os, syscall: support ill-formed UTF-16 strings on Windows

Windows UTF-16 strings can contain unpaired surrogates, which can't be
decoded into a valid UTF-8 string. This file defines a set of functions
that can be used to encode and decode potentially ill-formed UTF-16
strings by using the
[the WTF-8 encoding](https://simonsapin.github.io/wtf-8/).

WTF-8 is a strict superset of UTF-8, i.e. any string that is
well-formed in UTF-8 is also well-formed in WTF-8 and the content
is unchanged. Also, the conversion never fails and is lossless.

The benefit of using WTF-8 instead of UTF-8 when decoding a UTF-16
string is that the conversion is lossless even for ill-formed
UTF-16 strings. This property allows to read an ill-formed UTF-16
string, convert it to a Go string, and convert it back to the same
original UTF-16 string.

Fixes #59971

Change-Id: Id6007f6e537844913402b233e73d698688cd5ba6
Reviewed-on: https://go-review.googlesource.com/c/go/+/493036
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Paul Hampson <Paul.Hampson@Pobox.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/src/internal/syscall/execenv/execenv_windows.go b/src/internal/syscall/execenv/execenv_windows.go
index 46ba12efc5..2a89ed1f58 100644
--- a/src/internal/syscall/execenv/execenv_windows.go
+++ b/src/internal/syscall/execenv/execenv_windows.go
@@ -9,7 +9,6 @@ package execenv
 import (
 	"internal/syscall/windows"
 	"syscall"
-	"unicode/utf16"
 	"unsafe"
 )
 
@@ -41,7 +40,7 @@ func Default(sys *syscall.SysProcAttr) (env []string, err error) {
 		}
 
 		entry := unsafe.Slice(blockp, (uintptr(end)-uintptr(unsafe.Pointer(blockp)))/2)
-		env = append(env, string(utf16.Decode(entry)))
+		env = append(env, syscall.UTF16ToString(entry))
 		blockp = (*uint16)(unsafe.Add(end, size))
 	}
 	return