Record whether or not a repository contains ambiguous refs

A branch can't be a prefix (including a slash) of another branch. For
instance, you can't have a branch `a` and a branch `a/b`. Same with
tags. However, you can have a branch `a` and a tag `a/b`, which is
ambiguous.

In ExtractsRef, we do some work to handle these cases. That can involve
a fairly slow Redis SMEMBERS call. If a repository has no ambiguous refs
of this form (which should be the majority), we can perform some
optimisations.

For the purposes of this method, we only consider refs ambiguous before
the first slash. If we have a branch `a/b/c` and a tag `a/b`, this
method won't detect that.

app/models/repository.rb

@@ -43,7 +43,7 @@ class Repository
                       gitlab_ci_yml branch_names tag_names branch_count
                       tag_count avatar exists? root_ref merged_branch_names
                       has_visible_content? issue_template_names merge_request_template_names
-                      user_defined_metrics_dashboard_paths xcode_project?).freeze
+                      user_defined_metrics_dashboard_paths xcode_project? has_ambiguous_refs?).freeze
 
   # Methods that use cache_method but only memoize the value
   MEMOIZED_CACHED_METHODS = %i(license).freeze
@@ -196,6 +196,31 @@ class Repository
     tag_exists?(ref) && branch_exists?(ref)
   end
 
+  # It's possible for a tag name to be a prefix (including slash) of a branch
+  # name, or vice versa. For instance, a tag named `foo` means we can't create a
+  # tag `foo/bar`, but we _can_ create a branch `foo/bar`.
+  #
+  # If we know a repository has no refs of this type (which is the common case)
+  # then separating refs from paths - as in ExtractsRef - can be faster.
+  #
+  # This method only checks one level deep, so only prefixes that contain no
+  # slashes are considered. If a repository has a tag `foo/bar` and a branch
+  # `foo/bar/baz`, it will return false.
+  def has_ambiguous_refs?
+    return false unless branch_names.present? && tag_names.present?
+
+    with_slash, no_slash = (branch_names + tag_names).partition { |ref| ref.include?('/') }
+
+    return false if with_slash.empty?
+
+    prefixes = no_slash.map { |ref| Regexp.escape(ref) }.join('|')
+    prefix_regex = %r{^#{prefixes}/}
+
+    with_slash.any? do |ref|
+      prefix_regex.match?(ref)
+    end
+  end
+
   def expand_ref(ref)
     if tag_exists?(ref)
       Gitlab::Git::TAG_REF_PREFIX + ref

spec/models/repository_spec.rb

@@ -1245,6 +1245,32 @@ RSpec.describe Repository do
     end
   end
 
+  describe '#has_ambiguous_refs?' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:branch_names, :tag_names, :result) do
+      nil | nil | false
+      %w() | %w() | false
+      %w(a b) | %w() | false
+      %w() | %w(c d) | false
+      %w(a b) | %w(c d) | false
+      %w(a/b) | %w(c/d) | false
+      %w(a b) | %w(c d a/z) | true
+      %w(a b c/z) | %w(c d) | true
+      %w(a/b/z) | %w(a/b) | false # we only consider refs ambiguous before the first slash
+      %w(a/b/z) | %w(a/b a) | true
+    end
+
+    with_them do
+      it do
+        allow(repository).to receive(:branch_names).and_return(branch_names)
+        allow(repository).to receive(:tag_names).and_return(tag_names)
+
+        expect(repository.has_ambiguous_refs?).to eq(result)
+      end
+    end
+  end
+
   describe '#expand_ref' do
     let(:ref) { 'ref' }