runtime: make execution error panic values implement the Error interface

Make execution panics implement Error as
mandated by https://golang.org/ref/spec#Run_time_panics,
instead of panics with strings.

Fixes #14965

Change-Id: I7827f898b9b9c08af541db922cc24fa0800ff18a
Reviewed-on: https://go-review.googlesource.com/21214
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/chan.go

@@ -64,7 +64,7 @@ func makechan(t *chantype, size int64) *hchan {
 		throw("makechan: bad alignment")
 	}
 	if size < 0 || int64(uintptr(size)) != size || (elem.size > 0 && uintptr(size) > (_MaxMem-hchanSize)/elem.size) {
-		panic("makechan: size out of range")
+		panic(plainError("makechan: size out of range"))
 	}
 
 	var c *hchan
@@ -171,7 +171,7 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin
 
 	if c.closed != 0 {
 		unlock(&c.lock)
-		panic("send on closed channel")
+		panic(plainError("send on closed channel"))
 	}
 
 	if sg := c.recvq.dequeue(); sg != nil {
@@ -231,7 +231,7 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin
 		if c.closed == 0 {
 			throw("chansend: spurious wakeup")
 		}
-		panic("send on closed channel")
+		panic(plainError("send on closed channel"))
 	}
 	gp.param = nil
 	if mysg.releasetime > 0 {
@@ -302,13 +302,13 @@ func sendDirect(t *_type, sg *sudog, src unsafe.Pointer) {
 
 func closechan(c *hchan) {
 	if c == nil {
-		panic("close of nil channel")
+		panic(plainError("close of nil channel"))
 	}
 
 	lock(&c.lock)
 	if c.closed != 0 {
 		unlock(&c.lock)
-		panic("close of closed channel")
+		panic(plainError("close of closed channel"))
 	}
 
 	if raceenabled {

src/runtime/crash_test.go

@@ -273,6 +273,52 @@ func TestGoexitInPanic(t *testing.T) {
 	}
 }
 
+// Issue 14965: Runtime panics should be of type runtime.Error
+func TestRuntimePanicWithRuntimeError(t *testing.T) {
+	testCases := [...]func(){
+		0: func() {
+			var m map[uint64]bool
+			m[1234] = true
+		},
+		1: func() {
+			ch := make(chan struct{})
+			close(ch)
+			close(ch)
+		},
+		2: func() {
+			var ch = make(chan struct{})
+			close(ch)
+			ch <- struct{}{}
+		},
+		3: func() {
+			var s = make([]int, 2)
+			_ = s[2]
+		},
+		4: func() {
+			n := -1
+			_ = make(chan bool, n)
+		},
+		5: func() {
+			close((chan bool)(nil))
+		},
+	}
+
+	for i, fn := range testCases {
+		got := panicValue(fn)
+		if _, ok := got.(runtime.Error); !ok {
+			t.Errorf("test #%d: recovered value %v(type %T) does not implement runtime.Error", i, got, got)
+		}
+	}
+}
+
+func panicValue(fn func()) (recovered interface{}) {
+	defer func() {
+		recovered = recover()
+	}()
+	fn()
+	return
+}
+
 func TestPanicAfterGoexit(t *testing.T) {
 	// an uncaught panic should still work after goexit
 	output := runTestProg(t, "testprog", "PanicAfterGoexit")

src/runtime/error.go

@@ -50,6 +50,17 @@ func (e errorString) Error() string {
 	return "runtime error: " + string(e)
 }
 
+// plainError represents a runtime error described a string without
+// the prefix "runtime error: " after invoking errorString.Error().
+// See Issue #14965.
+type plainError string
+
+func (e plainError) RuntimeError() {}
+
+func (e plainError) Error() string {
+	return string(e)
+}
+
 type stringer interface {
 	String() string
 }
@@ -82,5 +93,5 @@ func printany(i interface{}) {
 
 // called from generated code
 func panicwrap(pkg, typ, meth string) {
-	panic("value method " + pkg + "." + typ + "." + meth + " called using nil *" + typ + " pointer")
+	panic(plainError("value method " + pkg + "." + typ + "." + meth + " called using nil *" + typ + " pointer"))
 }

src/runtime/hashmap.go

@@ -194,7 +194,7 @@ func makemap(t *maptype, hint int64, h *hmap, bucket unsafe.Pointer) *hmap {
 	}
 
 	if hint < 0 || int64(int32(hint)) != hint {
-		panic("makemap: size out of range")
+		panic(plainError("makemap: size out of range"))
 		// TODO: make hint an int, then none of this nonsense
 	}
 
@@ -428,7 +428,7 @@ func mapaccessK(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, unsafe
 
 func mapassign1(t *maptype, h *hmap, key unsafe.Pointer, val unsafe.Pointer) {
 	if h == nil {
-		panic("assignment to entry in nil map")
+		panic(plainError("assignment to entry in nil map"))
 	}
 	if raceenabled {
 		callerpc := getcallerpc(unsafe.Pointer(&t))

src/runtime/malloc.go

@@ -793,7 +793,7 @@ func newarray(typ *_type, n uintptr) unsafe.Pointer {
 		flags |= flagNoScan
 	}
 	if int(n) < 0 || (typ.size > 0 && n > _MaxMem/typ.size) {
-		panic("runtime: allocation size out of range")
+		panic(plainError("runtime: allocation size out of range"))
 	}
 	return mallocgc(typ.size*n, typ, flags)
 }

src/runtime/proc.go

@@ -381,7 +381,7 @@ func badmcall2(fn func(*g)) {
 }
 
 func badreflectcall() {
-	panic("runtime: arg size to reflect.call more than 1GB")
+	panic(plainError("arg size to reflect.call more than 1GB"))
 }
 
 func lockedOSThread() bool {

src/runtime/select.go

@@ -594,7 +594,7 @@ retc:
 sclose:
 	// send on closed channel
 	selunlock(scases, lockorder)
-	panic("send on closed channel")
+	panic(plainError("send on closed channel"))
 }
 
 func (c *hchan) sortkey() uintptr {