Implement -rpcwait and -rpcwaittimeout

This is a Bitcoin compatible implementation of -rpcwait and -rpcwaittimeout

Specifying -rpcwait for the rpc client will cause it to wait for the
time specified with -rpcwaittimeout (in seconds) with one second attempts
to connect. If -rpcwaittimeout is specified as zero or less, the wait is
indefinite. Note that the current default value of -rpcwaittimeout is zero.

src/init.cpp

@@ -48,6 +48,10 @@ extern bool fUseFastIndex;
 // Dump addresses to banlist.dat every 5 minutes (300 s)
 static constexpr int DUMP_BANS_INTERVAL = 300;
 
+// RPC client default timeout.
+extern constexpr int DEFAULT_WAIT_CLIENT_TIMEOUT = 0;
+
+
 std::unique_ptr<BanMan> g_banman;
 
 /**
@@ -534,6 +538,11 @@ void SetupServerArgs()
                    ArgsManager::ALLOW_ANY, OptionsCategory::RPC);
     argsman.AddArg("-rpcuser=<user>", "Username for JSON-RPC connections",
                    ArgsManager::ALLOW_ANY | ArgsManager::SENSITIVE, OptionsCategory::RPC);
+    argsman.AddArg("-rpcwait", "Wait for RPC server to start.",
+                   ArgsManager::ALLOW_ANY, OptionsCategory::RPC);
+    argsman.AddArg("-rpcwaittimeout=<n>", strprintf("Timeout in seconds to wait for the RPC server to start, or 0 for no "
+                                                    "timeout. (default: %d)", DEFAULT_WAIT_CLIENT_TIMEOUT),
+                   ArgsManager::ALLOW_INT, OptionsCategory::RPC);
     argsman.AddArg("-rpcpassword=<pw>", "Password for JSON-RPC connections",
                    ArgsManager::ALLOW_ANY | ArgsManager::SENSITIVE, OptionsCategory::RPC);
     argsman.AddArg("-rpcport=<port>", strprintf("Listen for JSON-RPC connections on <port> (default: %u, testnet: %u)",

src/rpc/client.cpp

@@ -31,6 +31,9 @@ using namespace std;
 using namespace boost;
 using namespace boost::asio;
 
+/** The default timeout for an RPC client to wait for the RPC server to start in seconds */
+static constexpr int DEFAULT_WAIT_CLIENT_TIMEOUT = 0;
+
 UniValue CallRPC(const string& strMethod, const UniValue& params)
 {
     if (!gArgs.IsArgSet("-rpcuser") || !gArgs.IsArgSet("-rpcpassword"))
@@ -47,8 +50,28 @@ UniValue CallRPC(const string& strMethod, const UniValue& params)
     asio::ssl::stream<asio::ip::tcp::socket> sslStream(io_context, context);
     SSLIOStreamDevice<asio::ip::tcp> d(sslStream, fUseSSL);
     iostreams::stream< SSLIOStreamDevice<asio::ip::tcp> > stream(d);
-    if (!d.connect(gArgs.GetArg("-rpcconnect", "127.0.0.1"), gArgs.GetArg("-rpcport", ToString(GetDefaultRPCPort()))))
-        throw runtime_error("couldn't connect to server");
+
+    bool fWait = gArgs.GetBoolArg("-rpcwait", false); // -rpcwait means try until server has started or timeout is reached
+    int timeout = gArgs.GetArg("-rpcwaittimeout", DEFAULT_WAIT_CLIENT_TIMEOUT); // The max time to wait
+
+    std::chrono::seconds deadline = GetTime<std::chrono::seconds>() + std::chrono::seconds{timeout};
+
+    do {
+        // If connection succeeds, immediately break. No need to wait.
+        if (d.connect(gArgs.GetArg("-rpcconnect", "127.0.0.1"),
+                      gArgs.GetArg("-rpcport", ToString(GetDefaultRPCPort())))) {
+            break;
+        }
+
+        std::chrono::seconds now = GetTime<std::chrono::seconds>();
+
+        // Note that timeout <= 0 means wait until connected with one second between connection attempts.
+        if (fWait && (timeout <= 0 || now < deadline)) {
+            UninterruptibleSleep(std::chrono::seconds{1});
+        } else {
+            throw runtime_error("couldn't connect to server");
+        }
+    } while (true);
 
     // HTTP basic authentication
     string strUserPass64 = EncodeBase64(gArgs.GetArg("-rpcuser", "dummy") + ":" + gArgs.GetArg("-rpcpassword", "dummy"));