From fa09322c0e0616a1ce5d56d0aabbbf6fb632f2a0 Mon Sep 17 00:00:00 2001
From: Nevkontakte <aleks@nevkontakte.com>
Date: Sun, 20 Feb 2022 21:21:53 +0000
Subject: [PATCH] Prevent non-blocking select from making function appear
 blocking.

In almost every place compiler checks whether a function is blocking by
checking `len(c.Blocking) > 0`, so assigning false to the map confuses
the check, causing unnecessary checkpointing prologue and epilogue to be
added to the function.

Fixes https://github.com/gopherjs/gopherjs/issues/1106.
---
 compiler/statements.go  |  4 +++-
 tests/goroutine_test.go | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/compiler/statements.go b/compiler/statements.go
index 25e5e51be..d845792ee 100644
--- a/compiler/statements.go
+++ b/compiler/statements.go
@@ -520,7 +520,9 @@ func (fc *funcContext) translateStmt(stmt ast.Stmt, label *types.Label) {
 			Fun:  fc.newIdent("$select", types.NewSignature(nil, types.NewTuple(types.NewVar(0, nil, "", types.NewInterface(nil, nil))), types.NewTuple(types.NewVar(0, nil, "", types.Typ[types.Int])), false)),
 			Args: []ast.Expr{fc.newIdent(fmt.Sprintf("[%s]", strings.Join(channels, ", ")), types.NewInterface(nil, nil))},
 		}, types.Typ[types.Int])
-		fc.Blocking[selectCall] = !hasDefault
+		if !hasDefault {
+			fc.Blocking[selectCall] = true
+		}
 		fc.Printf("%s = %s;", selectionVar, fc.translateExpr(selectCall))
 
 		if len(caseClauses) != 0 {
diff --git a/tests/goroutine_test.go b/tests/goroutine_test.go
index a4b5606c0..011e3e8af 100644
--- a/tests/goroutine_test.go
+++ b/tests/goroutine_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"runtime"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -289,3 +290,25 @@ func TestGoroutineJsObject(t *testing.T) {
 		runtime.Gosched()
 	}
 }
+
+func issue1106() {
+	select {
+	default:
+	}
+}
+
+func TestIssue1106(t *testing.T) {
+	// https://github.com/gopherjs/gopherjs/issues/1106#issuecomment-1046323374
+	var done int32 = 0
+	go func() {
+		f := issue1106
+		f()
+		atomic.AddInt32(&done, 1)
+	}()
+
+	// Will get stuck here if #1106 is not fixed.
+	for !atomic.CompareAndSwapInt32(&done, 1, 1) {
+		// Maintain one active goroutine to prevent Node from exiting.
+		runtime.Gosched()
+	}
+}