Relax tag requirements when generating changelogs

When generating a changelog and leaving out the `from` parameter, we no
longer require that the names of tags start with a "v". Instead, tags
can now be in the format `X.Y.Z` or `vX.Y.Z`.

This fixes https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1572

app/finders/repositories/previous_tag_finder.rb

@@ -16,12 +16,13 @@ module Repositories
   # This finder expects that all tags to consider meet the following
   # requirements:
   #
-  # * They start with the letter "v"
-  # * They use semantic versioning for the tag format
+  # * They start with the letter "v" followed by a version, or immediately start
+  #   with a version
+  # * They use semantic versioning for the version format
   #
   # Tags not meeting these requirements are ignored.
   class PreviousTagFinder
-    TAG_REGEX = /\Av(?<version>#{Gitlab::Regex.unbounded_semver_regex})\z/.freeze
+    TAG_REGEX = /\Av?(?<version>#{Gitlab::Regex.unbounded_semver_regex})\z/.freeze
 
     def initialize(project)
       @project = project

changelogs/unreleased/relax-changelog-tag-restriction.yml

+---
+title: Relax tag requirements when generating changelogs
+merge_request: 54832
+author:
+type: changed

doc/api/repositories.md

@@ -285,8 +285,8 @@ Example response:
 
 Generate changelog data based on commits in a repository.
 
-Given a version (using semantic versioning) and a range of commits,
-GitLab generates a changelog for all commits that use a particular
+Given a version (using [semantic versioning](https://semver.org/)) and a range
+of commits, GitLab generates a changelog for all commits that use a particular
 [Git trailer](https://git-scm.com/docs/git-interpret-trailers).
 
 The output of this process is a new section in a changelog file in the Git
@@ -312,15 +312,15 @@ Supported attributes:
 
 If the `from` attribute is unspecified, GitLab uses the Git tag of the last
 version that came before the version specified in the `version` attribute. For
-this to work, your project must create Git tags for versions using the
-following format:
+this to work, your project must create Git tags for versions using one of the
+following formats:
 
-```plaintext
-vX.Y.Z
-```
+- `vX.Y.Z`
+- `X.Y.Z`
 
-Where `X.Y.Z` is a version that follows semantic versioning. For example,
-consider a project with the following tags:
+Where `X.Y.Z` is a version that follows [semantic
+versioning](https://semver.org/). For example, consider a project with the
+following tags:
 
 - v1.0.0
 - v1.1.0

spec/finders/repositories/previous_tag_finder_spec.rb

@@ -12,16 +12,19 @@ RSpec.describe Repositories::PreviousTagFinder do
         tag1 = double(:tag1, name: 'v1.0.0')
         tag2 = double(:tag2, name: 'v1.1.0')
         tag3 = double(:tag3, name: 'v2.0.0')
-        tag4 = double(:tag4, name: '1.0.0')
+        tag4 = double(:tag4, name: '0.9.0')
+        tag5 = double(:tag4, name: 'v0.8.0-pre1')
 
         allow(project.repository)
           .to receive(:tags)
-          .and_return([tag1, tag3, tag2, tag4])
+          .and_return([tag1, tag3, tag2, tag4, tag5])
 
         expect(finder.execute('2.1.0')).to eq(tag3)
         expect(finder.execute('2.0.0')).to eq(tag2)
         expect(finder.execute('1.5.0')).to eq(tag2)
         expect(finder.execute('1.0.1')).to eq(tag1)
+        expect(finder.execute('1.0.0')).to eq(tag4)
+        expect(finder.execute('0.9.0')).to eq(tag5)
       end
     end