Why Does `defer recover()` Sometimes Fail to Recover Panics in Go?

Understanding the Limitations of defer recover()

In Go, using the defer recover() pattern can catch panics and prevent program termination. However, as demonstrated in the example, a direct call to defer recover() fails to recover from panics. This behavior hinges on the specific implementation of recover().

According to the official documentation, recover() only suppresses panics if called within a deferred function. When called directly, it fails to prevent program termination. This distinction stems from the fact that the deferred function, in the case of defer recover(), is the recover() call itself.

To illustrate this, consider the following example:

package main

func main() {
    defer func() { recover() }() // Recoverable panic
    panic("panic")
}

In this scenario, the anonymous function serves as the deferred function, and the recover() call within it successfully catches the panic.

However, in the following code:

package main

func main() {
    defer recover() // Direct call to recover()
    panic("panic")
}

recover() is called directly as the deferred function, resulting in the program panicking instead of recovering.

An Interesting Variation

An intriguing variation to consider is the following code, which does not panic:

package main

func main() {
    var recover = func() { recover() } // Function type variable
    defer recover()
    panic("panic")
}

In this example, we assign an anonymous function to a variable named recover. This function calls the built-in recover() function. The deferred function is then set to call the value of the recover variable, effectively calling recover(). As a result, the panic is caught and the program continues execution.

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