rpc: Expose g_is_mempool_loaded via getmempoolinfo and /rest/mempool/info.json

And use it to fix a race condition in mempool_persist.py:
https://travis-ci.org/Empact/bitcoin/jobs/487577243

Since e.g. getrawmempool returns errors based on this status, this
enables users to test it for readiness.

doc/REST-interface.md

@@ -101,6 +101,7 @@ $ curl localhost:18332/rest/getutxos/checkmempool/b2cdfd7b89def827ff8af7cd9bff76
 
 Returns various information about the TX mempool.
 Only supports JSON as output format.
+* loaded : (boolean) if the mempool is fully loaded
 * size : (numeric) the number of transactions in the TX mempool
 * bytes : (numeric) size of the TX mempool in bytes
 * usage : (numeric) total TX mempool memory usage

src/rpc/blockchain.cpp

@@ -1484,6 +1484,7 @@ static UniValue getchaintips(const JSONRPCRequest& request)
 UniValue MempoolInfoToJSON(const CTxMemPool& pool)
 {
     UniValue ret(UniValue::VOBJ);
+    ret.pushKV("loaded", g_is_mempool_loaded);
     ret.pushKV("size", (int64_t)pool.size());
     ret.pushKV("bytes", (int64_t)pool.GetTotalTxSize());
     ret.pushKV("usage", (int64_t)pool.DynamicMemoryUsage());
@@ -1504,6 +1505,7 @@ static UniValue getmempoolinfo(const JSONRPCRequest& request)
                 {},
                 RPCResult{
             "{\n"
+            "  \"loaded\": true|false         (boolean) True if the mempool is fully loaded\n"
             "  \"size\": xxxxx,               (numeric) Current tx count\n"
             "  \"bytes\": xxxxx,              (numeric) Sum of all virtual transaction sizes as defined in BIP 141. Differs from actual serialized size because witness data is discounted\n"
             "  \"usage\": xxxxx,              (numeric) Total memory usage for the mempool\n"

test/functional/mempool_persist.py

@@ -37,7 +37,6 @@ Test is as follows:
 """
 from decimal import Decimal
 import os
-import time
 
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal, assert_raises_rpc_error, wait_until
@@ -83,9 +82,10 @@ class MempoolPersistTest(BitcoinTestFramework):
         self.start_node(1, extra_args=["-persistmempool=0"])
         self.start_node(0)
         self.start_node(2)
-        # Give bitcoind a second to reload the mempool
-        wait_until(lambda: len(self.nodes[0].getrawmempool()) == 5, timeout=1)
-        wait_until(lambda: len(self.nodes[2].getrawmempool()) == 5, timeout=1)
+        wait_until(lambda: self.nodes[0].getmempoolinfo()["loaded"], timeout=1)
+        wait_until(lambda: self.nodes[2].getmempoolinfo()["loaded"], timeout=1)
+        assert_equal(len(self.nodes[0].getrawmempool()), 5)
+        assert_equal(len(self.nodes[2].getrawmempool()), 5)
         # The others have loaded their mempool. If node_1 loaded anything, we'd probably notice by now:
         assert_equal(len(self.nodes[1].getrawmempool()), 0)
 
@@ -100,14 +100,14 @@ class MempoolPersistTest(BitcoinTestFramework):
         self.log.debug("Stop-start node0 with -persistmempool=0. Verify that it doesn't load its mempool.dat file.")
         self.stop_nodes()
         self.start_node(0, extra_args=["-persistmempool=0"])
-        # Give bitcoind a second to reload the mempool
-        time.sleep(1)
+        wait_until(lambda: self.nodes[0].getmempoolinfo()["loaded"])
         assert_equal(len(self.nodes[0].getrawmempool()), 0)
 
         self.log.debug("Stop-start node0. Verify that it has the transactions in its mempool.")
         self.stop_nodes()
         self.start_node(0)
-        wait_until(lambda: len(self.nodes[0].getrawmempool()) == 5)
+        wait_until(lambda: self.nodes[0].getmempoolinfo()["loaded"])
+        assert_equal(len(self.nodes[0].getrawmempool()), 5)
 
         mempooldat0 = os.path.join(self.nodes[0].datadir, 'regtest', 'mempool.dat')
         mempooldat1 = os.path.join(self.nodes[1].datadir, 'regtest', 'mempool.dat')
@@ -120,7 +120,8 @@ class MempoolPersistTest(BitcoinTestFramework):
         os.rename(mempooldat0, mempooldat1)
         self.stop_nodes()
         self.start_node(1, extra_args=[])
-        wait_until(lambda: len(self.nodes[1].getrawmempool()) == 5)
+        wait_until(lambda: self.nodes[1].getmempoolinfo()["loaded"])
+        assert_equal(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 creating a tmp folder called mempool.dat.new