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jeremybenn |
/* go-deferred-recover.c -- support for a deferred recover function.
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Copyright 2010 The Go Authors. All rights reserved.
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Use of this source code is governed by a BSD-style
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license that can be found in the LICENSE file. */
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#include <stddef.h>
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#include "runtime.h"
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#include "go-panic.h"
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#include "go-defer.h"
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/* This is called when a call to recover is deferred. That is,
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something like
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defer recover()
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We need to handle this specially. In 6g/8g, the recover function
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looks up the stack frame. In particular, that means that a
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deferred recover will not recover a panic thrown in the same
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function that defers the recover. It will only recover a panic
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thrown in a function that defers the deferred call to recover.
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In other words:
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func f1() {
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defer recover() // does not stop panic
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panic(0)
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}
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func f2() {
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defer func() {
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defer recover() // stops panic(0)
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}()
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panic(0)
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}
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func f3() {
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defer func() {
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defer recover() // does not stop panic
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panic(0)
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}()
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panic(1)
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}
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func f4() {
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defer func() {
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defer func() {
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defer recover() // stops panic(0)
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}()
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panic(0)
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}()
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panic(1)
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}
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The interesting case here is f3. As can be seen from f2, the
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deferred recover could pick up panic(1). However, this does not
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happen because it is blocked by the panic(0).
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When a function calls recover, then when we invoke it we pass a
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hidden parameter indicating whether it should recover something.
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This parameter is set based on whether the function is being
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invoked directly from defer. The parameter winds up determining
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whether __go_recover or __go_deferred_recover is called at all.
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In the case of a deferred recover, the hidden parameter which
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controls the call is actually the one set up for the function which
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runs the defer recover() statement. That is the right thing in all
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the cases above except for f3. In f3 the function is permitted to
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call recover, but the deferred recover call is not. We address
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that here by checking for that specific case before calling
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recover. If this function was deferred when there is already a
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panic on the panic stack, then we can only recover that panic, not
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any other.
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Note that we can get away with using a special function here
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because you are not permitted to take the address of a predeclared
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function like recover. */
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struct __go_empty_interface
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__go_deferred_recover ()
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{
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G *g;
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g = runtime_g ();
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if (g->defer == NULL || g->defer->__panic != g->panic)
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{
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struct __go_empty_interface ret;
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ret.__type_descriptor = NULL;
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ret.__object = NULL;
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return ret;
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}
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return __go_recover ();
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}
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