blockchain: add case for pruning stale blocks

Pruning stale blocks will make the block validation fail for the block
that the prune was triggered on as BlockHeightByHash will not return the
height for blocks that are not in the main chain.

We add a test case to ensure that the test fails in the above case.

blockchain/utxocache_test.go

@@ -729,18 +729,33 @@ func TestFlushOnPrune(t *testing.T) {
 	}
 
 	syncBlocks := func() {
+		// Modify block 1 to be a different hash.  This is to artificially
+		// create a stale branch in the chain.
+		staleMsgBlock := blocks[1].MsgBlock().Copy()
+		staleMsgBlock.Header.Nonce = 0
+		staleBlock := btcutil.NewBlock(staleMsgBlock)
+
+		// Add the stale block here to create a chain view like so. The
+		// block will be the main chain at first but become stale as we
+		// keep adding blocks.  BFNoPoWCheck is given as the pow check will
+		// fail.
+		//
+		// (genesis block) -> 1 -> 2 -> 3 -> ...
+		//                \-> 1a
+		_, _, err = chain.ProcessBlock(staleBlock, BFNoPoWCheck)
+		if err != nil {
+			t.Fatal(err)
+		}
+
 		for i, block := range blocks {
 			if i == 0 {
 				// Skip the genesis block.
 				continue
 			}
-			isMainChain, _, err := chain.ProcessBlock(block, BFNone)
+			_, _, err = chain.ProcessBlock(block, BFNone)
 			if err != nil {
-				t.Fatal(err)
-			}
-
-			if !isMainChain {
-				t.Fatalf("expected block %s to be on the main chain", block.Hash())
+				t.Fatalf("Failed to process block %v(%v). %v",
+					block.Hash().String(), block.Height(), err)
 			}
 		}
 	}