Merge branch 'fix-multiple-ref-prefix' into 'master'

Extract the longest-matching ref from a commit path when multiple matches occur

### What does this MR do?

This MR extracts the longest-matching ref from a commit path. In cases when there are multiple refs that prefix the path, the ref name is ambiguous. Using the heuristic that the longest-matching ref seems like a sensible default.

### Why was this MR needed?

Suppose there is a branch named `release/app` and a tag named `release/app/v1.0.0`. Suppose `README.md` exists in the root directory.

Let's suppose the path passed in is `release/app/v1.0.0/README.md`. There are two possible ways to interpret the ref and path:

1. ref = `release/app`, path = `v1.0.0/README.md`
2. ref = `release/app/v1.0.0`, path = `README.md`

The crux of the issue is that there is ambiguity which one is correct; both could be real possibilities. In the current implementation of `extract_ref`, GitLab gets confused and tries neither: it uses ref = `release` and path = `app/v1.0.0/README.md`. Since the file does not exist, it returns 404.

### What are the relevant issue numbers?

Closes #1839

See merge request !859

CHANGELOG

@@ -10,6 +10,7 @@ v 7.13.0 (unreleased)
   - Fix downloading of patches on public merge requests when user logged out (Stan Hu)
   - The password for the default administrator (root) account has been changed from "5iveL!fe" to "password".
   - Fix Error 500 when relative submodule resolves to a namespace that has a different name from its path (Stan Hu)
+  - Extract the longest-matching ref from a commit path when multiple matches occur (Stan Hu)
   - Update maintenance documentation to explain no need to recompile asssets for omnibus installations (Stan Hu)
   - Support commenting on diffs in side-by-side mode (Stan Hu)
   - Fix JavaScript error when clicking on the comment button on a diff line that has a comment already (Stan Hu)

lib/extracts_path.rb

@@ -55,12 +55,16 @@ module ExtractsPath
       valid_refs = @project.repository.ref_names
       valid_refs.select! { |v| id.start_with?("#{v}/") }
 
-      if valid_refs.length != 1
+      if valid_refs.length == 0
         # No exact ref match, so just try our best
         pair = id.match(/([^\/]+)(.*)/).captures
       else
+        # There is a distinct possibility that multiple refs prefix the ID.
+        # Use the longest match to maximize the chance that we have the
+        # right ref.
+        best_match = valid_refs.max_by(&:length)
         # Partition the string into the ref and the path, ignoring the empty first value
-        pair = id.partition(valid_refs.first)[1..-1]
+        pair = id.partition(best_match)[1..-1]
       end
     end
 

spec/lib/extracts_path_spec.rb

@@ -10,7 +10,8 @@ describe ExtractsPath do
   before do
     @project = project
 
-    repo = double(ref_names: ['master', 'foo/bar/baz', 'v1.0.0', 'v2.0.0'])
+    repo = double(ref_names: ['master', 'foo/bar/baz', 'v1.0.0', 'v2.0.0',
+                              'release/app', 'release/app/v1.0.0'])
     allow(project).to receive(:repository).and_return(repo)
     allow(project).to receive(:path_with_namespace).
       and_return('gitlab/gitlab-ci')
@@ -54,11 +55,17 @@ describe ExtractsPath do
       it "falls back to a primitive split for an invalid ref" do
         expect(extract_ref('stable')).to eq(['stable', ''])
       end
+
+      it "extracts the longest matching ref" do
+        expect(extract_ref('release/app/v1.0.0/README.md')).to eq(
+          ['release/app/v1.0.0', 'README.md'])
+      end
     end
 
     context "with a path" do
       it "extracts a valid branch" do
-        expect(extract_ref('foo/bar/baz/CHANGELOG')).to eq(['foo/bar/baz', 'CHANGELOG'])
+        expect(extract_ref('foo/bar/baz/CHANGELOG')).to eq(
+          ['foo/bar/baz', 'CHANGELOG'])
       end
 
       it "extracts a valid tag" do