DEV: Introduce rake task to validate discourse-compatibility file (#26158)

spec/tasks/compatibility_spec.rb

@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+RSpec.describe "compatibility:validate" do
+  before do
+    Rake::Task.clear
+    Discourse::Application.load_tasks
+  end
+
+  def invoke(content)
+    file = Tempfile.new("discourse-compat-validate")
+    file.write content
+    file.close
+    error = nil
+    stdout =
+      capture_stdout do
+        Rake::Task["compatibility:validate"].invoke(file.path)
+      rescue => e
+        error = e
+      end
+    [error, stdout]
+  ensure
+    file.unlink
+  end
+
+  it "passes for a valid .discourse-compatibility file" do
+    error, stdout = invoke <<~CONTENT
+      2.5.0.beta6: c4a6c17
+      2.5.0.beta4: d1d2d3f
+    CONTENT
+    expect(error).to eq(nil)
+    expect(stdout).to include("Compatibility file is valid")
+  end
+
+  it "passes for empty file" do
+    error, stdout = invoke ""
+    expect(error).to eq(nil)
+    expect(stdout).to include("Compatibility file is valid")
+  end
+
+  it "fails for invalid YAML" do
+    error, stdout = invoke <<~CONTENT
+      2.5.0.beta6 c4a6c17
+    CONTENT
+    expect(error).to be_a(Discourse::InvalidVersionListError)
+    expect(stdout).to include("Invalid version list")
+  end
+
+  it "fails for invalid version specifier" do
+    error, stdout = invoke <<~CONTENT
+      > 2.5.0.beta6: c4a6c17
+    CONTENT
+    expect(error).to be_a(Discourse::InvalidVersionListError)
+    expect(stdout).to include("Invalid version list")
+  end
+
+  it "fails when matching current core version" do
+    error, stdout = invoke <<~CONTENT
+      #{Discourse::VERSION::STRING}: c4a6c17
+    CONTENT
+    expect(error).to be_a(CoreTooRecentError)
+    expect(stdout).to include(
+      "Compatibility file has an entry which matches the current version of Discourse core",
+    )
+  end
+end