syscall: remove support for O_NONBLOCK and O_SYNC on js/wasm

This commit removes O_NONBLOCK on js/wasm. O_SYNC can't be
removed, because it is referenced by the os package, so instead
its use returns an error.

On Windows, the options O_NONBLOCK and O_SYNC are not available
when opening a file with Node.js. This caused the initialization
of the syscall package to panic.

The simplest solution is to not support these two options on js/wasm
at all. Code written for js/wasm is supposed to be portable,
so platform-specific options should not be used.

Fixes #26524.

Change-Id: I366aa3cdcfa59dfa9dc513368259f363ca090f00
Reviewed-on: https://go-review.googlesource.com/126600Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/wasm/wasm_exec.js

@@ -37,7 +37,7 @@
 
 		let outputBuf = "";
 		global.fs = {
-			constants: { O_WRONLY: -1, O_RDWR: -1, O_CREAT: -1, O_TRUNC: -1, O_APPEND: -1, O_EXCL: -1, O_NONBLOCK: -1, O_SYNC: -1 }, // unused
+			constants: { O_WRONLY: -1, O_RDWR: -1, O_CREAT: -1, O_TRUNC: -1, O_APPEND: -1, O_EXCL: -1 }, // unused
 			writeSync(fd, buf) {
 				outputBuf += decoder.decode(buf);
 				const nl = outputBuf.lastIndexOf("\n");

src/syscall/fs_js.go

@@ -7,6 +7,7 @@
 package syscall
 
 import (
+	"errors"
 	"io"
 	"sync"
 	"syscall/js"
@@ -26,8 +27,6 @@ var (
 	nodeTRUNC  = constants.Get("O_TRUNC").Int()
 	nodeAPPEND = constants.Get("O_APPEND").Int()
 	nodeEXCL   = constants.Get("O_EXCL").Int()
-	nodeNONBLOCK = constants.Get("O_NONBLOCK").Int()
-	nodeSYNC     = constants.Get("O_SYNC").Int()
 )
 
 type jsFile struct {
@@ -78,11 +77,8 @@ func Open(path string, openmode int, perm uint32) (int, error) {
 	if openmode&O_EXCL != 0 {
 		flags |= nodeEXCL
 	}
-	if openmode&O_NONBLOCK != 0 {
-		flags |= nodeNONBLOCK
-	}
 	if openmode&O_SYNC != 0 {
-		flags |= nodeSYNC
+		return 0, errors.New("syscall.Open: O_SYNC is not supported by js/wasm")
 	}
 
 	jsFD, err := fsCall("openSync", path, flags, perm)

src/syscall/syscall_js.go

@@ -108,7 +108,6 @@ const (
 	O_TRUNC  = 01000
 	O_APPEND = 02000
 	O_EXCL   = 0200
-	O_NONBLOCK = 04000
 	O_SYNC   = 010000
 
 	O_CLOEXEC = 0