syscall: try non-blocking stdio on wasip1

Try to set stdio to non-blocking mode before the os package
calls NewFile for each fd. NewFile queries the non-blocking flag
but doesn't change it, even if the runtime supports non-blocking
stdio. Since WebAssembly modules are single-threaded, blocking
system calls temporarily halt execution of the module. If the
runtime supports non-blocking stdio, the Go runtime is able to
use the WASI net poller to poll for read/write readiness and is
able to schedule goroutines while waiting.

Change-Id: I1e3ce68a414e3c5960ce6a27fbfd38556e59c3dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/498196
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Auto-Submit: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Achille Roussel <achille.roussel@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/syscall/fs_wasip1.go

@@ -11,6 +11,20 @@ import (
 	"unsafe"
 )
 
+func init() {
+	// Try to set stdio to non-blocking mode before the os package
+	// calls NewFile for each fd. NewFile queries the non-blocking flag
+	// but doesn't change it, even if the runtime supports non-blocking
+	// stdio. Since WebAssembly modules are single-threaded, blocking
+	// system calls temporarily halt execution of the module. If the
+	// runtime supports non-blocking stdio, the Go runtime is able to
+	// use the WASI net poller to poll for read/write readiness and is
+	// able to schedule goroutines while waiting.
+	SetNonblock(0, true)
+	SetNonblock(1, true)
+	SetNonblock(2, true)
+}
+
 type uintptr32 = uint32
 type size = uint32
 type fdflags = uint32