postgres.git / summary / log / commit / refs

commit    b40b4dd9e10ea701c8d47ccba9407fc32ed384e5
Author:   Robert Haas <rhaas@postgresql.org>
Date:     Tue Nov 15 21:30:35 2016 +0000

    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.


 src/backend/storage/ipc/dsm.c | 4 +++-
 src/include/storage/dsm.h     | 3 +++
 2 files changed, 6 insertions(+), 1 deletion(-)

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;
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);


[parent: 0a7481930c78]