wasm: correctly return from run() in wasm_exec.js

Instead of hanging forever, it should return the exit code from os.Exit.

src/runtime/runtime_tinygowasm.go

@@ -80,12 +80,17 @@ func abort() {
 
 //go:linkname syscall_Exit syscall.Exit
 func syscall_Exit(code int) {
-	// TODO: should we call __stdio_exit here?
-	// It's a low-level exit (syscall.Exit) so doing any libc stuff seems
-	// unexpected, but then where else should stdio buffers be flushed?
+	// Flush stdio buffers.
+	__stdio_exit()
+
+	// Exit the program.
 	proc_exit(uint32(code))
 }
 
+func mainReturnExit() {
+	syscall_Exit(0)
+}
+
 // TinyGo does not yet support any form of parallelism on WebAssembly, so these
 // can be left empty.
 

src/runtime/runtime_tinygowasm_unknown.go

@@ -31,6 +31,10 @@ func abort() {
 
 //go:linkname syscall_Exit syscall.Exit
 func syscall_Exit(code int) {
+	// Because this is the "unknown" target we can't call an exit function.
+	// But we also can't just return since the program will likely expect this
+	// function to never return. So we panic instead.
+	runtimePanic("unsupported: syscall.Exit")
 }
 
 // There is not yet any support for any form of parallelism on WebAssembly, so these

src/runtime/runtime_tinygowasmp2.go

@@ -60,6 +60,13 @@ func syscall_Exit(code int) {
 	exit.Exit(code != 0)
 }
 
+func mainReturnExit() {
+	// WASIp2 does not use _start, instead it uses _initialize and a custom
+	// WASIp2-specific main function. So this should never be called in
+	// practice.
+	runtimePanic("unreachable: _start was called")
+}
+
 // TinyGo does not yet support any form of parallelism on WebAssembly, so these
 // can be left empty.
 

src/runtime/runtime_wasip1.go

@@ -91,10 +91,6 @@ func ticks() timeUnit {
 	return timeUnit(nano)
 }
 
-func beforeExit() {
-	__stdio_exit()
-}
-
 // Implementations of WASI APIs
 
 //go:wasmimport wasi_snapshot_preview1 args_get

src/runtime/runtime_wasip2.go

@@ -52,6 +52,3 @@ func sleepTicks(d timeUnit) {
 func ticks() timeUnit {
 	return timeUnit(monotonicclock.Now())
 }
-
-func beforeExit() {
-}

src/runtime/runtime_wasm_js.go

@@ -32,7 +32,3 @@ func sleepTicks(d timeUnit)
 
 //go:wasmimport gojs runtime.ticks
 func ticks() timeUnit
-
-func beforeExit() {
-	__stdio_exit()
-}

src/runtime/runtime_wasm_unknown.go

@@ -34,5 +34,8 @@ func ticks() timeUnit {
 	return timeUnit(0)
 }
 
-func beforeExit() {
+func mainReturnExit() {
+	// Don't exit explicitly here. We can't (there is no environment with an
+	// exit call) but also it's not needed. We can just let _start and main.main
+	// return to the caller.
 }

src/runtime/runtime_wasmentry.go

@@ -19,7 +19,8 @@ func wasmEntryCommand() {
 	heapEnd = uintptr(wasm_memory_size(0) * wasmPageSize)
 	run()
 	if mainExited {
-		beforeExit()
+		// To make sure wasm_exec.js knows that we've exited, exit explicitly.
+		mainReturnExit()
 	}
 }