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Impossible interface nil check

ID: go/impossible-interface-nil-check
Kind: problem
Security severity: 
Severity: warning
Precision: high
Tags:
   - correctness
Query suites:
   - go-security-and-quality.qls

Click to see the query in the CodeQL repository

Interface values in Go are type tagged, that is, they are essentially pairs of the form (value, type), where value is a non-interface value with the given type. Such a pair is never nil, even if value is nil.

In particular, if a non-interface value v is assigned to a variable x whose type is an interface, then x will never be nil, regardless of v. Comparing x to nil is pointless, and may indicate a misunderstanding of Go interface values or some other underlying bug.

Recommendation

Carefully inspect the comparison to ensure it is not a symptom of a bug.

Example

The following example shows a declaration of a function fetch which fetches the contents of a URL, returning either the contents or an error value, which is a pointer to a custom error type RequestError (not shown). The function niceFetch wraps this function, printing out either the URL contents or an error message.

package main

import "fmt"

func fetch(url string) (string, *RequestError)

func niceFetch(url string) {
	var s string
	var e error
	s, e = fetch(url)
	if e != nil {
		fmt.Printf("Unable to fetch URL: %v\n", e)
	} else {
		fmt.Printf("URL contents: %s\n", s)
	}
}

However, because e is declared to be of type error, which is an interface, the nil check will never succeed, since e can never be nil.

In this case, the problem can be solved by using a short variable declaration using operator :=, which will automatically infer the more precise non-interface type *ResourceError for e, making the nil check behave as expected.

package main

import "fmt"

func niceFetchGood(url string) {
	s, e := fetch(url)
	if e != nil {
		fmt.Printf("Unable to fetch URL: %v\n", e)
	} else {
		fmt.Printf("URL contents: %s\n", s)
	}
}

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