chain: refactor alt chain saving.

lib/chain/chain.js

@@ -913,6 +913,37 @@ Chain.prototype.setBestChain = co(function* setBestChain(entry, block, prev) {
   this.emit('tip', entry);
 });
 
+/**
+ * Save block on an alternate chain.
+ * @private
+ * @param {ChainEntry} entry
+ * @param {Block|MerkleBlock} block
+ * @param {ChainEntry} prev
+ * @returns {Promise}
+ */
+
+Chain.prototype.saveAlternate = co(function* saveAlternate(entry, block, prev) {
+  try {
+    // Do as much verification
+    // as we can before saving.
+    yield this.verify(block, prev);
+  } catch (e) {
+    // Couldn't verify block.
+    // Revert the height.
+    block.setHeight(-1);
+
+    if (e.type === 'VerifyError') {
+      if (!e.malleated)
+        this.invalid[entry.hash] = true;
+      this.emit('invalid', block, entry.height);
+    }
+
+    throw e;
+  }
+
+  yield this.db.save(entry, block);
+});
+
 /**
  * Reset the chain to the desired height. This
  * is useful for replaying the blockchain download
@@ -1188,19 +1219,10 @@ Chain.prototype._add = co(function* add(block) {
     // our tip's. Add the block but do _not_
     // connect the inputs.
     if (entry.chainwork.cmp(this.tip.chainwork) <= 0) {
-      try {
+      // Save block to an alternate chain.
-        yield this.verify(block, prev);
+      yield this.saveAlternate(entry, block, prev);
-      } catch (e) {
-        if (e.type === 'VerifyError') {
-          if (!e.malleated)
-            this.invalid[entry.hash] = true;
-          this.emit('invalid', block, entry.height);
-        }
-        throw e;
-      }
-
-      yield this.db.save(entry, block);
 
+      // Emit as a "competitor" block.
       this.emit('competitor', block, entry);
 
       if (!initial)