Allow single-character repository paths

[semx]

checkRepository requires at least two characters, so a repository whose path is a single character cannot be parsed — even though the spec allows it and registries serve it.

The reference is valid, and everything else accepts it

The distribution spec's grammar for a repository name is

[a-z0-9]+((\.|_|__|-+)[a-z0-9]+)*(/[a-z0-9]+((\.|_|__|-+)[a-z0-9]+)*)*

[a-z0-9]+ is one or more, so a is a valid path component. distribution/reference parses example.com/a into domain example.com, path a.

Against a local registry:2 with an image pushed by docker as 127.0.0.1:5555/a:v1:

$ curl -s http://127.0.0.1:5555/v2/_catalog
{"repositories":["a"]}
$ curl -s http://127.0.0.1:5555/v2/a/tags/list
{"name":"a","tags":["v1"]}

$ crane ls 127.0.0.1:5555/a
Error: parsing repo "127.0.0.1:5555/a": repository must be between 2 and 255 characters in length: a
$ crane digest 127.0.0.1:5555/a:v1
Error: parsing reference "127.0.0.1:5555/a:v1": could not parse reference: 127.0.0.1:5555/a:v1

docker push and docker pull both round-trip that name; only this library refuses it, and it refuses at parse time, so every consumer — crane, ko, kaniko, cosign, skaffold — is cut off from the image.

With this change:

$ crane ls 127.0.0.1:5555/a
v1
$ crane digest 127.0.0.1:5555/a:v1
sha256:45e09956dc667c5eff3583c9d94830261fb1ca0be10a0a7db36266edf5de9e1d

which is the digest docker push reported for the same image.

Why the minimum isn't load-bearing

It reads like a guard against degenerate names, but it isn't one — the current release accepts all of these:

--        ..        __        //        -.
example.com/--      example.com/..      example.com///

So two-character garbage passes today; the floor only excluded the single-character subset of exactly the same class, and took every valid single-character repository with it. Character-level validation is unchanged by this PR, and the existing TODO(dekkagaijin): use the docker/distribution regexes for validation still describes the way to tighten the whole class properly. checkTag already uses a minimum of 1; the repository was the only element with a 2.

What changes, exactly

I ran 10,133 generated references — hosts (none, docker.io, localhost, ports, IPv6, uppercase), paths (1…256 chars, separators, mixed case), and suffixes (tags, digests, both) — through ParseReference before and after:

inputs	10,133
results that change	225
direction	every one of them error → parses; nothing that parsed before stops parsing
single-character alphanumeric repositories (the point of the change)	216
single-character -, ., _, /	9

The 9 are the degenerate class quoted above, now consistent with their two-character forms rather than singled out by length. Nothing else moves.

Tests

example.com/a, example.com/a/b, a and 0 are added to the existing fixtures. They are not vacuous — with the 2 restored, the suite fails:

--- FAIL: TestNewRepositoryStrictValidation
    repository_test.go:56: `example.com/a` should be a valid Repository name,
        got error: repository must be between 2 and 255 characters in length: a
--- FAIL: TestNewRepository
    repository_test.go:74: `example.com/a` should be a valid repository name, ...

go test ./... is green across all 37 packages.

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[semx]

@googlebot I signed it!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.42%. Comparing base (d3bca10) to head (5127c3e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2407   +/-   ##
=======================================
  Coverage   57.42%   57.42%           
=======================================
  Files         166      166           
  Lines       11558    11558           
=======================================
  Hits         6637     6637           
  Misses       4142     4142           
  Partials      779      779           

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