Make ipfs add -r follow symlinks

[mildred]

Fix issue described in #616, symlinks are now followed by default.

[mappum]

If we're following symlinks, we might need cycle detection (what if you linked to the parent directory?). Also, might be better to only follow symlinks when a CLI flag is used (ipfs add -r -l).

[whyrusleeping]

Yeah, agreed with @mappum.

[mildred]

Ok, so if we want to default to not follow symlinks, we'll have first to implement symlinks in unixfs.

As for cycle detection, I'm sure this could be a good idea, but I don't think any other program out there does it. You shouldn't have cycles on your filesystem, period. Following symlinks is a natural behaviour, and you generally have to be explicit if you suspect there is a symlink and you don't want to follow it. Go is a kind of exception here with its os.Readdir function that does a lstat() instead of the more conventional stat() function.

So I think adding cycle detection would slow things down a lot and add too much complexity to the code to be worth implementing.

[whyrusleeping]

@mildred most unix programs do actually do a sort of 'cycle detection' in counting how many symlinks theyve seen in the current resolution and failing if that number goes over a certain threshold. It costs pretty much nothing as its done in-line during the resolve.

Implementing symlinks in unixfs is feasible, and should just be as simple as storing a path in the dag node.

[jbenet]

@mildred thanks for the PR! agree on both:

• follow symlinks with a flag (default operation should just add the symlink, like git does)
• cycle detection (either the symlink count, or could be done with a map of absolute paths)

I'll let @whyrusleeping shepherd this one, as he's the unixfs pro

[jbenet]

ping @whyrusleeping

[whyrusleeping]

If we can add a sharness test or two around this, that would be awesome. Otherwise, LGTM

[whyrusleeping]

closing due to inactivity, please reopen as necessary

note: all pull requests older than three weeks may be closed in an effort to keep our open pull requests more focused.