Allow Repositories API GET endpoints to be requested anonymously

Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased/4269-public-repositories-api.yml

+---
+title: Allow Repositories API GET endpoints to be requested anonymously
+merge_request:
+author:

doc/api/repositories.md

@@ -2,7 +2,8 @@
 
 ## List repository tree
 
-Get a list of repository files and directories in a project.
+Get a list of repository files and directories in a project. This endpoint can
+be accessed without authentication if the repository is publicly accessible.
 
 ```
 GET /projects/:id/repository/tree
@@ -71,7 +72,8 @@ Parameters:
 
 ## Raw file content
 
-Get the raw file contents for a file by commit SHA and path.
+Get the raw file contents for a file by commit SHA and path. This endpoint can
+be accessed without authentication if the repository is publicly accessible.
 
 ```
 GET /projects/:id/repository/blobs/:sha
@@ -85,7 +87,8 @@ Parameters:
 
 ## Raw blob content
 
-Get the raw file contents for a blob by blob SHA.
+Get the raw file contents for a blob by blob SHA. This endpoint can be accessed
+without authentication if the repository is publicly accessible.
 
 ```
 GET /projects/:id/repository/raw_blobs/:sha
@@ -98,7 +101,8 @@ Parameters:
 
 ## Get file archive
 
-Get an archive of the repository
+Get an archive of the repository. This endpoint can be accessed without
+authentication if the repository is publicly accessible.
 
 ```
 GET /projects/:id/repository/archive
@@ -111,6 +115,9 @@ Parameters:
 
 ## Compare branches, tags or commits
 
+This endpoint can be accessed without authentication if the repository is
+publicly accessible.
+
 ```
 GET /projects/:id/repository/compare
 ```
@@ -163,7 +170,8 @@ Response:
 
 ## Contributors
 
-Get repository contributors list
+Get repository contributors list. This endpoint can be accessed without
+authentication if the repository is publicly accessible.
 
 ```
 GET /projects/:id/repository/contributors

lib/api/repositories.rb

@@ -2,7 +2,6 @@ require 'mime/types'
 
 module API
   class Repositories < Grape::API
-    before { authenticate! }
     before { authorize! :download_code, user_project }
 
     params do
@@ -79,8 +78,6 @@ module API
         optional :format, type: String, desc: 'The archive format'
       end
       get ':id/repository/archive', requirements: { format: Gitlab::Regex.archive_formats_regex } do
-        authorize! :download_code, user_project
-
         begin
           send_git_archive user_project.repository, ref: params[:sha], format: params[:format]
         rescue
@@ -96,7 +93,6 @@ module API
         requires :to, type: String, desc: 'The commit, branch name, or tag name to stop comparison'
       end
       get ':id/repository/compare' do
-        authorize! :download_code, user_project
         compare = Gitlab::Git::Compare.new(user_project.repository.raw_repository, params[:from], params[:to])
         present compare, with: Entities::Compare
       end
@@ -105,8 +101,6 @@ module API
         success Entities::Contributor
       end
       get ':id/repository/contributors' do
-        authorize! :download_code, user_project
-
         begin
           present user_project.repository.contributors,
                   with: Entities::Contributor