Can Panicking in Go\'s `defer` Functions Interfere with Error Handling?

Panic in Defer Functions: Is It a Concern?

Panicking within a deferred function raises questions about the consequences of interrupting the unwinding process. This article will delve into the behavior of panic functions in defer statements and explore potential concerns associated with this practice.

Behavior of Panic in Deferred Functions

Panicking from a deferred function doesn't initiate a new panic state; instead, it continues the existing panic sequence. While the panic value from the deferred function may overwrite the initial panic value, recover() will still yield the original value, indicating that the panic sequence was not interrupted.

Validity of Panicking in Defer Functions

Panicking in defer functions is generally not a problem. It allows developers to clean up resources or perform additional operations in the event of an unexpected error. Furthermore, all deferred functions will execute, regardless of whether panic() is called within them.

Example

Consider the following code:

<code class="go">func sub() {
    defer func() {
        panic(2)
    }()
    panic(1)
}

func main() {
    defer func() {
        x := recover()
        println(x.(int))
    }()
    sub()
}</code>

When executed, this code first panics with the value 1, then within the deferred function in sub(), it panics with the value 2. However, the recover() in main() still retrieves the original panic value of 1, demonstrating that the second panic did not overwrite it.

Exceptions

It's worth noting that even if panic() is called multiple times in defer functions, all the deferred functions will still execute. The panic sequences will be "wrapped," with the latest panic value appearing as the top-level error.

For instance:

<code class="go">func main() {
    defer func() {
        fmt.Println("Checkpoint 1")
        panic(1)
    }()
    defer func() {
        fmt.Println("Checkpoint 2")
        panic(2)
    }()
    panic(999)
}</code>

Output:

Checkpoint 2
Checkpoint 1
panic: 999
...
panic: 2
...
panic: 1

Conclusion:

Panicking in defer functions is an acceptable practice in Go. It allows for resource cleanup and additional operations during panic handling. While multiple panics in defer functions may result in wrapped errors, all deferred functions will execute, and recover() will still retrieve the original panic value.

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