ipc, refactor: Add ProcessId type alias and use it

Use ProcessId type instead of int to represent process ids to be
compatible with an upcoming version of libmultiprocess which adds
windows support.

src/ipc/interfaces.cpp

@@ -62,7 +62,7 @@ public:
     }
     std::unique_ptr<interfaces::Init> spawnProcess(const char* new_exe_name) override
     {
-        int pid;
+        mp::ProcessId pid;
         int fd = m_process->spawn(new_exe_name, m_process_argv0, pid);
         LogDebug(::BCLog::IPC, "Process %s pid %i launched\n", new_exe_name, pid);
         auto init = m_protocol->connect(fd);

src/ipc/process.cpp

@@ -32,7 +32,7 @@ namespace {
 class ProcessImpl : public Process
 {
 public:
-    int spawn(const std::string& new_exe_name, const fs::path& argv0_path, int& pid) override
+    int spawn(const std::string& new_exe_name, const fs::path& argv0_path, mp::ProcessId& pid) override
     {
         return mp::SpawnProcess(pid, [&](int fd) {
             fs::path path = argv0_path;
@@ -41,7 +41,7 @@ public:
             return std::vector<std::string>{fs::PathToString(path), "-ipcfd", strprintf("%i", fd)};
         });
     }
-    int waitSpawned(int pid) override { return mp::WaitProcess(pid); }
+    int waitSpawned(mp::ProcessId pid) override { return mp::WaitProcess(pid); }
     bool checkSpawned(int argc, char* argv[], int& fd) override
     {
         // If this process was not started with a single -ipcfd argument, it is

src/ipc/process.h

@@ -8,6 +8,7 @@
 #include <util/fs.h>
 
 #include <memory>
+#include <ipc/util.h>
 #include <string>
 
 namespace ipc {
@@ -25,10 +26,10 @@ public:
 
     //! Spawn process and return socket file descriptor for communicating with
     //! it.
-    virtual int spawn(const std::string& new_exe_name, const fs::path& argv0_path, int& pid) = 0;
+    virtual int spawn(const std::string& new_exe_name, const fs::path& argv0_path, mp::ProcessId& pid) = 0;
 
     //! Wait for spawned process to exit and return its exit code.
-    virtual int waitSpawned(int pid) = 0;
+    virtual int waitSpawned(mp::ProcessId pid) = 0;
 
     //! Parse command line and determine if current process is a spawned child
     //! process. If so, return true and a file descriptor for communicating

src/ipc/util.h

@@ -0,0 +1,21 @@
+// Copyright (c) The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_IPC_UTIL_H
+#define BITCOIN_IPC_UTIL_H
+
+#include <cstdint>
+#include <mp/util.h>
+#include <mp/version.h>
+
+namespace mp {
+// Definitions that can be deleted when libmultiprocess subtree is updated to
+// v14. Having these allows Bitcoin Core changes to be decoupled from
+// libmultiprocess changes so they don't have to be reviewed in a single PR.
+#if MP_MAJOR_VERSION < 14
+using ProcessId = int;
+#endif
+} // namespace mp
+
+#endif // BITCOIN_IPC_UTIL_H