standardize absolute path spec

[AkihiroSuda]

https://github.com/containerd/continuity/pull/82#issuecomment-308214355

• should we use / separator on both Unix and Windows?
• "absolute path" does not contain drive letter (C:), right?

[AkihiroSuda]

ptal @dmcgowan @stevvooe

[stevvooe]

@AkihiroSuda I am not sure. What will work for windows?

[AkihiroSuda]

So, a manifest will contain paths like /Program Files/Foo/Bar. When it is applied on C:, the path will be treated as C:\Program Files\Foo\Bar

[stevvooe]

For manifests, they usually target a single platform, so having OS specific paths seems fine, which means we can avoid the conversion.

[dmcgowan]

@AkihiroSuda do you know if \Program Files\Foo\Bar is considered a valid absolute path on Windows?

The advantage I see of using / for both Windows and Linux is that some content may not target a single platform, making it the responsibility of the unpacker to correctly interpret and convert the separator. As soon as the unpacker is doing some interpretation, it is allowing multiple forms for the same content which is against the design goals of continuity.

[stevvooe]

@tonistiigi PTAL

I think we should probably standardize on unix-style paths. In general, the manifests should be relocatable.

[stevvooe]

@AkihiroSuda Are we good on this? What are the next steps?