Deferred call may return an error

When calling a function which may return an error, we should check whether an error has occurred and handle it in some meaningful way. Deferring a function call does not allow us to perform such a check. Therefore, calls to functions which may return errors should not be deferred.

Recommendation

Examine the deferred function call carefully to check whether any errors that may arise should be handled explicitly.

Example

In the following example, a call to io.WriteLine is deferred with the intention of appendending some data to a file after some other data has been written to it. However, the call may not succeed and return and error, as demonstrated in the other use of io.WriteLine.

package main

import (
	"io"
	"log"
	"os"
)

func example() {
	defer io.WriteString(os.Stdout, "All done!")

	if _, err := io.WriteString(os.Stdout, "Hello"); err != nil {
		log.Fatal(err)
	}
}

To correct this issue, do not defer the function call that may return an error and handle the error explicitly instead:

package main

import (
	"io"
	"log"
	"os"
)

func example() {
	if _, err := io.WriteString(os.Stdout, "Hello"); err != nil {
		log.Fatal(err)
	}

	if _, err := io.WriteString(os.Stdout, "All done!"); err != nil {
		log.Fatal(err)
	}
}

References

• The Go Programming Language Specification: Defer statements.
• The Go Programming Language Specification: Errors.