Merge #12653: Allow to optional specify the directory for the blocks storage

a192636 -blocksdir: keep blockindex leveldb database in datadir (Jonas Schnelli)
f38e4fd QA: Add -blocksdir test (Jonas Schnelli)
386a6b6 Allow to optional specify the directory for the blocks storage (Jonas Schnelli)

Pull request description:

  Since the actual block files taking up more and more space, it may be desirable to have them stored in a different location then the data directory (use case: SSD for chainstate, etc., HD for blocks).

  This PR adds a `-blocksdir` option that allows one to keep the blockfiles and the blockindex external from the data directory (instead of creating symlinks).

  I fist had an option to keep the blockindex within the datadir, but seems to make no sense since accessing the index will (always) lead to access (r/w) the block files.

Tree-SHA512: f8b9e1a681679eac25076dc30e45e6e12d4b2d9ac4be907cbea928a75af081dbcb0f1dd3e97169ab975f73d0bd15824c00c2a34638f3b284b39017171fce2409

test/functional/feature_blocksdir.py

@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+# Copyright (c) 2018 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test the blocksdir option.
+"""
+
+from test_framework.test_framework import BitcoinTestFramework, initialize_datadir
+
+import shutil
+import os
+
+class BlocksdirTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.setup_clean_chain = True
+        self.num_nodes = 1
+
+    def run_test(self):
+        self.stop_node(0)
+        node0path = os.path.join(self.options.tmpdir, "node0")
+        shutil.rmtree(node0path)
+        initialize_datadir(self.options.tmpdir, 0)
+        self.log.info("Starting with non exiting blocksdir ...")
+        self.assert_start_raises_init_error(0, ["-blocksdir="+self.options.tmpdir+ "/blocksdir"], "Specified blocks director")
+        os.mkdir(self.options.tmpdir+ "/blocksdir")
+        self.log.info("Starting with exiting blocksdir ...")
+        self.start_node(0, ["-blocksdir="+self.options.tmpdir+ "/blocksdir"])
+        self.log.info("mining blocks..")
+        self.nodes[0].generate(10)
+        assert(os.path.isfile(os.path.join(self.options.tmpdir, "blocksdir", "regtest", "blocks", "blk00000.dat")))
+        assert(os.path.isdir(os.path.join(self.options.tmpdir, "node0", "regtest", "blocks", "index")))
+
+if __name__ == '__main__':
+    BlocksdirTest().main()

test/functional/test_runner.py

@@ -136,6 +136,7 @@ BASE_SCRIPTS= [
     'p2p_unrequested_blocks.py',
     'feature_logging.py',
     'p2p_node_network_limited.py',
+    'feature_blocksdir.py',
     'feature_config_args.py',
     # Don't append tests at the end to avoid merge conflicts
     # Put them in a random line within the section that fits their approximate run-time