Merge bitcoin#23398: rpc: add return message to savemempool RPC

aa1a4c9 Add file validation to savemempool RPC test (lsilva01)
871e64d Add filename to savemempool RPC result (lsilva01)

Pull request description:

  Currently, if the user calls the `savemempool` RPC method, there is no way to know
  where the file was created (unless the user knows internal implementation details).

  This PR adds a return message stating the file name and path where the mempool was saved and changes `mempool_persist.py` to validate this new return message.

ACKs for top commit:
  laanwj:
    Code review ACK aa1a4c9

Tree-SHA512: e8b1dd0a8976e5eb15f7476c9651e492d2c621a67e0b726721fa7a2ae0ddd272ee28b87a2d0c650bd635e07fa96bdefe77bece4deb6486ef3ee9a4f83423a840

Author: laanwj

src/rpc/blockchain.cpp

@@ -2202,7 +2202,11 @@ static RPCHelpMan savemempool()
     return RPCHelpMan{"savemempool",
                 "\nDumps the mempool to disk. It will fail until the previous dump is fully loaded.\n",
                 {},
-                RPCResult{RPCResult::Type::NONE, "", ""},
+                RPCResult{
+                    RPCResult::Type::OBJ, "", "",
+                    {
+                        {RPCResult::Type::STR, "filename", "the directory and file where the mempool was saved"},
+                    }},
                 RPCExamples{
                     HelpExampleCli("savemempool", "")
             + HelpExampleRpc("savemempool", "")
@@ -2211,6 +2215,8 @@ static RPCHelpMan savemempool()
 {
     const CTxMemPool& mempool = EnsureAnyMemPool(request.context);
 
+    const NodeContext& node = EnsureAnyNodeContext(request.context);
+
     if (!mempool.IsLoaded()) {
         throw JSONRPCError(RPC_MISC_ERROR, "The mempool was not loaded yet");
     }
@@ -2219,7 +2225,10 @@ static RPCHelpMan savemempool()
         throw JSONRPCError(RPC_MISC_ERROR, "Unable to dump mempool to disk");
     }
 
-    return NullUniValue;
+    UniValue ret(UniValue::VOBJ);
+    ret.pushKV("filename", fs::path((node.args->GetDataDirNet() / "mempool.dat")).u8string());
+
+    return ret;
 },
     };
 }

test/functional/mempool_persist.py

@@ -149,8 +149,9 @@ def run_test(self):
         mempooldat1 = os.path.join(self.nodes[1].datadir, self.chain, '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()
+        result0 = self.nodes[0].savemempool()
         assert os.path.isfile(mempooldat0)
+        assert_equal(result0['filename'], mempooldat0)
 
         self.log.debug("Stop nodes, make node1 use mempool.dat from node0. Verify it has 6 transactions")
         os.rename(mempooldat0, mempooldat1)