Standardize use of `content` parameter in snippets API

There was some confusion over whether `code` or `content` is the right
parameter for snippets. Internally, the database stores
`content`. However:

1. Project snippets use `code`. `code` gets remapped in `content` in
`lib/api/project_snippets.rb`.

2. Personal snippets use `content`.

To unify these APIs, allow an alias of `content` to work for project
snippets.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66673

changelogs/unreleased/sh-support-content-for-snippets-api.yml

+---
+title: Standardize use of `content` parameter in snippets API
+merge_request: 32296
+author:
+type: changed

lib/api/project_snippets.rb

@@ -51,16 +51,18 @@ module API
       params do
         requires :title, type: String, desc: 'The title of the snippet'
         requires :file_name, type: String, desc: 'The file name of the snippet'
-        requires :code, type: String, allow_blank: false, desc: 'The content of the snippet'
+        optional :code, type: String, allow_blank: false, desc: 'The content of the snippet (deprecated in favor of "content")'
+        optional :content, type: String, allow_blank: false, desc: 'The content of the snippet'
         optional :description, type: String, desc: 'The description of a snippet'
         requires :visibility, type: String,
                               values: Gitlab::VisibilityLevel.string_values,
                               desc: 'The visibility of the snippet'
+        mutually_exclusive :code, :content
       end
       post ":id/snippets" do
         authorize! :create_project_snippet, user_project
-        snippet_params = declared_params.merge(request: request, api: true)
-        snippet_params[:content] = snippet_params.delete(:code)
+        snippet_params = declared_params(include_missing: false).merge(request: request, api: true)
+        snippet_params[:content] = snippet_params.delete(:code) if snippet_params[:code].present?
 
         snippet = CreateSnippetService.new(user_project, current_user, snippet_params).execute
 
@@ -80,12 +82,14 @@ module API
         requires :snippet_id, type: Integer, desc: 'The ID of a project snippet'
         optional :title, type: String, desc: 'The title of the snippet'
         optional :file_name, type: String, desc: 'The file name of the snippet'
-        optional :code, type: String, allow_blank: false, desc: 'The content of the snippet'
+        optional :code, type: String, allow_blank: false, desc: 'The content of the snippet (deprecated in favor of "content")'
+        optional :content, type: String, allow_blank: false, desc: 'The content of the snippet'
         optional :description, type: String, desc: 'The description of a snippet'
         optional :visibility, type: String,
                               values: Gitlab::VisibilityLevel.string_values,
                               desc: 'The visibility of the snippet'
-        at_least_one_of :title, :file_name, :code, :visibility_level
+        at_least_one_of :title, :file_name, :code, :content, :visibility_level
+        mutually_exclusive :code, :content
       end
       # rubocop: disable CodeReuse/ActiveRecord
       put ":id/snippets/:snippet_id" do

spec/requests/api/project_snippets_spec.rb

@@ -108,6 +108,29 @@ describe API::ProjectSnippets do
       expect(snippet.visibility_level).to eq(Snippet::PUBLIC)
     end
 
+    it 'creates a new snippet with content parameter' do
+      params[:content] = params.delete(:code)
+
+      post api("/projects/#{project.id}/snippets/", admin), params: params
+
+      expect(response).to have_gitlab_http_status(201)
+      snippet = ProjectSnippet.find(json_response['id'])
+      expect(snippet.content).to eq(params[:content])
+      expect(snippet.description).to eq(params[:description])
+      expect(snippet.title).to eq(params[:title])
+      expect(snippet.file_name).to eq(params[:file_name])
+      expect(snippet.visibility_level).to eq(Snippet::PUBLIC)
+    end
+
+    it 'returns 400 when both code and content parameters specified' do
+      params[:content] = params[:code]
+
+      post api("/projects/#{project.id}/snippets/", admin), params: params
+
+      expect(response).to have_gitlab_http_status(400)
+      expect(json_response['error']).to eq('code, content are mutually exclusive')
+    end
+
     it 'returns 400 for missing parameters' do
       params.delete(:title)
 
@@ -175,6 +198,25 @@ describe API::ProjectSnippets do
       expect(snippet.description).to eq(new_description)
     end
 
+    it 'updates snippet with content parameter' do
+      new_content = 'New content'
+      new_description = 'New description'
+
+      put api("/projects/#{snippet.project.id}/snippets/#{snippet.id}/", admin), params: { content: new_content, description: new_description }
+
+      expect(response).to have_gitlab_http_status(200)
+      snippet.reload
+      expect(snippet.content).to eq(new_content)
+      expect(snippet.description).to eq(new_description)
+    end
+
+    it 'returns 400 when both code and content parameters specified' do
+      put api("/projects/#{snippet.project.id}/snippets/1234", admin), params: { code: 'some content', content: 'other content' }
+
+      expect(response).to have_gitlab_http_status(400)
+      expect(json_response['error']).to eq('code, content are mutually exclusive')
+    end
+
     it 'returns 404 for invalid snippet id' do
       put api("/projects/#{snippet.project.id}/snippets/1234", admin), params: { title: 'foo' }