On Sat, Feb 27, 2021 at 7:03 PM Deiter <hwaterf...@gmail.com> wrote:
>
> Go: go1.15.8 darwin/amd64
> OS: MacOS 11.2.1
>
> It makes sense to me that a function can have arguments that are interfaces -
> so long as the argument provides the methods that the function requires, it
> will be happy. However, I’m having a difficult time understanding functions
> that return an interface. I posted code here for context. Note that I’m not
> concerned about the deadlock that occurs in the playground - it works fine on
> my Mac.
>
> The code comes from the example provided in the Cmd.StdoutPipe documentation,
> which indicates:
> func (c *Cmd) StdoutPipe() (io.ReadCloser, error)
>
> The example uses a “short assignment” statement, so it’s not obvious what
> “concrete” type c.StdoutPipe() returns, but according to the io.ReadCloser
> documentation, it will have to provide these methods:
> type ReadCloser interface {
> Reader
> Closer
> }
>
> My (clearly flawed) interpretation of interface types suggested that I could
> assign the first result from c.StdoutPipe() to anything that implements
> io.ReadCloser. The io.PipeReader documentation indicates that it provides
> both a Read() and a Close() function with the appropriate signatures, so I
> figured I’d try connecting up both ends of a pipe:
>
> rPipe, _ := io.Pipe()
> p, err := cmd.StdoutPipe()
> if err != nil {
> log.Fatal(err)
> }
> rPipe = p
>
> The first clue that I was misguided was when the compiler indicated that I
> needed a type assertion. I put one in to see what would happen:
> rPipe = p.(*io.PipeReader)
>
> Everything compiled, but not surprising, it panics:
> panic: interface conversion: io.ReadCloser is *os.File, not *io.PipeReader
>
> I suppose it makes sense that an assignment operator requires more than
> simply interface compatibility. A couple of questions:
>
> How is one supposed to know what type is required in a situation like this?
> Is a type assertion panic the only way to discover that a *os.File type is
> required?!
The general expectation is that you will store the first result of
StdoutPipe in a variable of type io.ReadCloser. Then you would call
Read and Close methods on that value as appropriate. You are not
expected to store the result in a variable of type *os.File, and in
particular there is no guarantee that future releases of Go will
continue returning a *os.File from StdoutPIpe.
> Any suggestions on how I can get a io.PipeReader to connect with the
> cmd.StdoutPipe()?
I don't understand why you would want to do this. You could start a
goroutine that calls io.Copy from the cmd.StdoutPipe result to the
PipeWriter returned by io.Pipe, and then you could read from the
PipeReader, but that seems pointless.
Ian
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