syscall: introduce SysProcAttr.AdditionalInheritedHandles on Windows

This allows users to specify handles that they explicitly want to be
inherited by the new process. These handles must already be marked as
inheritable.

Updates #44011.
Updates #21085.

Change-Id: Ib18322e7dc2909e68c4209e80385492804fa15d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/288298
Trust: Jason A. Donenfeld <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>

Author: zx2c4

src/syscall/exec_windows.go

@@ -235,13 +235,14 @@ type ProcAttr struct {
 }
 
 type SysProcAttr struct {
-	HideWindow        bool
-	CmdLine           string // used if non-empty, else the windows command line is built by escaping the arguments passed to StartProcess
-	CreationFlags     uint32
-	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
+	HideWindow                 bool
+	CmdLine                    string // used if non-empty, else the windows command line is built by escaping the arguments passed to StartProcess
+	CreationFlags              uint32
+	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
+	AdditionalInheritedHandles []Handle            // a list of additional handles, already marked as inheritable, that will be inherited by the new process
 }
 
 var zeroProcAttr ProcAttr
@@ -337,6 +338,7 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 	si.StdOutput = fd[1]
 	si.StdErr = fd[2]
 
+	fd = append(fd, sys.AdditionalInheritedHandles...)
 	// Do not accidentally inherit more than these handles.
 	err = updateProcThreadAttribute(si.ProcThreadAttributeList, 0, _PROC_THREAD_ATTRIBUTE_HANDLE_LIST, uintptr(unsafe.Pointer(&fd[0])), uintptr(len(fd))*unsafe.Sizeof(fd[0]), 0, nil)
 	if err != nil {