Add savemempool RPC

test/functional/mempool_persist.py

@@ -28,8 +28,14 @@ Test is as follows:
   - Restart node0 with -persistmempool. Verify that it has 5
     transactions in its mempool. This tests that -persistmempool=0
     does not overwrite a previously valid mempool stored on disk.
+  - Remove node0 mempool.dat and verify savemempool RPC recreates it
+    and verify that node1 can load it and has 5 transaction in its
+    mempool.
+  - Verify that savemempool throws when the RPC is called if
+    node1 can't write to disk.
 
 """
+import os
 import time
 
 from test_framework.test_framework import BitcoinTestFramework
@@ -78,5 +84,27 @@ class MempoolPersistTest(BitcoinTestFramework):
         self.start_node(0)
         wait_until(lambda: len(self.nodes[0].getrawmempool()) == 5)
 
+        mempooldat0 = os.path.join(self.options.tmpdir, 'node0', 'regtest', 'mempool.dat')
+        mempooldat1 = os.path.join(self.options.tmpdir, 'node1', 'regtest', 'mempool.dat')
+        self.log.debug("Remove the mempool.dat file. Verify that savemempool to disk via RPC re-creates it")
+        os.remove(mempooldat0)
+        self.nodes[0].savemempool()
+        assert os.path.isfile(mempooldat0)
+
+        self.log.debug("Stop nodes, make node1 use mempool.dat from node0. Verify it has 5 transactions")
+        os.rename(mempooldat0, mempooldat1)
+        self.stop_nodes()
+        self.start_node(1, extra_args=[])
+        wait_until(lambda: len(self.nodes[1].getrawmempool()) == 5)
+
+        self.log.debug("Prevent bitcoind from writing mempool.dat to disk. Verify that `savemempool` fails")
+        # to test the exception we are setting bad permissions on a tmp file called mempool.dat.new
+        # which is an implementation detail that could change and break this test
+        mempooldotnew1 = mempooldat1 + '.new'
+        with os.fdopen(os.open(mempooldotnew1, os.O_CREAT, 0o000), 'w'):
+            pass
+        assert_raises_jsonrpc(-1, "Unable to dump mempool to disk", self.nodes[1].savemempool)
+        os.remove(mempooldotnew1)
+
 if __name__ == '__main__':
     MempoolPersistTest().main()