syscall: add SysProcAttr.PseudoConsole on Windows

This allows the user to pass a ConPty handle to run a process on a
ConPty session.

See: https://learn.microsoft.com/en-us/windows/console/creating-a-pseudoconsole-session
Fixes: https://learn.microsoft.com/en-us/windows/console/creating-a-pseudoconsole-session

src/syscall/exec_windows.go

@@ -251,6 +251,7 @@ type SysProcAttr struct {
 	NoInheritHandles           bool                // if set, no handles are inherited by the new process, not even the standard handles, contained in ProcAttr.Files, nor the ones contained in AdditionalInheritedHandles
 	AdditionalInheritedHandles []Handle            // a list of additional handles, already marked as inheritable, that will be inherited by the new process
 	ParentProcess              Handle              // if non-zero, the new process regards the process given by this handle as its parent process, and AdditionalInheritedHandles, if set, should exist in this parent process
+	PseudoConsole              Handle              // if non-zero, the new process will be attached to the console represented by this handle, any AdditionalInheritedHandles will be ignored, this implies NoInheritHandles
 }
 
 var zeroProcAttr ProcAttr
@@ -371,9 +372,13 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 			return 0, 0, err
 		}
 	}
-	si.StdInput = fd[0]
-	si.StdOutput = fd[1]
-	si.StdErr = fd[2]
+
+	// If a PseudoConsole is specified, then there is nothing we need to do with the handles since the process will inherit the other end of the PseudoConsole.
+	if sys.PseudoConsole == 0 {
+		si.StdInput = fd[0]
+		si.StdOutput = fd[1]
+		si.StdErr = fd[2]
+	}
 
 	fd = append(fd, sys.AdditionalInheritedHandles...)
 
@@ -396,7 +401,7 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 	}
 	fd = fd[:j]
 
-	willInheritHandles := len(fd) > 0 && !sys.NoInheritHandles
+	willInheritHandles := len(fd) > 0 && !sys.NoInheritHandles && sys.PseudoConsole == 0
 
 	// Do not accidentally inherit more than these handles.
 	if willInheritHandles {
@@ -406,6 +411,13 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 		}
 	}
 
+	if sys.PseudoConsole != 0 {
+		err = updateProcThreadAttribute(si.ProcThreadAttributeList, 0, _PROC_THREAD_ATTRIBUTE_PSEUDOCONSOLE, unsafe.Pointer(sys.PseudoConsole), unsafe.Sizeof(sys.PseudoConsole), nil, nil)
+		if err != nil {
+			return 0, 0, err
+		}
+	}
+
 	envBlock, err := createEnvBlock(attr.Env)
 	if err != nil {
 		return 0, 0, err

src/syscall/types_windows.go

@@ -497,6 +497,7 @@ type _PROC_THREAD_ATTRIBUTE_LIST struct {
 const (
 	_PROC_THREAD_ATTRIBUTE_PARENT_PROCESS = 0x00020000
 	_PROC_THREAD_ATTRIBUTE_HANDLE_LIST    = 0x00020002
+	_PROC_THREAD_ATTRIBUTE_PSEUDOCONSOLE  = 0x00020016
 )
 
 type _STARTUPINFOEXW struct {