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, 0 deletions

diff --git a/src/include/storage/dsm.h b/src/include/storage/dsm.h
index 8be7c9aeeb5..bc91be62131 100644
--- a/src/include/storage/dsm.h
+++ b/src/include/storage/dsm.h
@@ -19,6 +19,9 @@ typedef struct dsm_segment dsm_segment;
 
 #define DSM_CREATE_NULL_IF_MAXSEGMENTS			0x0001
 
+/* A sentinel value for an invalid DSM handle. */
+#define DSM_HANDLE_INVALID 0
+
 /* Startup and shutdown functions. */
 struct PGShmemHeader;			/* avoid including pg_shmem.h */
 extern void dsm_cleanup_using_control_segment(dsm_handle old_control_handle);