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Reserve zero as an invalid DSM handle.

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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.
This commit is contained in:
Robert Haas 2016-11-15 16:30:35 -05:00
parent 0a7481930c
commit b40b4dd9e1
2 changed files with 6 additions and 1 deletions
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4
src/backend/storage/ipc/dsm.c
View File

@ -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;

3
src/include/storage/dsm.h
View File

@ -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);