runtime: correctly print panics before fatal-ing on defer

Fixes #67792

Change-Id: I93d16580cb31e54cee7c8490212404e4d0dec446
Reviewed-on: https://go-review.googlesource.com/c/go/+/613757
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>

src/runtime/panic.go

@@ -1235,10 +1235,12 @@ func throw(s string) {
 //
 //go:nosplit
 func fatal(s string) {
+	p := getg()._panic
 	// Everything fatal does should be recursively nosplit so it
 	// can be called even when it's unsafe to grow the stack.
 	printlock() // Prevent multiple interleaved fatal reports. See issue 69447.
 	systemstack(func() {
+		printPreFatalDeferPanic(p)
 		print("fatal error: ")
 		printindented(s) // logically printpanicval(s), but avoids convTstring write barrier
 		print("\n")
@@ -1248,6 +1250,27 @@ func fatal(s string) {
 	printunlock()
 }
 
+// printPreFatalDeferPanic prints the panic
+// when fatal occurs in panics while running defer.
+func printPreFatalDeferPanic(p *_panic) {
+	// Don`t call preprintpanics, because
+	// don't want to call String/Error on the panicked values.
+	// When we fatal we really want to just print and exit,
+	// no more executing user Go code.
+	for x := p; x != nil; x = x.link {
+		if x.link != nil && *efaceOf(&x.link.arg) == *efaceOf(&x.arg) {
+			// This panic contains the same value as the next one in the chain.
+			// Mark it as repanicked. We will skip printing it twice in a row.
+			x.link.repanicked = true
+		}
+	}
+	if p != nil {
+		printpanics(p)
+		// make fatal have the same indentation as non-first panics.
+		print("\t")
+	}
+}
+
 // runningPanicDefers is non-zero while running deferred functions for panic.
 // This is used to try hard to get a panic stack trace out when exiting.
 var runningPanicDefers atomic.Uint32

src/runtime/panic_test.go

@@ -34,6 +34,8 @@ func TestPanicWithDirectlyPrintableCustomTypes(t *testing.T) {
 		{"panicCustomUint32", `panic: main.MyUint32(93)`},
 		{"panicCustomUint64", `panic: main.MyUint64(93)`},
 		{"panicCustomUintptr", `panic: main.MyUintptr(93)`},
+		{"panicDeferFatal", "panic: runtime.errorString(\"invalid memory address or nil pointer dereference\")\n\tfatal error: sync: unlock of unlocked mutex"},
+		{"panicDoublieDeferFatal", "panic: runtime.errorString(\"invalid memory address or nil pointer dereference\") [recovered, repanicked]\n\tfatal error: sync: unlock of unlocked mutex"},
 	}
 
 	for _, tt := range tests {

src/runtime/testdata/testprog/panicprint.go

@@ -4,6 +4,8 @@
 
 package main
 
+import "sync"
+
 type MyBool bool
 type MyComplex128 complex128
 type MyComplex64 complex64
@@ -90,6 +92,23 @@ func panicCustomFloat32() {
 	panic(MyFloat32(-93.70))
 }
 
+func panicDeferFatal() {
+	var mu sync.Mutex
+	defer mu.Unlock()
+	var i *int
+	*i = 0
+}
+
+func panicDoublieDeferFatal() {
+	var mu sync.Mutex
+	defer mu.Unlock()
+	defer func() {
+		panic(recover())
+	}()
+	var i *int
+	*i = 0
+}
+
 func init() {
 	register("panicCustomComplex64", panicCustomComplex64)
 	register("panicCustomComplex128", panicCustomComplex128)
@@ -108,4 +127,6 @@ func init() {
 	register("panicCustomUint32", panicCustomUint32)
 	register("panicCustomUint64", panicCustomUint64)
 	register("panicCustomUintptr", panicCustomUintptr)
+	register("panicDeferFatal", panicDeferFatal)
+	register("panicDoublieDeferFatal", panicDoublieDeferFatal)
 }