!362 fix standby node cannot be connected when startup, due to t_thrd.xact_cxt.ShmemVariableCache->recentGlobalXmin is invalid

Merge pull request !362 from xiong_xjun/master_1028

src/gausskernel/storage/ipc/procarray.cpp

@@ -904,6 +904,11 @@ RunningTransactions GetRunningTransactionData(void)
 
     oldestRunningXid = t_thrd.xact_cxt.ShmemVariableCache->nextXid;
 
+    /* xmax is always latestCompletedXid + 1 */
+    TransactionId xmax = latestCompletedXid;
+    TransactionIdAdvance(xmax);
+    TransactionId globalXmin = xmax;
+
     /*
      * Spin over procArray collecting all xids and subxids.
      */
@@ -914,6 +919,13 @@ RunningTransactions GetRunningTransactionData(void)
         TransactionId xid;
         int nxids;
 
+        /* Update globalxmin to be the smallest valid xmin */
+        xid = pgxact->xmin; /* fetch just once */
+
+        if (TransactionIdIsNormal(xid) && TransactionIdPrecedes(xid, globalXmin)) {
+            globalXmin = xid;
+        }
+
         /* Fetch xid just once - see GetNewTransactionId */
         xid = pgxact->xid;
 
@@ -955,6 +967,16 @@ RunningTransactions GetRunningTransactionData(void)
              */
         }
     }
+
+    /*
+     * Update globalxmin to include actual process xids.  This is a slightly
+     * different way of computing it than GetOldestXmin uses, but should give
+     * the same result.
+     */
+    if (TransactionIdPrecedes(oldestRunningXid, globalXmin)) {
+        globalXmin = oldestRunningXid;
+    }
+
     /*
      * It's important *not* to include the limits set by slots here because
      * snapbuild.c uses oldestRunningXid to manage its xmin horizon. If those
@@ -968,6 +990,7 @@ RunningTransactions GetRunningTransactionData(void)
     CurrentRunningXacts->nextXid = t_thrd.xact_cxt.ShmemVariableCache->nextXid;
     CurrentRunningXacts->oldestRunningXid = oldestRunningXid;
     CurrentRunningXacts->latestCompletedXid = latestCompletedXid;
+    CurrentRunningXacts->globalXmin = globalXmin;
 
     Assert(TransactionIdIsValid(CurrentRunningXacts->nextXid));
     Assert(TransactionIdIsValid(CurrentRunningXacts->oldestRunningXid));

src/gausskernel/storage/ipc/standby.cpp

@@ -784,7 +784,7 @@ static XLogRecPtr LogCurrentRunningXacts(RunningTransactions CurrRunningXacts)
 {
     xl_running_xacts xlrec;
     XLogRecPtr recptr;
-    TransactionId recentXmin = t_thrd.xact_cxt.ShmemVariableCache->recentGlobalXmin;
+    TransactionId recentXmin = CurrRunningXacts->globalXmin;
 
     XLogBeginInsert();
     XLogRegisterData((char*)(&recentXmin), sizeof(TransactionId));

src/include/storage/standby.h

@@ -118,6 +118,7 @@ typedef struct RunningTransactionsData {
     bool subxid_overflow;             /* snapshot overflowed, subxids missing */
     TransactionId nextXid;            /* copy of ShmemVariableCache->nextXid */
     TransactionId oldestRunningXid;   /* *not* oldestXmin */
+    TransactionId globalXmin;         /* running xacts's snapshot xmin */
     TransactionId latestCompletedXid; /* copy of ShmemVariableCache-> latestCompletedXid*/
     TransactionId* xids;              /* array of (sub)xids still running */
 } RunningTransactionsData;