Add compatibility macros for MyProcNumber and PGIOAlignedBlock (#12715)

There were a few uses of these already, so collect them to the
compatibility header to avoid the repetition and scattered #ifdefs.

The definition of MyProcNumber is a little different from what was used
before, but the end result is the same. (PGPROC->pgprocno values were
just assigned sequentially to all PGPROC array members, see
InitProcGlobal(). That's a bit silly, which is why it was removed in
v17.)

pgxn/neon/communicator.c

@@ -79,10 +79,6 @@
 #include "access/xlogrecovery.h"
 #endif
 
-#if PG_VERSION_NUM < 160000
-typedef PGAlignedBlock PGIOAlignedBlock;
-#endif
-
 #define NEON_PANIC_CONNECTION_STATE(shard_no, elvl, message, ...) \
 	neon_shard_log(shard_no, elvl, "Broken connection state: " message, \
 				   ##__VA_ARGS__)

pgxn/neon/neon_perf_counters.h

@@ -167,11 +167,7 @@ extern neon_per_backend_counters *neon_per_backend_counters_shared;
  */
 #define NUM_NEON_PERF_COUNTER_SLOTS (MaxBackends + NUM_AUXILIARY_PROCS)
 
-#if PG_VERSION_NUM >= 170000
 #define MyNeonCounters (&neon_per_backend_counters_shared[MyProcNumber])
-#else
-#define MyNeonCounters (&neon_per_backend_counters_shared[MyProc->pgprocno])
-#endif
 
 extern void inc_getpage_wait(uint64 latency);
 extern void inc_page_cache_read_wait(uint64 latency);

pgxn/neon/neon_pgversioncompat.h

@@ -9,6 +9,10 @@
 #include "fmgr.h"
 #include "storage/buf_internals.h"
 
+#if PG_MAJORVERSION_NUM < 16
+typedef PGAlignedBlock PGIOAlignedBlock;
+#endif
+
 #if PG_MAJORVERSION_NUM < 17
 #define NRelFileInfoBackendIsTemp(rinfo) (rinfo.backend != InvalidBackendId)
 #else
@@ -158,6 +162,10 @@ InitBufferTag(BufferTag *tag, const RelFileNode *rnode,
 #define AmAutoVacuumWorkerProcess() (IsAutoVacuumWorkerProcess())
 #endif
 
+#if PG_MAJORVERSION_NUM < 17
+#define	MyProcNumber (MyProc - &ProcGlobal->allProcs[0])
+#endif
+
 #if PG_MAJORVERSION_NUM < 15
 extern void InitMaterializedSRF(FunctionCallInfo fcinfo, bits32 flags);
 extern TimeLineID GetWALInsertionTimeLine(void);

pgxn/neon/pagestore_smgr.c

@@ -72,10 +72,6 @@
 #include "access/xlogrecovery.h"
 #endif
 
-#if PG_VERSION_NUM < 160000
-typedef PGAlignedBlock PGIOAlignedBlock;
-#endif
-
 #include "access/nbtree.h"
 #include "storage/bufpage.h"
 #include "access/xlog_internal.h"