--- /dev/null
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Tests verifying the semantics of the select statement
+// for basic empty/non-empty cases.
+
+package main
+
+import "time"
+
+const always = "function did not"
+const never = "function did"
+
+
+func unreachable() {
+ panic("control flow shouldn't reach here")
+}
+
+
+// Calls f and verifies that f always/never panics depending on signal.
+func testPanic(signal string, f func()) {
+ defer func() {
+ s := never
+ if recover() != nil {
+ s = always // f panicked
+ }
+ if s != signal {
+ panic(signal + " panic")
+ }
+ }()
+ f()
+}
+
+
+// Calls f and empirically verifies that f always/never blocks depending on signal.
+func testBlock(signal string, f func()) {
+ c := make(chan string)
+ go func() {
+ f()
+ c <- never // f didn't block
+ }()
+ go func() {
+ time.Sleep(1e8) // 0.1s seems plenty long
+ c <- always // f blocked always
+ }()
+ if <-c != signal {
+ panic(signal + " block")
+ }
+}
+
+
+func main() {
+ const async = 1 // asynchronous channels
+ var nilch chan int
+
+ // sending/receiving from a nil channel outside a select panics
+ testPanic(always, func() {
+ nilch <- 7
+ })
+ testPanic(always, func() {
+ <-nilch
+ })
+
+ // sending/receiving from a nil channel inside a select never panics
+ testPanic(never, func() {
+ select {
+ case nilch <- 7:
+ unreachable()
+ default:
+ }
+ })
+ testPanic(never, func() {
+ select {
+ case <-nilch:
+ unreachable()
+ default:
+ }
+ })
+
+ // sending to an async channel with free buffer space never blocks
+ testBlock(never, func() {
+ ch := make(chan int, async)
+ ch <- 7
+ })
+
+ // receiving from a non-ready channel always blocks
+ testBlock(always, func() {
+ ch := make(chan int)
+ <-ch
+ })
+
+ // TODO(gri) remove this if once 6g accepts empty selects
+ enabled := false
+ if enabled {
+ // empty selects always block
+ testBlock(always, func() {
+ select {
+ case <-make(chan int): // remove this once 6g accepts empty selects
+ }
+ })
+
+ // selects with only nil channels always block
+ testBlock(always, func() {
+ select {
+ case <-nilch:
+ unreachable()
+ }
+ })
+ testBlock(always, func() {
+ select {
+ case nilch <- 7:
+ unreachable()
+ }
+ })
+ testBlock(always, func() {
+ select {
+ case <-nilch:
+ unreachable()
+ case nilch <- 7:
+ unreachable()
+ }
+ })
+ }
+
+ // selects with non-ready non-nil channels always block
+ testBlock(always, func() {
+ ch := make(chan int)
+ select {
+ case <-ch:
+ unreachable()
+ }
+ })
+
+ // selects with default cases don't block
+ testBlock(never, func() {
+ select {
+ default:
+ }
+ })
+ testBlock(never, func() {
+ select {
+ case <-nilch:
+ unreachable()
+ default:
+ }
+ })
+ testBlock(never, func() {
+ select {
+ case nilch <- 7:
+ unreachable()
+ default:
+ }
+ })
+
+ // selects with ready channels don't block
+ testBlock(never, func() {
+ ch := make(chan int, async)
+ select {
+ case ch <- 7:
+ default:
+ unreachable()
+ }
+ })
+ testBlock(never, func() {
+ ch := make(chan int, async)
+ ch <- 7
+ select {
+ case <-ch:
+ default:
+ unreachable()
+ }
+ })
+}