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Commitb40b4dd

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Reserve zero as an invalid DSM handle.
Previously, the handle for the control segment could not be zero, butsome other DSM segment could potentially have a handle value of zero.However, that means that if someone wanted to store a dsm_handle thatmight or might not be valid, they would need a separate boolean tokeep 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 toadd an malloc-like allocator for dynamic shared memory, but this partseems like a good idea independently of the rest of the patch.
1 parent0a74819 commitb40b4dd

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2 files changed

+6
-1
lines changed

2 files changed

+6
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lines changed

‎src/backend/storage/ipc/dsm.c

Lines changed: 3 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -182,7 +182,7 @@ dsm_postmaster_startup(PGShmemHeader *shim)
182182
Assert(dsm_control_address==NULL);
183183
Assert(dsm_control_mapped_size==0);
184184
dsm_control_handle=random();
185-
if (dsm_control_handle==0)
185+
if (dsm_control_handle==DSM_HANDLE_INVALID)
186186
continue;
187187
if (dsm_impl_op(DSM_OP_CREATE,dsm_control_handle,segsize,
188188
&dsm_control_impl_private,&dsm_control_address,
@@ -476,6 +476,8 @@ dsm_create(Size size, int flags)
476476
{
477477
Assert(seg->mapped_address==NULL&&seg->mapped_size==0);
478478
seg->handle=random();
479+
if (seg->handle==DSM_HANDLE_INVALID)/* Reserve sentinel */
480+
continue;
479481
if (dsm_impl_op(DSM_OP_CREATE,seg->handle,size,&seg->impl_private,
480482
&seg->mapped_address,&seg->mapped_size,ERROR))
481483
break;

‎src/include/storage/dsm.h

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -19,6 +19,9 @@ typedef struct dsm_segment dsm_segment;
1919

2020
#defineDSM_CREATE_NULL_IF_MAXSEGMENTS0x0001
2121

22+
/* A sentinel value for an invalid DSM handle. */
23+
#defineDSM_HANDLE_INVALID 0
24+
2225
/* Startup and shutdown functions. */
2326
structPGShmemHeader;/* avoid including pg_shmem.h */
2427
externvoiddsm_cleanup_using_control_segment(dsm_handleold_control_handle);

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