x/blog: Error in defer-panic-and-recover article #34690

Xerock · 2019-10-04T10:33:58Z

What did you expect to see?

func CopyFile(dstName, srcName string) (written int64, err error) {
    src, err := os.Open(srcName)
    defer src.Close()
    if err != nil {
        return
    }

    dst, err := os.Create(dstName)
    defer dst.Close()
    if err != nil {
        return
    }

    return io.Copy(dst, src)
}

What did you see instead?

func CopyFile(dstName, srcName string) (written int64, err error) {
    src, err := os.Open(srcName)
    if err != nil {
        return
    }
    defer src.Close()

    dst, err := os.Create(dstName)
    if err != nil {
        return
    }
    defer dst.Close()

    return io.Copy(dst, src)
}

With the current code example, in the case of a non-nil error, the defer isn't stacked before the return occurs, so the function will return without closing the source file.

Little program to prove my point : https://play.golang.org/p/UAdMVffys_P

The text was updated successfully, but these errors were encountered:

bcmills · 2019-10-04T13:29:20Z

This aspect of the example is correct and idiomatic as written.

Like most Go functions, os.Open and os.Create return zero-values (in this case, nil pointers) for the other arguments when they return a non-nil error, and calling Close on a nil receiver is generally nonsensical.

In this case, the nonsensical call is masked by the permissiveness of (*os.File).Close. Rather than panicking (as many Close implementations would for nil receivers), it returns ErrInvalid:

go/src/os/file_unix.go

Lines 230 to 232 in fc70527

    
           if f == nil { 
        
           	return ErrInvalid 
        
           }

gopherbot added this to the Unreleased milestone Oct 4, 2019

bcmills closed this as completed Oct 4, 2019

golang locked and limited conversation to collaborators Oct 3, 2020

gopherbot added the FrozenDueToAge label Oct 3, 2020

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

x/blog: Error in defer-panic-and-recover article #34690

x/blog: Error in defer-panic-and-recover article #34690

Xerock commented Oct 4, 2019 •

edited

bcmills commented Oct 4, 2019 •

edited

x/blog: Error in defer-panic-and-recover article #34690

x/blog: Error in defer-panic-and-recover article #34690

Comments

Xerock commented Oct 4, 2019 • edited

What did you expect to see?

What did you see instead?

bcmills commented Oct 4, 2019 • edited

Xerock commented Oct 4, 2019 •

edited

bcmills commented Oct 4, 2019 •

edited