Avoid type cheats for invalid dsa_handles and dshash_table_handles.

Invent separate macros for "invalid" values of these types, so that
we needn't embed knowledge of their representations into calling code.
These are all zeroes anyway ATM, so this is not fixing any live bug,
but it makes the code cleaner and more future-proof.

I (tgl) also chose to move DSM_HANDLE_INVALID into dsm_impl.h,
since it seems like it should live beside the typedef for dsm_handle.

Hou Zhijie, Nathan Bossart, Kyotaro Horiguchi, Tom Lane

Discussion: https://postgr.es/m/OS0PR01MB5716860B1454C34E5B179B6694C99@OS0PR01MB5716.jpnprd01.prod.outlook.com

src/backend/replication/logical/launcher.c

@@ -922,8 +922,8 @@ ApplyLauncherShmemInit(void)
 
 		memset(LogicalRepCtx, 0, ApplyLauncherShmemSize());
 
-		LogicalRepCtx->last_start_dsa = DSM_HANDLE_INVALID;
-		LogicalRepCtx->last_start_dsh = DSM_HANDLE_INVALID;
+		LogicalRepCtx->last_start_dsa = DSA_HANDLE_INVALID;
+		LogicalRepCtx->last_start_dsh = DSHASH_HANDLE_INVALID;
 
 		/* Initialize memory and spin locks for each worker slot. */
 		for (slot = 0; slot < max_logical_replication_workers; slot++)
@@ -947,7 +947,7 @@ logicalrep_launcher_attach_dshmem(void)
 	MemoryContext oldcontext;
 
 	/* Quick exit if we already did this. */
-	if (LogicalRepCtx->last_start_dsh != DSM_HANDLE_INVALID &&
+	if (LogicalRepCtx->last_start_dsh != DSHASH_HANDLE_INVALID &&
 		last_start_times != NULL)
 		return;
 
@@ -957,7 +957,7 @@ logicalrep_launcher_attach_dshmem(void)
 	/* Be sure any local memory allocated by DSA routines is persistent. */
 	oldcontext = MemoryContextSwitchTo(TopMemoryContext);
 
-	if (LogicalRepCtx->last_start_dsh == DSM_HANDLE_INVALID)
+	if (LogicalRepCtx->last_start_dsh == DSHASH_HANDLE_INVALID)
 	{
 		/* Initialize dynamic shared hash table for last-start times. */
 		last_start_times_dsa = dsa_create(LWTRANCHE_LAUNCHER_DSA);

src/backend/storage/ipc/dsm.c

@@ -173,9 +173,8 @@ dsm_postmaster_startup(PGShmemHeader *shim)
 
 	/*
 	 * Loop until we find an unused identifier for the new control segment. We
-	 * sometimes use 0 as a sentinel value indicating that no control segment
-	 * is known to exist, so avoid using that value for a real control
-	 * segment.
+	 * sometimes use DSM_HANDLE_INVALID as a sentinel value indicating "no
+	 * control segment", so avoid generating that value for a real handle.
 	 */
 	for (;;)
 	{

src/backend/utils/mmgr/dsa.c

@@ -505,7 +505,7 @@ dsa_create_in_place(void *place, size_t size,
 dsa_handle
 dsa_get_handle(dsa_area *area)
 {
-	Assert(area->control->handle != DSM_HANDLE_INVALID);
+	Assert(area->control->handle != DSA_HANDLE_INVALID);
 	return area->control->handle;
 }
 
@@ -554,7 +554,7 @@ dsa_attach_in_place(void *place, dsm_segment *segment)
 {
 	dsa_area   *area;
 
-	area = attach_internal(place, NULL, DSM_HANDLE_INVALID);
+	area = attach_internal(place, NULL, DSA_HANDLE_INVALID);
 
 	/*
 	 * Clean up when the control segment detaches, if a containing DSM segment

src/include/lib/dshash.h

@@ -23,6 +23,9 @@ typedef struct dshash_table dshash_table;
 /* A handle for a dshash_table which can be shared with other processes. */
 typedef dsa_pointer dshash_table_handle;
 
+/* Sentinel value to use for invalid dshash_table handles. */
+#define DSHASH_HANDLE_INVALID ((dshash_table_handle) InvalidDsaPointer)
+
 /* The type for hash values. */
 typedef uint32 dshash_hash;
 

src/include/storage/dsm.h

@@ -19,9 +19,6 @@ 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);

src/include/storage/dsm_impl.h

@@ -54,6 +54,9 @@ extern PGDLLIMPORT int min_dynamic_shared_memory;
 /* A "name" for a dynamic shared memory segment. */
 typedef uint32 dsm_handle;
 
+/* Sentinel value to use for invalid DSM handles. */
+#define DSM_HANDLE_INVALID ((dsm_handle) 0)
+
 /* All the shared-memory operations we know about. */
 typedef enum
 {

src/include/utils/dsa.h

@@ -99,6 +99,10 @@ typedef pg_atomic_uint64 dsa_pointer_atomic;
  */
 typedef dsm_handle dsa_handle;
 
+/* Sentinel value to use for invalid dsa_handles. */
+#define DSA_HANDLE_INVALID ((dsa_handle) DSM_HANDLE_INVALID)
+
+
 extern dsa_area *dsa_create(int tranche_id);
 extern dsa_area *dsa_create_in_place(void *place, size_t size,
 									 int tranche_id, dsm_segment *segment);