dbLink: backward link tracking

src/ioc/db/dbCommon.dbd

@@ -92,6 +92,12 @@
 		interest(4)
 		extra("ELLLIST             mlis")
 	}
+	field(BKLNK,DBF_NOACCESS) {
+		prompt("Backwards link tracking")
+		special(SPC_NOMOD)
+		interest(4)
+		extra("ELLLIST             bklnk")
+	}
 	field(DISP,DBF_UCHAR) {
 		prompt("Disable putField")
 	}

src/ioc/db/dbLink.c

@@ -141,6 +141,7 @@ static long dbDbInitLink(struct link *plink, short dbfType)
     pdbAddr = dbCalloc(1, sizeof(struct dbAddr));
     *pdbAddr = dbaddr; /* structure copy */
     plink->value.pv_link.pvt = pdbAddr;
+    ellAdd(&dbaddr.precord->bklnk, &plink->value.pv_link.backlinknode);
     /* merging into the same lockset is deferred to the caller.
      * cf. initPVLinks()
      */
@@ -154,6 +155,7 @@ static void dbDbRemoveLink(dbLocker *locker, struct dbCommon *prec, struct link
     plink->value.pv_link.getCvt = 0;
     plink->value.pv_link.lastGetdbrType = 0;
     plink->type = PV_LINK;
+    ellDelete(&pdbAddr->precord->bklnk, &plink->value.pv_link.backlinknode);
     dbLockSetSplit(locker, prec, pdbAddr->precord);
     free(pdbAddr);
 }
@@ -438,6 +440,7 @@ void dbAddLink(dbLocker *locker, struct dbCommon *precord, struct link *plink, s
 
         plink->type = DB_LINK;
         plink->value.pv_link.pvt = ptargetaddr;
+        ellAdd(&ptargetaddr->precord->bklnk, &plink->value.pv_link.backlinknode);
 
         /* target record is already locked in dbPutFieldLink() */
         dbLockSetMerge(locker, plink->value.pv_link.precord, ptargetaddr->precord);

src/ioc/dbStatic/link.h

@@ -17,6 +17,7 @@
 #define INC_link_H
 
 #include "dbDefs.h"
+#include "ellLib.h"
 #include "shareLib.h"
 
 #ifdef __cplusplus
@@ -79,6 +80,7 @@ struct dbCommon;
 struct pvlet;
 
 struct pv_link {
+	ELLNODE backlinknode;
 	char	*pvname;		/* pvname link points to */
 	struct dbCommon *precord;	/* Address of record owning link */
 	void	*pvt;			/* CA or DB private */