eth/protocols/eth: stop serving on unavailable responses (#34787)

This is an alternative PR for
https://github.com/ethereum/go-ethereum/pull/34746.
This PR implements the second approach among the two possible solutions
mentioned in the above PR.

Requests for unavailable items are possible when the peer is following a
different fork from us. However this is not expected to happen
frequently. Considering the amount of complexity added to the codebase,
the simpler approach (this PR) can be preferred.

eth/protocols/eth/handler_test.go

@@ -424,16 +424,20 @@ func testGetBlockBodies(t *testing.T, protocol uint) {
 		{0, []common.Hash{backend.chain.CurrentBlock().Hash()}, []bool{true}, 1}, // The chains head block should be retrievable
 		{0, []common.Hash{{}}, []bool{false}, 0},                                 // A non existent block should not be returned
 
-		// Existing and non-existing blocks interleaved should not cause problems
+		// Existing blocks followed by a non-existing one should stop at the gap
+		{0, []common.Hash{
+			backend.chain.GetBlockByNumber(1).Hash(),
+			backend.chain.GetBlockByNumber(10).Hash(),
+			backend.chain.GetBlockByNumber(100).Hash(),
+			{},
+		}, []bool{true, true, true, false}, 3},
+
+		// A non-existing block at the start should return nothing
 		{0, []common.Hash{
 			{},
 			backend.chain.GetBlockByNumber(1).Hash(),
-			{},
 			backend.chain.GetBlockByNumber(10).Hash(),
-			{},
-			backend.chain.GetBlockByNumber(100).Hash(),
-			{},
-		}, []bool{false, true, false, true, false, true, false}, 3},
+		}, []bool{false, true, true}, 0},
 	}
 	// Run each of the tests and verify the results against the chain
 	for i, tt := range tests {

eth/protocols/eth/handlers.go

@@ -238,10 +238,12 @@ func ServiceGetBlockBodiesQuery(chain *core.BlockChain, query GetBlockBodiesRequ
 			lookups >= 2*maxBodiesServe {
 			break
 		}
-		if data := chain.GetBodyRLP(hash); len(data) != 0 {
-			bodies = append(bodies, data)
-			bytes += len(data)
+		data := chain.GetBodyRLP(hash)
+		if len(data) == 0 {
+			break // If we don't have this block's body, stop serving.
 		}
+		bodies = append(bodies, data)
+		bytes += len(data)
 	}
 	return bodies
 }
@@ -281,16 +283,16 @@ func ServiceGetReceiptsQuery69(chain *core.BlockChain, query GetReceiptsRequest)
 		// Retrieve the requested block's receipts
 		results := chain.GetReceiptsRLP(hash)
 		if results == nil {
-			continue // Can't retrieve the receipts, so we just skip this block.
+			break // Don't have this block's receipts, stop serving.
 		}
 		body := chain.GetBodyRLP(hash)
 		if body == nil {
-			continue // The block body is missing, we also have to skip.
+			break // The block body is missing, stop serving.
 		}
 		results, _, err := blockReceiptsToNetwork(results, body, receiptQueryParams{})
 		if err != nil {
 			log.Error("Error in block receipts conversion", "hash", hash, "err", err)
-			continue
+			break
 		}
 		receipts.AppendRaw(results)
 		bytes += len(results)
@@ -312,12 +314,13 @@ func serviceGetReceiptsQuery70(chain *core.BlockChain, query GetReceiptsRequest,
 			break
 		}
 		results := chain.GetReceiptsRLP(hash)
+		// If we don't have this block's receipts or body, stop serving.
 		if results == nil {
-			continue // Can't retrieve the receipts, so we just skip this block.
+			break
 		}
 		body := chain.GetBodyRLP(hash)
 		if body == nil {
-			continue // The block body is missing, we also have to skip.
+			break
 		}
 		q := receiptQueryParams{sizeLimit: uint64(maxPacketSize - bytes)}
 		if i == 0 {
@@ -326,7 +329,7 @@ func serviceGetReceiptsQuery70(chain *core.BlockChain, query GetReceiptsRequest,
 		results, incomplete, err := blockReceiptsToNetwork(results, body, q)
 		if err != nil {
 			log.Error("Error in block receipts conversion", "hash", hash, "err", err)
-			continue
+			break
 		}
 		if results == nil {
 			// This case triggers when the first receipt of the block receipts list doesn't