util, gen: Fix Windows process launching support

Author: Sjors

CMakeLists.txt

@@ -144,6 +144,9 @@ target_include_directories(mputil PRIVATE
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
   $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/include>)
 target_link_libraries(mputil PUBLIC CapnProto::kj)
+if(WIN32)
+  target_link_libraries(mputil PUBLIC ws2_32)
+endif()
 
 # libmultiprocess.a runtime library
 set(MP_PUBLIC_HEADERS

include/mp/util.h

@@ -291,6 +291,10 @@ SocketId StartSpawned(const ConnectInfo& connect_info);
 //! between parent and child processes.
 std::array<SocketId, 2> SocketPair();
 
+//! Start a child process and return a handle that can be passed to
+//! WaitProcess().
+ProcessId StartProcess(const std::vector<std::string>& args);
+
 //! Call execvp with vector args.
 //! Not safe to call in a post-fork child of a multi-threaded process.
 //! Currently only used by mpgen at build time.

src/mp/gen.cpp

@@ -27,7 +27,6 @@
 #include <stdexcept>
 #include <string>
 #include <system_error>
-#include <unistd.h>
 #include <utility>
 #include <vector>
 
@@ -180,13 +179,7 @@ static void Generate(kj::StringPtr src_prefix,
     }
     args.emplace_back("--output=" capnp_PREFIX "/bin/capnpc-c++");
     args.emplace_back(src_file);
-    const int pid = fork();
-    if (pid == -1) {
-        throw std::system_error(errno, std::system_category(), "fork");
-    }
-    if (!pid) {
-        mp::ExecProcess(args);
-    }
+    const mp::ProcessId pid = mp::StartProcess(args);
     const int status = mp::WaitProcess(pid);
     if (status) {
         throw std::runtime_error("Invoking " capnp_PREFIX "/bin/capnp failed");

src/mp/util.cpp

@@ -283,7 +283,7 @@ SocketId StartSpawned(const ConnectInfo& connect_info)
     CloseHandle(pipe);
 
     WSADATA dontcare;
-    KJ_WIN32(WSAStartup(MAKEWORD(2, 2), &dontcare) != 0, "WSAStartup() failed");
+    KJ_WIN32(WSAStartup(MAKEWORD(2, 2), &dontcare) == 0, "WSAStartup() failed");
 
     SOCKET socket{WSASocketA(FROM_PROTOCOL_INFO, FROM_PROTOCOL_INFO, FROM_PROTOCOL_INFO, &info, 0, WSA_FLAG_OVERLAPPED | WSA_FLAG_NO_HANDLE_INHERIT)};
     KJ_WINSOCK(socket, "WSASocket(FROM_PROTOCOL_INFO) failed");
@@ -303,6 +303,28 @@ std::array<SocketId, 2> SocketPair()
     return {pair[0], pair[1]};
 }
 
+ProcessId StartProcess(const std::vector<std::string>& args)
+{
+#ifndef WIN32
+    ProcessId pid = fork();
+    if (pid == -1) {
+        throw std::system_error(errno, std::system_category(), "fork");
+    }
+    if (!pid) {
+        ExecProcess(args);
+    }
+    return pid;
+#else
+    std::string cmd{CommandLineFromArgv(args)};
+    STARTUPINFOA si{};
+    si.cb = sizeof(si);
+    PROCESS_INFORMATION pi{};
+    KJ_WIN32(CreateProcessA(nullptr, const_cast<char*>(cmd.c_str()), nullptr, nullptr, FALSE, 0, nullptr, nullptr, &si, &pi), "CreateProcess failed");
+    CloseHandle(pi.hThread);
+    return reinterpret_cast<ProcessId>(pi.hProcess);
+#endif
+}
+
 void ExecProcess(const std::vector<std::string>& args)
 {
     const std::vector<char*> argv{MakeArgv(args)};
@@ -326,7 +348,7 @@ int WaitProcess(ProcessId pid)
 #else
     HANDLE handle{reinterpret_cast<HANDLE>(pid)};
     DWORD result{WaitForSingleObject(handle, INFINITE)};
-    KJ_WIN32(result != WAIT_OBJECT_0, "WaitForSingleObject(child) failed");
+    KJ_WIN32(result == WAIT_OBJECT_0, "WaitForSingleObject(child) failed");
     KJ_WIN32(GetExitCodeProcess(handle, &result), "GetExitCodeProcess failed");
     CloseHandle(handle);
     return result;