ipc, refactor: Update mp::SpawnProcess call

diff --git a/src/ipc/interfaces.cpp b/src/ipc/interfaces.cpp
index 75a854c2..40cddb4b 100644
--- a/src/ipc/interfaces.cpp
+++ b/src/ipc/interfaces.cpp
@@ -62,10 +62,9 @@ public:
     }
     std::unique_ptr<interfaces::Init> spawnProcess(const char* new_exe_name) override
     {
-        mp::ProcessId pid;
-        mp::SocketId fd = m_process->spawn(new_exe_name, m_process_argv0, pid);
+        const auto [pid, socket] = m_process->spawn(new_exe_name, m_process_argv0);
         LogDebug(::BCLog::IPC, "Process %s pid %i launched\n", new_exe_name, pid);
-        auto init = m_protocol->connect(m_protocol->makeStream(fd));
+        auto init = m_protocol->connect(m_protocol->makeStream(socket));
         Ipc::addCleanup(*init, [this, new_exe_name, pid] {
             int status = m_process->waitSpawned(pid);
             LogDebug(::BCLog::IPC, "Process %s pid %i exited with status %i\n", new_exe_name, pid, status);
diff --git a/src/ipc/process.cpp b/src/ipc/process.cpp
index d7f04078..a9aa47ae 100644
--- a/src/ipc/process.cpp
+++ b/src/ipc/process.cpp
@@ -32,13 +32,13 @@ namespace {
 class ProcessImpl : public Process
 {
 public:
-    mp::SocketId spawn(const std::string& new_exe_name, const fs::path& argv0_path, mp::ProcessId& pid) override
+    std::tuple<mp::ProcessId, mp::SocketId> spawn(const std::string& new_exe_name, const fs::path& argv0_path) override
     {
-        return mp::SpawnProcess(pid, [&](int fd) {
+        return mp::SpawnProcess([&](std::string connect_info) {
             fs::path path = argv0_path;
             path.remove_filename();
             path /= fs::PathFromString(new_exe_name);
-            return std::vector<std::string>{fs::PathToString(path), "-ipcfd", strprintf("%i", fd)};
+            return std::vector<std::string>{fs::PathToString(path), "-ipcfd", std::move(connect_info)};
         });
     }
     int waitSpawned(mp::ProcessId pid) override { return mp::WaitProcess(pid); }
@@ -56,11 +56,11 @@ public:
         // in combination with other arguments because the parent process
         // should be able to control the child process through the IPC protocol
         // without passing information out of band.
-        const auto maybe_fd{ToIntegral<int32_t>(argv[2])};
-        if (!maybe_fd) {
-            throw std::runtime_error(strprintf("Invalid -ipcfd number '%s'", argv[2]));
+        try {
+           socket = mp::StartSpawned(argv[2]);
+        } catch (const std::exception& e) {
+           throw std::runtime_error(strprintf("Invalid -ipcfd number '%s' (%s)", argv[2], e.what()));
         }
-        socket = *maybe_fd;
         return true;
     }
     mp::SocketId connect(const fs::path& data_dir,
diff --git a/src/ipc/process.h b/src/ipc/process.h
index 54ca204c..ac597cb0 100644
--- a/src/ipc/process.h
+++ b/src/ipc/process.h
@@ -25,7 +25,7 @@ public:
     virtual ~Process() = default;
 
     //! Spawn process and return socket id for communicating with it.
-    virtual mp::SocketId spawn(const std::string& new_exe_name, const fs::path& argv0_path, mp::ProcessId& pid) = 0;
+    virtual std::tuple<mp::ProcessId, mp::SocketId> spawn(const std::string& new_exe_name, const fs::path& argv0_path) = 0;
 
     //! Wait for spawned process to exit and return its exit code.
     virtual int waitSpawned(mp::ProcessId pid) = 0;
diff --git a/src/ipc/util.h b/src/ipc/util.h
index ec36b418..3fd3ff16 100644
--- a/src/ipc/util.h
+++ b/src/ipc/util.h
@@ -5,8 +5,12 @@
 #ifndef BITCOIN_IPC_UTIL_H
 #define BITCOIN_IPC_UTIL_H
 
+#include <tinyformat.h>
+#include <util/strencodings.h>
+
 #include <array>
 #include <cstdint>
+#include <functional>
 #include <kj/debug.h>
 #include <mp/util.h>
 #include <mp/version.h>
@@ -34,6 +38,20 @@ inline std::array<SocketId, 2> SocketPair()
     KJ_SYSCALL(socketpair(AF_UNIX, SOCK_STREAM, 0, pair));
     return {pair[0], pair[1]};
 }
+
+inline std::tuple<ProcessId, SocketId> SpawnProcess(const std::function<std::vector<std::string>(std::string)>& spawn_argv)
+{
+    ProcessId pid;
+    SocketId socket = SpawnProcess(pid, [&](int fd) { return spawn_argv(strprintf("%d", fd)); });
+    return {pid, socket};
+}
+
+inline SocketId StartSpawned(const std::string& connect_info)
+{
+    auto socket = ToIntegral<SocketId>(connect_info);
+    if (!socket) throw std::invalid_argument(strprintf("Invalid socket descriptor '%s'", connect_info));
+    return *socket;
+}
 #endif
 } // namespace mp