Validation for paths

[jchadwick-buf]

• Rejects paths with backslashes
• Limits length to 4096 (will be enforced in v1alpha1 as well.)
• Enforces relative paths (no preceding slash.)
• Enforces no trailing slash
• Enforces normalized paths (no . or .. elements)

Explanation

Regular expression breakdown:

Path := PathSegment(/PathSegment)*
PathSegment := [^/.][^/]?|[^/][^/.]|[^/]{3,}

• Path enforces normalized, relative paths by ensuring that there is one or more path segments with one and only one / between them and nowhere else.
• PathSegment enforces each path segment can't be . or .. and must be non-empty. It does this by matching any 1 character segment that isn't ., any 2 character segment that isn't .., and any 3 character segment.

Overall path length is then enforced with max_len and Windows-style paths are rejected using not_contains to generally disallow backslashes in paths.