Spoof PID across execve() on Windows

It's now possible with cosmo and redbean, to deliver a signal to a child
process after it has called execve(). However the executed program needs
to be compiled using cosmocc. The cosmo runtime WinMain() implementation
now intercepts a _COSMO_PID environment variable that's set by execve().
It ensures the child process will use the same C:\ProgramData\cosmo\sigs
file, which is where kill() will place the delivered signal. We are able
to do this on Windows even better than NetBSD, which has a bug with this

Fixes #1334

libc/proc/execve-nt.greg.c

@@ -17,13 +17,14 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/assert.h"
+#include "libc/calls/calls.h"
 #include "libc/calls/internal.h"
+#include "libc/calls/sig.internal.h"
 #include "libc/calls/struct/sigset.internal.h"
 #include "libc/calls/syscall-nt.internal.h"
 #include "libc/errno.h"
 #include "libc/fmt/itoa.h"
 #include "libc/intrin/fds.h"
-#include "libc/intrin/kprintf.h"
 #include "libc/mem/mem.h"
 #include "libc/nt/enum/processaccess.h"
 #include "libc/nt/enum/startf.h"
@@ -33,8 +34,10 @@
 #include "libc/nt/runtime.h"
 #include "libc/nt/struct/processinformation.h"
 #include "libc/nt/struct/startupinfo.h"
+#include "libc/nt/thunk/msabi.h"
 #include "libc/proc/describefds.internal.h"
 #include "libc/proc/ntspawn.h"
+#include "libc/runtime/internal.h"
 #include "libc/str/str.h"
 #include "libc/sysv/consts/at.h"
 #include "libc/sysv/consts/o.h"
@@ -43,23 +46,37 @@
 #include "libc/thread/thread.h"
 #ifdef __x86_64__
 
+__msabi extern typeof(TerminateProcess) *const __imp_TerminateProcess;
+
+extern pthread_mutex_t __sig_worker_lock;
+
+static void sys_execve_nt_abort(sigset_t sigmask) {
+  _pthread_unlock();
+  pthread_mutex_unlock(&__sig_worker_lock);
+  __sig_unblock(sigmask);
+}
+
 textwindows int sys_execve_nt(const char *program, char *const argv[],
                               char *const envp[]) {
 
   // execve() needs to be @asyncsignalsafe
   sigset_t sigmask = __sig_block();
-  _pthread_lock();
+  pthread_mutex_lock(&__sig_worker_lock);  // order matters
+  _pthread_lock();                         // order matters
 
   // new process should be a child of our parent
   int64_t hParentProcess;
   int ppid = sys_getppid_nt();
   if (!(hParentProcess = OpenProcess(
             kNtProcessDupHandle | kNtProcessCreateProcess, false, ppid))) {
-    _pthread_unlock();
-    __sig_unblock(sigmask);
+    sys_execve_nt_abort(sigmask);
     return -1;
   }
 
+  // inherit pid
+  char pidvar[11 + 21];
+  FormatUint64(stpcpy(pidvar, "_COSMO_PID="), __pid);
+
   // inherit signal mask
   char maskvar[6 + 21];
   FormatUint64(stpcpy(maskvar, "_MASK="), sigmask);
@@ -84,22 +101,26 @@ textwindows int sys_execve_nt(const char *program, char *const argv[],
   if (!(fdspec = __describe_fds(g_fds.p, g_fds.n, &si, hParentProcess,
                                 &lpExplicitHandles, &dwExplicitHandleCount))) {
     CloseHandle(hParentProcess);
-    _pthread_unlock();
-    __sig_unblock(sigmask);
+    sys_execve_nt_abort(sigmask);
     return -1;
   }
 
+  // inherit pending signals
+  atomic_fetch_or_explicit(
+      __sig.process,
+      atomic_load_explicit(&__get_tls()->tib_sigpending, memory_order_acquire),
+      memory_order_release);
+
   // launch the process
   struct NtProcessInformation pi;
   int rc = ntspawn(&(struct NtSpawnArgs){
-      AT_FDCWD, program, argv, envp, (char *[]){fdspec, maskvar, 0}, 0, 0,
-      hParentProcess, lpExplicitHandles, dwExplicitHandleCount, &si, &pi});
+      AT_FDCWD, program, argv, envp, (char *[]){fdspec, maskvar, pidvar, 0}, 0,
+      0, hParentProcess, lpExplicitHandles, dwExplicitHandleCount, &si, &pi});
   __undescribe_fds(hParentProcess, lpExplicitHandles, dwExplicitHandleCount);
   if (rc == -1) {
     free(fdspec);
     CloseHandle(hParentProcess);
-    _pthread_unlock();
-    __sig_unblock(sigmask);
+    sys_execve_nt_abort(sigmask);
     if (GetLastError() == kNtErrorSharingViolation) {
       return etxtbsy();
     } else {
@@ -112,12 +133,13 @@ textwindows int sys_execve_nt(const char *program, char *const argv[],
   if (DuplicateHandle(GetCurrentProcess(), pi.hProcess, hParentProcess, &handle,
                       0, false, kNtDuplicateSameAccess)) {
     unassert(!(handle & 0xFFFFFFFFFF000000));
-    TerminateThisProcess(0x23000000u | handle);
+    __imp_TerminateProcess(-1, 0x23000000u | handle);
   } else {
     // TODO(jart): Why does `make loc` print this?
     // kprintf("DuplicateHandle failed w/ %d\n", GetLastError());
-    TerminateThisProcess(ECHILD);
+    __imp_TerminateProcess(-1, ECHILD);
   }
+  __builtin_unreachable();
 }
 
 #endif /* __x86_64__ */