syscall: expose bInheritHandles of CreateProcess

Certain use cases require this parameter to be false. This includes
spawning a child process in a different windows session than session 0.

Docs regarding the behavior of this parameter to CreateProcess:
https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createprocessa

Fixes #42098

Change-Id: If998f57d6f2962824aacbee75e1b508b255ab293
GitHub-Last-Rev: 584eb13
GitHub-Pull-Request: #41957
Reviewed-on: https://go-review.googlesource.com/c/go/+/261917
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Alex Brainman <alex.brainman@gmail.com>

doc/go1.16.html

@@ -378,6 +378,14 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl><!-- runtime/debug -->
 
+<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
+  <dd>
+    <p><!-- CL 261917 -->
+      <a href="/pkg/syscall/#SysProcAttr"><code>SysProcAttr</code></a> on Windows has a new NoInheritHandles field that disables inheriting handles when creating a new process.
+    </p>
+  </dd>
+</dl><!-- syscall -->
+
 <dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
   <dd>
     <p><!-- CL 260858 -->

src/syscall/exec_windows.go

@@ -241,6 +241,7 @@ type SysProcAttr struct {
 	Token             Token               // if set, runs new process in the security context represented by the token
 	ProcessAttributes *SecurityAttributes // if set, applies these security attributes as the descriptor for the new process
 	ThreadAttributes  *SecurityAttributes // if set, applies these security attributes as the descriptor for the main thread of the new process
+	NoInheritHandles  bool                // if set, each inheritable handle in the calling process is not inherited by the new process
 }
 
 var zeroProcAttr ProcAttr
@@ -341,9 +342,9 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 
 	flags := sys.CreationFlags | CREATE_UNICODE_ENVIRONMENT
 	if sys.Token != 0 {
-		err = CreateProcessAsUser(sys.Token, argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, true, flags, createEnvBlock(attr.Env), dirp, si, pi)
+		err = CreateProcessAsUser(sys.Token, argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, !sys.NoInheritHandles, flags, createEnvBlock(attr.Env), dirp, si, pi)
 	} else {
-		err = CreateProcess(argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, true, flags, createEnvBlock(attr.Env), dirp, si, pi)
+		err = CreateProcess(argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, !sys.NoInheritHandles, flags, createEnvBlock(attr.Env), dirp, si, pi)
 	}
 	if err != nil {
 		return 0, 0, err