Panic and recover mechanism has been introduced before, and several panic/recover use cases are shown in the last article. This current article will explain panic/recover mechanism in detail. Exiting phases of function calls will also be explained in detail.
In Go, a function call may undergo an exiting phase before it fully exits. In the exiting phase, the deferred function calls pushed into the deferred call queue during executing the function call will be executed (in the inverse pushing order). When all of the deferred calls fully exit, the exiting phase ends and the function call also fully exits.
Exiting phases might also be called returning phases elsewhere.
A function call may enter its exiting phase (or exit directly) through three ways:runtime.Goexit function is called and fully exits in the call.
f0 or f1
will enter its existing phase after it returns normally.
f2 will enter its exiting phase
after the divided-by-zero panic happens.
f3 will enter its exiting phase
after the runtime.Goexit function call fully exits.
import (
"fmt"
"runtime"
)
func f0() int {
var x = 1
defer fmt.Println("exits normally:", x)
x++
return x
}
func f1() {
var x = 1
defer fmt.Println("exits normally:", x)
x++
}
func f2() {
var x, y = 1, 0
defer fmt.Println("exits for panicking:", x)
x = x / y // will panic
x++ // unreachable
}
func f3() int {
x := 1
defer fmt.Println("exits for Goexiting:", x)
x++
runtime.Goexit()
return x+x // unreachable
}
BTW, the runtime.Goexit() function is not intended to be called in the main goroutine of a program.
When a panic occurs directly in a function call, we say the (unrecovered) panic starts associating with the function call.
Similarly, when the runtime.Goexit function is called in a function call,
we say a Goexit signal starts associating with the function call after the runtime.Goexit call fully exits.
As explained in the last section,
associating either a panic or a Goexit signal with a function call
will make the function call enter its exiting phase immediately.
We have learned that panics can be recovered. However, there are no ways to cancel a Goexit signal.
At any give time, a function call may associate with at most one unrecovered panic. If a call is associating with an unrecovered panic, thenmain function call.
package main
import "fmt"
func main() {
defer func() {
fmt.Println(recover()) // 3
}()
defer panic(3) // will replace panic 2
defer panic(2) // will replace panic 1
defer panic(1) // will replace panic 0
panic(0)
}
As Goexit signals can't be cancelled, arguing whether a function call may associate with at most one or more than one Goexit signal is unnecessary.
Although it is unusual, there might be multiple unrecovered panics coexisting in a goroutine at a time. Each one associates with one non-exited function call in the call stack of the goroutine. When a nested call fully exits and it still associates with an unrecovered panic, the unrecovered panic will spread to the nesting call (the caller of the nested call). The effect is the same as a panic occurs directly in the nesting call. That says,So, when a goroutine finishes to exit, there may be at most one unrecovered panic in the goroutine. If a goroutine exits with an unrecovered panic, the whole program crashes, and the information of the unrecovered panic will be reported. Otherwise, the goroutine exits normally (peacefully).
When a function is invoked, there is neither a panic nor Goexit signals associating with its call initially,
no matter whether its caller (the nesting call) has entered exiting phase or not.
Surely, panics might occur or the runtime.Goexit function might be called later
in the process of executing the call, so panics and Goexit signals might associate with the call later.
package main
func main() {
// The new goroutine.
go func() {
// This is an anonymous deferred call.
// When it fully exits, the panic 2 will spread
// to the entry function call of the new
// goroutine, and replace the panic 0. The
// panic 2 will never be recovered.
defer func() {
// As explained in the last example,
// panic 2 will replace panic 1.
defer panic(2)
// When the anonymous function call fully
// exits, panic 1 will spread to (and
// associate with) the nesting anonymous
// deferred call.
func () {
// Once the panic 1 occurs, there will
// be two unrecovered panics coexisting
// in the new goroutine. One (panic 0)
// associates with the entry function
// call of the new goroutine, the other
// (panic 1) associates with the
// current anonymous function call.
panic(1)
}()
}()
panic(0)
}()
select{}
}
panic: 0
panic: 1
panic: 2
...
The format of the output is not perfect, it is prone to make some people think that the panic 0 is the final unrecovered panic, whereas the final unrecovered panic is actually panic 2.
Similarly, when a nested call fully exits and it is associating with a Goexit signal, then the Goexit signal will also spread to (and associate with) the nesting call. This will make the nesting call enter (if it hasn't entered) its exiting phase immediately.
When a Goexit signal associates with a function call, if the function call is associating with an unrecovered panic, then the panic will be recovered. For example, the following program will exit peacefully and print<nil>,
because the bye panic will be recovered by the Goexit signal.
package main
import (
"fmt"
"runtime"
)
func f() {
defer func() {
fmt.Println(recover())
}()
// The Goexit signal will disable the "bye" panic.
defer runtime.Goexit()
panic("bye")
}
func main() {
go f()
for runtime.NumGoroutine() > 1 {
runtime.Gosched()
}
}
recover Calls Are No-Opsrecover function must be called at proper places to take effect.
Otherwise, the calls are no-ops.
For example, none of the recover calls in the following example recover the bye panic.
package main
func main() {
defer func() {
defer func() {
recover() // no-op
}()
}()
defer func() {
func() {
recover() // no-op
}()
}()
func() {
defer func() {
recover() // no-op
}()
}()
func() {
defer recover() // no-op
}()
func() {
recover() // no-op
}()
recover() // no-op
defer recover() // no-op
panic("bye")
}
We have already known that the following
recover call takes effect.
package main
func main() {
defer func() {
recover() // take effect
}()
panic("bye")
}
Then why don't those
recover calls in the first example of the current section take effect?
Let's read the current version of Go specification:
recover is nil if any of the following conditions holds:
panic's argument was nil;recover was not called directly by a deferred function.There is an example showing the first condition case in the last article.
Most of therecover calls in the first example of the current section satisfy
either the second or the third conditions mentioned in Go specification,
except the first call. Yes, here, the current descriptions are not precise yet.
The thrid condition should be described as
recover was not called directly by a deferred function call which was called directly
by the function call associating with the expected to-be-recovered panic.
In the first example of the current section, the expected to-be-recovered panic is associating with
the main function call. The first recover call is called
directly by a deferred function call but the deferred function call is not called directly
by the main function call. This is why the first recover call is a no-op.
recover call (by code line order), which is expected to recover panic 1,
in the following example doesn't take effect.
// This program exits without recovering panic 1.
package main
func demo() {
defer func() {
defer func() {
recover() // this one recovers panic 2
}()
defer recover() // no-op
panic(2)
}()
panic(1)
}
func main() {
demo()
}
What Go specification doesn't mention is that,
each recover call is viewed as an attempt
to recover the newest unrecovered panic in the current goroutine.
Go runtime thinks the second recover call in the above example attempts to recover
the newest unrecovered panic, panic 2, which is associating with the caller call
of the second recover call. The second recover call
is not called directly by a deferred function call which is called by the associating function call.
Instead, it is directly called by the associating function call.
This is why the second recover call is a no-op.
recover calls will take effect:
recover call takes effect only if the direct caller of
the recover call is a deferred call and
the direct caller of the deferred call associates with
the newest unrecovered panic in the current goroutine.
An effective recover call disassociates the
newest unrecovered panic from its associating function call,
and returns the value passed to the panic call
which produced the newest unrecovered panic.
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