&seg->mapped_address, &seg->mapped_size, ERROR))
 			break;