Proper way to release resources with defer in a loop?

Execution of a deferred function is not only delayed, deferred to the moment the surrounding function returns, it is also executed even if the enclosing function terminates abruptly, e.g. panics. Spec: Defer statements:

A “defer” statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.

Whenever you create a value or a resource that provides means to properly close it / dispose of it, you should always use a defer statement to make sure it is released even if your other code panics to prevent leaking memory or other system resources.

It’s true that if you’re allocating resources in a loop you should not simply use defer, as then releasing resources will not happen as early as it could and should (at the end of each iteration), only after the for statement (only after all iterations).

What you should do is that if you have a snippet that allocates such resources, wrap it in a function –either an anonymous or a named function–, and in that function you may use defer, and resources will be freed as soon as they are no longer needed, and what’s important is that even if there is a bug in your code which may panic.

Example:

for rows.Next() {
    func() {
        fields, err := db.Query(...)
        if err != nil {
            // Handle error and return
            return
        }
        defer fields.Close()

        // do something with `fields`
    }()
}

Or if put in a named function:

func foo(rs *db.Rows) {
    fields, err := db.Query(...)
    if err != nil {
        // Handle error and return
        return
    }
    defer fields.Close()

    // do something with `fields`
}

And calling it:

for rows.Next() {
    foo(rs)
}

Also if you’d want to terminate on the first error, you could return the error from foo():

func foo(rs *db.Rows) error {
    fields, err := db.Query(...)
    if err != nil {
        return fmt.Errorf("db.Query error: %w", err)
    }
    defer fields.Close()

    // do something with `fields`
    return nil
}

And calling it:

for rows.Next() {
    if err := foo(rs); err != nil {
        // Handle error and return
        return
    }
}

Also note that Rows.Close() returns an error which when called using defer is discarded. If we want to check the returned error, we can use an anonymous function like this:

func foo(rs *db.Rows) (err error) {
    fields, err := db.Query(...)
    if err != nil {
        return fmt.Errorf("db.Query error: %w", err)
    }
    defer func() {
        if err = fields.Close(); err != nil {
            err = fmt.Errorf("Rows.Close() error: %w", err)
        }
    }()

    // do something with `fields`
    return nil
}