descriptor validation

[xiekeyang]

Function (d *descriptor) validate() is for descriptor variable, with which the exact cas file path is clear. So in this function, it is safely to open the target file, needn't to walk again.
Other walking mechanism is working properly, to need overall refactor, so I don't touch them hastily.

This PR refers to #5 , by @wking, on reading file part. I'm not very sure if this PR is valuable to be accepted individually. Otherwise we should close it and wait for #5 , or please @wking amend this PR with his singed-off?

As to me, I hope a cas reading/writing API can be used to code.

Signed-off-by: xiekeyang keyang.xie@gmail.com

cc @wking @stevvooe

[vbatts]

is the named loop entirely needed? I'm not opposed to them, but I don't see any inner loops to distinguish from

[xiekeyang]

@vbatts

loop:
        for {
                hdr, err := tr.Next()
                switch err {
                case io.EOF:
                        break loop
                case nil:
                // success, continue below
                default:
                        return nil, err 
                }   
                if hdr.Name == filepath.Join("blobs", desc.algo(), desc.hash()) &&
                        !hdr.FileInfo().IsDir() {
                        buf, err := ioutil.ReadAll(tr)
                        if err != nil {
                                return nil, err 
                        }   
                        return ioutil.NopCloser(bytes.NewReader(buf)), nil 
                }   
        }   

break is located in switch-case block, but it want to break the for-loop. So it should point the label( as break loop). Otherwise, the break will jump out switch-case block.
Of course, we can use if checking instead of switch-case, which not use label. But it is same effect.
If my explain has covered your question?

[vbatts]

You're right. My apologies.

[xiekeyang]

PTAL.

[vbatts]

LGTM