db: implement hot backups.

lib/db/lowlevelup.js

@@ -106,6 +106,23 @@ LowlevelUp.prototype.repair = function repair(callback) {
   this.backend.repair(this.location, callback);
 };
 
+/**
+ * Backup the database.
+ * @param {String} path
+ * @param {Function} callback
+ */
+
+LowlevelUp.prototype.backup = function backup(path, callback) {
+  assert(!this.loading);
+  assert(!this.closing);
+  assert(this.loaded);
+
+  if (!this.binding.backup)
+    return this.clone(path, callback);
+
+  this.binding.backup(path, callback);
+};
+
 /**
  * Retrieve a record from the database.
  * @param {String} key
@@ -379,6 +396,63 @@ LowlevelUp.prototype.checkVersion = function checkVersion(key, version, callback
   });
 };
 
+/**
+ * Clone the database.
+ * @param {String} path
+ * @param {Function} callback
+ */
+
+LowlevelUp.prototype.clone = function clone(path, callback) {
+  var self = this;
+  var options = { keys: true, values: true };
+  var tmp, batch;
+
+  assert(!this.loading);
+  assert(!this.closing);
+  assert(this.loaded);
+
+  this.options.createIfMissing = true;
+  this.options.errorIfExists = true;
+
+  tmp = new LowlevelUp(path, this.options);
+
+  function done(err) {
+    tmp.close(function(e) {
+      if (e)
+        return callback(e);
+      callback(err);
+    });
+  }
+
+  tmp.open(function(err) {
+    if (err)
+      return callback(err);
+
+    batch = tmp.batch();
+
+    self.each(options, function(key, value, next, i) {
+      batch.put(key, value);
+
+      if (i % 1000 === 0) {
+        batch.write(function(err) {
+          if (err)
+            return next(err);
+          batch = tmp.batch();
+          next();
+        });
+        return;
+      }
+
+      next();
+    }, function(err) {
+      if (err)
+        return done(err);
+
+      batch.write(done);
+    });
+  });
+};
+
 /*
  * Helpers
  */

lib/http/rpc.js

@@ -2574,10 +2574,14 @@ RPC.prototype.addwitnessaddress = function addwitnessaddress(args, callback) {
 };
 
 RPC.prototype.backupwallet = function backupwallet(args, callback) {
+  var dest;
+
   if (args.help || args.length !== 1)
     return callback(new RPCError('backupwallet "destination"'));
-  // Unlikely to be implemented.
-  callback(new Error('Not implemented.'));
+
+  dest = toString(args[0]);
+
+  this.walletdb.db.backup(dest, callback);
 };
 
 RPC.prototype.dumpprivkey = function dumpprivkey(args, callback) {