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>

3 files changed, 3 insertions, 9 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
diff --git a/src/internal/syscall/windows/registry/value.go b/src/internal/syscall/windows/registry/value.go
index 025574015f..7dfee0330f 100644
--- a/src/internal/syscall/windows/registry/value.go
+++ b/src/internal/syscall/windows/registry/value.go
@@ -217,7 +217,7 @@ func (k Key) GetStringsValue(name string) (val []string, valtype uint32, err err
 	from := 0
 	for i, c := range p {
 		if c == 0 {
-			val = append(val, string(utf16.Decode(p[from:i])))
+			val = append(val, syscall.UTF16ToString(p[from:i]))
 			from = i + 1
 		}
 	}
diff --git a/src/internal/syscall/windows/syscall_windows.go b/src/internal/syscall/windows/syscall_windows.go
index cfe4695258..53d32a14a0 100644
--- a/src/internal/syscall/windows/syscall_windows.go
+++ b/src/internal/syscall/windows/syscall_windows.go
@@ -7,7 +7,6 @@ package windows
 import (
 	"sync"
 	"syscall"
-	"unicode/utf16"
 	"unsafe"
 )
 
@@ -17,17 +16,13 @@ func UTF16PtrToString(p *uint16) string {
 	if p == nil {
 		return ""
 	}
-	// Find NUL terminator.
 	end := unsafe.Pointer(p)
 	n := 0
 	for *(*uint16)(end) != 0 {
 		end = unsafe.Pointer(uintptr(end) + unsafe.Sizeof(*p))
 		n++
 	}
-	// Turn *uint16 into []uint16.
-	s := unsafe.Slice(p, n)
-	// Decode []uint16 into string.
-	return string(utf16.Decode(s))
+	return syscall.UTF16ToString(unsafe.Slice(p, n))
 }
 
 const (