Explain Panic/Recover Mechanism in Detail

Exiting Phases of Function Calls

1. after the call returns normally.
2. when a panic occurs in the call.
3. after the runtime.Goexit function is called and fully exits in the call.

• a call to the function f0 or f1 will enter its existing phase after it returns normally.
• a call to the function f2 will enter its exiting phase after the divided-by-zero panic happens.
• a call to the function 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
}

Associating Panics and Goexit Signals of Function Calls

• the call will associate with no panics when the unrecovered panic is recovered.
• when a new panic occurs in the function call, the new one will replace the old one to become the associating unrecovered panic of the 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)
}

• if there was an old unrecovered panic associating with the nesting call before, the old one will be replaced by the spread one. For this case, the nesting call must had already entered its exiting phase for sure, so the next deferred function call in its defer-call stack will be invoked.
• if there was not an unrecovered panic associating with the nesting call before, the spread one will associates with the nesting call. For this case, the nesting call might have entered its exiting phase or not. If it hasn't, it will enter its exiting phase immediately.

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

...

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()
	}
}

Some recover Calls Are No-Ops

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")
}

package main

func main() {
	defer func() {
		recover() // take effect
	}()

	panic("bye")
}

• 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.

// 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()
}

Summary