select statement: initial set of test cases for corner cases

R=r, rsc
CC=golang-dev
https://golang.org/cl/1772043

test/chan/select3.go

+// $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()
+		}
+	})
+}