runtime, sycall/js: add support for callbacks from JavaScript

This commit adds support for JavaScript callbacks back into
WebAssembly. This is experimental API, just like the rest of the
syscall/js package. The time package now also uses this mechanism
to properly support timers without resorting to a busy loop.

JavaScript code can call into the same entry point multiple times.
The new RUN register is used to keep track of the program's
run state. Possible values are: starting, running, paused and exited.
If no goroutine is ready any more, the scheduler can put the
program into the "paused" state and the WebAssembly code will
stop running. When a callback occurs, the JavaScript code puts
the callback data into a queue and then calls into WebAssembly
to allow the Go code to continue running.

Updates #18892
Updates #25506

Change-Id: Ib8701cfa0536d10d69bd541c85b0e2a754eb54fb
Reviewed-on: https://go-review.googlesource.com/114197
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index e3549d367a..36c74a1e8c 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -263,6 +263,7 @@ func forcegchelper() {
 // Gosched yields the processor, allowing other goroutines to run. It does not
 // suspend the current goroutine, so execution resumes automatically.
 func Gosched() {
+	checkTimeouts()
 	mcall(gosched_m)
 }
 
@@ -282,6 +283,9 @@ func goschedguarded() {
 // Reasons should be unique and descriptive.
 // Do not re-use reasons, add new ones.
 func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceEv byte, traceskip int) {
+	if reason != waitReasonSleep {
+		checkTimeouts() // timeouts may expire while two goroutines keep the scheduler busy
+	}
 	mp := acquirem()
 	gp := mp.curg
 	status := readgstatus(gp)
@@ -2361,6 +2365,14 @@ stop:
 		return gp, false
 	}
 
+	// wasm only:
+	// Check if a goroutine is waiting for a callback from the WebAssembly host.
+	// If yes, pause the execution until a callback was triggered.
+	if pauseSchedulerUntilCallback() {
+		// A callback was triggered and caused at least one goroutine to wake up.
+		goto top
+	}
+
 	// Before we drop our P, make a snapshot of the allp slice,
 	// which can change underfoot once we no longer block
 	// safe-points. We don't need to snapshot the contents because