Redefine backend ID to be an index into the proc array

Previously, backend ID was an index into the ProcState array, in the
shared cache invalidation manager (sinvaladt.c). The entry in the
ProcState array was reserved at backend startup by scanning the array
for a free entry, and that was also when the backend got its backend
ID. Things become slightly simpler if we redefine backend ID to be the
index into the PGPROC array, and directly use it also as an index to
the ProcState array. This uses a little more memory, as we reserve a
few extra slots in the ProcState array for aux processes that don't
need them, but the simplicity is worth it.

Aux processes now also have a backend ID. This simplifies the
reservation of BackendStatusArray and ProcSignal slots.

You can now convert a backend ID into an index into the PGPROC array
simply by subtracting 1. We still use 0-based "pgprocnos" in various
places, for indexes into the PGPROC array, but the only difference now
is that backend IDs start at 1 while pgprocnos start at 0. (The next
commmit will get rid of the term "backend ID" altogether and make
everything 0-based.)

There is still a 'backendId' field in PGPROC, now part of 'vxid' which
encapsulates the backend ID and local transaction ID together. It's
needed for prepared xacts. For regular backends, the backendId is
always equal to pgprocno + 1, but for prepared xact PGPROC entries,
it's the ID of the original backend that processed the transaction.

Reviewed-by: Andres Freund, Reid Thompson
Discussion: https://www.postgresql.org/message-id/[email protected]

7 files changed, 44 insertions, 21 deletions

diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index 756d144c323..519ef8ad684 100644
--- a/src/include/miscadmin.h
+++ b/src/include/miscadmin.h
@@ -454,8 +454,6 @@ typedef enum
 	WalWriterProcess,
 	WalReceiverProcess,
 	WalSummarizerProcess,
-
-	NUM_AUXPROCTYPES			/* Must be last! */
 } AuxProcType;
 
 extern PGDLLIMPORT AuxProcType MyAuxProcType;
diff --git a/src/include/storage/backendid.h b/src/include/storage/backendid.h
index 50ac982da19..01387723f79 100644
--- a/src/include/storage/backendid.h
+++ b/src/include/storage/backendid.h
@@ -14,11 +14,15 @@
 #ifndef BACKENDID_H
 #define BACKENDID_H
 
-/* ----------------
- *		-cim 8/17/90
- * ----------------
+/*
+ * BackendId uniquely identifies an active backend or auxiliary process.  It's
+ * assigned at backend startup after authentication.  Note that a backend ID
+ * can be reused for a different backend immediately after a backend exits.
+ *
+ * Backend IDs are assigned starting from 1. For historical reasons, BackendId
+ * 0 is unused, but InvalidBackendId is defined as -1.
  */
-typedef int BackendId;			/* unique currently active backend identifier */
+typedef int BackendId;
 
 #define InvalidBackendId		(-1)
 
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ed6071f3286..13b81228e5b 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -74,9 +74,9 @@ typedef struct
 #define SetInvalidVirtualTransactionId(vxid) \
 	((vxid).backendId = InvalidBackendId, \
 	 (vxid).localTransactionId = InvalidLocalTransactionId)
-#define GET_VXID_FROM_PGPROC(vxid, proc) \
-	((vxid).backendId = (proc).backendId, \
-	 (vxid).localTransactionId = (proc).lxid)
+#define GET_VXID_FROM_PGPROC(vxid_dst, proc) \
+	((vxid_dst).backendId = (proc).vxid.backendId, \
+		 (vxid_dst).localTransactionId = (proc).vxid.lxid)
 
 /* MAX_LOCKMODES cannot be larger than the # of bits in LOCKMASK */
 #define MAX_LOCKMODES		10
@@ -454,8 +454,7 @@ typedef struct LockInstanceData
 	LOCKTAG		locktag;		/* tag for locked object */
 	LOCKMASK	holdMask;		/* locks held by this PGPROC */
 	LOCKMODE	waitLockMode;	/* lock awaited by this PGPROC, if any */
-	BackendId	backend;		/* backend ID of this PGPROC */
-	LocalTransactionId lxid;	/* local transaction ID of this PGPROC */
+	VirtualTransactionId vxid;	/* virtual transaction ID of this PGPROC */
 	TimestampTz waitStart;		/* time at which this PGPROC started waiting
 								 * for lock */
 	int			pid;			/* pid of this PGPROC */
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 4453c6df877..1e804463370 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -186,16 +186,31 @@ struct PGPROC
 								 * vacuum must not remove tuples deleted by
 								 * xid >= xmin ! */
 
-	LocalTransactionId lxid;	/* local id of top-level transaction currently
-								 * being executed by this proc, if running;
-								 * else InvalidLocalTransactionId */
 	int			pid;			/* Backend's process ID; 0 if prepared xact */
 
 	int			pgxactoff;		/* offset into various ProcGlobal->arrays with
 								 * data mirrored from this PGPROC */
 
+	/*
+	 * Currently running top-level transaction's virtual xid. Together these
+	 * form a VirtualTransactionId, but we don't use that struct because this
+	 * is not atomically assignable as whole, and we want to enforce code to
+	 * consider both parts separately.  See comments at VirtualTransactionId.
+	 */
+	struct
+	{
+		BackendId	backendId;	/* For regular backends, equal to
+								 * GetBackendIdFromPGProc(proc).  For prepared
+								 * xacts, ID of the original backend that
+								 * processed the transaction. For unused
+								 * PGPROC entries, InvalidBackendID. */
+		LocalTransactionId lxid;	/* local id of top-level transaction
+									 * currently * being executed by this
+									 * proc, if running; else
+									 * InvalidLocaltransactionId */
+	}			vxid;
+
 	/* These fields are zero while a backend is still starting up: */
-	BackendId	backendId;		/* This backend's backend ID (if assigned) */
 	Oid			databaseId;		/* OID of database this backend is using */
 	Oid			roleId;			/* OID of role using this backend */
 
@@ -406,9 +421,16 @@ extern PGDLLIMPORT PROC_HDR *ProcGlobal;
 
 extern PGDLLIMPORT PGPROC *PreparedXactProcs;
 
-/* Accessor for PGPROC given a pgprocno, and vice versa. */
+/*
+ * Accessors for getting PGPROC given a pgprocno or BackendId, and vice versa.
+ *
+ * For historical reasons, some code uses 0-based "proc numbers", while other
+ * code uses 1-based backend IDs.
+ */
 #define GetPGProcByNumber(n) (&ProcGlobal->allProcs[(n)])
 #define GetNumberFromPGProc(proc) ((proc) - &ProcGlobal->allProcs[0])
+#define GetPGProcByBackendId(n) (&ProcGlobal->allProcs[(n) - 1])
+#define GetBackendIdFromPGProc(proc) (GetNumberFromPGProc(proc) + 1)
 
 /*
  * We set aside some extra PGPROC structures for auxiliary processes,
diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index f3eba9b7640..3af7577e8c6 100644
--- a/src/include/storage/procarray.h
+++ b/src/include/storage/procarray.h
@@ -64,6 +64,10 @@ extern VirtualTransactionId *GetVirtualXIDsDelayingChkpt(int *nvxids, int type);
 extern bool HaveVirtualXIDsDelayingChkpt(VirtualTransactionId *vxids,
 										 int nvxids, int type);
 
+extern PGPROC *BackendIdGetProc(int backendID);
+extern void BackendIdGetTransactionIds(int backendID, TransactionId *xid,
+									   TransactionId *xmin, int *nsubxid,
+									   bool *overflowed);
 extern PGPROC *BackendPidGetProc(int pid);
 extern PGPROC *BackendPidGetProcWithLock(int pid);
 extern int	BackendXidGetPid(TransactionId xid);
diff --git a/src/include/storage/procsignal.h b/src/include/storage/procsignal.h
index 52dcb4c2adf..febdda3611c 100644
--- a/src/include/storage/procsignal.h
+++ b/src/include/storage/procsignal.h
@@ -62,7 +62,7 @@ typedef enum
 extern Size ProcSignalShmemSize(void);
 extern void ProcSignalShmemInit(void);
 
-extern void ProcSignalInit(int pss_idx);
+extern void ProcSignalInit(void);
 extern int	SendProcSignal(pid_t pid, ProcSignalReason reason,
 						   BackendId backendId);
 
diff --git a/src/include/storage/sinvaladt.h b/src/include/storage/sinvaladt.h
index aa3d203efca..c3c97b3f8b7 100644
--- a/src/include/storage/sinvaladt.h
+++ b/src/include/storage/sinvaladt.h
@@ -31,10 +31,6 @@
 extern Size SInvalShmemSize(void);
 extern void CreateSharedInvalidationState(void);
 extern void SharedInvalBackendInit(bool sendOnly);
-extern PGPROC *BackendIdGetProc(int backendID);
-extern void BackendIdGetTransactionIds(int backendID, TransactionId *xid,
-									   TransactionId *xmin, int *nsubxid,
-									   bool *overflowed);
 
 extern void SIInsertDataEntries(const SharedInvalidationMessage *data, int n);
 extern int	SIGetDataEntries(SharedInvalidationMessage *data, int datasize);