Don't consider stubbed terminal block hash terminal (fixes #4094) (#4096

)

beacon_chain/eth1/eth1_monitor.nim

@@ -134,6 +134,7 @@ type
     depositsChain: Eth1Chain
     eth1Progress: AsyncEvent
 
+    exchangedConfiguration*: bool
     terminalBlockHash*: Option[BlockHash]
     terminalBlockNumber*: Option[Quantity]
 
@@ -588,8 +589,17 @@ proc exchangeTransitionConfiguration*(p: Eth1Monitor): Future[EtcStatus] {.async
          localValue = consensusCfg.terminalTotalDifficulty
     return EtcStatus.mismatch
 
+  if not p.exchangedConfiguration:
+    # Log successful engine configuration exchange once at startup
+    p.exchangedConfiguration = true
+    info "Exchanged engine configuration",
+      ttd = executionCfg.terminalTotalDifficulty,
+      terminalBlockHash = executionCfg.terminalBlockHash,
+      terminalBlockNumber = executionCfg.terminalBlockNumber.uint64
+
   if p.terminalBlockNumber.isSome and p.terminalBlockHash.isSome:
     var res = EtcStatus.match
+
     if consensusCfg.terminalBlockNumber != executionCfg.terminalBlockNumber:
       warn "Engine API reporting different terminal block number",
             engineAPI_value = executionCfg.terminalBlockNumber.uint64,
@@ -602,6 +612,13 @@ proc exchangeTransitionConfiguration*(p: Eth1Monitor): Future[EtcStatus] {.async
       res = EtcStatus.mismatch
     return res
   else:
+    if executionCfg.terminalBlockHash == default BlockHash:
+      # If TERMINAL_BLOCK_HASH is stubbed with
+      # 0x0000000000000000000000000000000000000000000000000000000000000000 then
+      # TERMINAL_BLOCK_HASH and TERMINAL_BLOCK_NUMBER parameters MUST NOT take
+      # an effect.
+      return EtcStatus.match
+
     p.terminalBlockNumber = some executionCfg.terminalBlockNumber
     p.terminalBlockHash = some executionCfg.terminalBlockHash
     return EtcStatus.localConfigurationUpdated