Some Panic/Recover Use Cases

Panic and recover have been introduced before. The following of the current article will introduce some (good and bad) panic/recover use cases.

Use Case 1: Avoid Panics Crashing Programs

This should be the most popular use case of panic/recover. The use case is used commonly in concurrent programs, especially client-server programs.

An example:

package main

import "errors"
import "log"
import "net"

func main() {
	listener, err := net.Listen("tcp", ":12345")
	if err != nil {
		log.Fatalln(err)
	}
	for {
		conn, err := listener.Accept()
		if err != nil {
			log.Println(err)
		}
		// Handle each client connection in a new goroutine.
		go ClientHandler(conn)
	}
}

func ClientHandler(c net.Conn) {
	defer func() {
		if v := recover(); v != nil {
			log.Println("capture a panic:", v)
			log.Println("avoid crashing the program")
		}
		c.Close()
	}()
	panic(errors.New("just a demo.")) // a demo-purpose panic
}

Start the server and run telnet localhost 12345 in another terminal, we can observe that the server will not crash down for the panics created in each client handler goroutine.

If we don't recover the potential panic in each client handler goroutine, the potential panic will crash the program.

Use Case 2: Automatically Restart A Crashed Goroutine

When a panic is detected in a goroutine, we can create a new goroutine for it. An example:
package main

import "log"
import "time"

func shouldNotExit() {
	for {
		time.Sleep(time.Second) // simulate a workload
		// Simulate an unexpected panic.
		if time.Now().UnixNano() & 0x3 == 0 {
			panic("unexpected situation")
		}
	}
}

func NeverExit(name string, f func()) {
	defer func() {
		if v := recover(); v != nil { // a panic is detected.
			log.Println(name, "is crashed. Restart it now.")
			go NeverExit(name, f) // restart
		}
	}()
	f()
}

func main() {
	log.SetFlags(0)
	go NeverExit("job#A", shouldNotExit)
	go NeverExit("job#B", shouldNotExit)
	select{} // blocks here for ever
}

Use Case 3: Use panic/recover Calls To Simulate Long Jump Statements

Sometimes, we can use panic/recover as a way to simulate crossing-function long jump statements and crossing-function returns, though generally this way is not recommended to use. This way does harm for both code readability and execution efficiency. The only benefit is sometimes it can make code look less verbose.

In the following example, once a panic is created in an inner function, the execution will jump to the deferred call.
package main

import "fmt"

func main() {
	n := func () (result int)  {
		defer func() {
			if v := recover(); v != nil {
				if n, ok := v.(int); ok {
					result = n
				}
			}
		}()

		func () {
			func () {
				func () {
					// ...
					panic(123) // panic on succeeded
				}()
				// ...
			}()
		}()
		// ...
		return 0
	}()
	fmt.Println(n) // 123
}

Use Case 4: Use panic/recover Calls To Reduce Error Checks

An example:
package main

import "fmt"

func doTask(n int) {
	if n%2 != 0 {
		// Create a demo-purpose panic.
		panic(fmt.Errorf("bad number: %v", n))
	}
	return
}

func doSomething() (err error) {
	defer func() {
		// The second optional return must be present here,
		// otherwise, the assertion will panic if no errors occur.
		err, _ = recover().(error)
	}()

	doTask(22)
	doTask(98)
	doTask(100)
	doTask(53)
	return nil
}

func main() {
	fmt.Println(doSomething()) // bad number: 53
}

The above code is less verbose than the following one.

func doTask(n int) error {
	if n%2 != 0 {
		return fmt.Errorf("bad number: %v", n)
	}
	return nil
}

func doSomething() (err error) {
	err = doTask(22)
	if err != nil {
		return
	}
	err = doTask(98)
	if err != nil {
		return
	}
	err = doTask(100)
	if err != nil {
		return
	}
	err = doTask(53)
	if err != nil {
		return
	}
	return
}

However, usually, this panic/recover use pattern is not recommended to use. It is less readable and less efficient.


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