Add test for concurrent fetching

Library/Homebrew/test/cmd/fetch_spec.rb

@@ -14,4 +14,26 @@ RSpec.describe Homebrew::Cmd::FetchCmd do
     expect(HOMEBREW_CACHE/"testball--0.1.tbz").to be_a_symlink
     expect(HOMEBREW_CACHE/"testball--0.1.tbz").to exist
   end
+
+  it "concurrently downloads formula URLs", :integration_test do
+    setup_test_formula "testball1"
+    setup_test_formula "testball2"
+
+    expect { brew "fetch", "testball1", "testball2", "--concurrency=2" }.to be_a_success
+
+    expect(HOMEBREW_CACHE/"testball1--0.1.tbz").to be_a_symlink
+    expect(HOMEBREW_CACHE/"testball1--0.1.tbz").to exist
+    expect(HOMEBREW_CACHE/"testball2--0.1.tbz").to be_a_symlink
+    expect(HOMEBREW_CACHE/"testball2--0.1.tbz").to exist
+  end
+
+  it "errors when concurrently downloading to the same destination", :integration_test do
+    # This test will fail if the fetch is not performed concurrently.
+    # N.B. this test relies on how we set up test formulae, see the `integration_test` shared context.
+    setup_test_formula "testball1"
+    setup_test_formula "testball3"
+
+    expect { brew "fetch", "testball1", "testball3", "--concurrency=2" }.to be_a_failure
+      .and output(/Error:.*process has already locked/).to_stderr
+  end
 end

Library/Homebrew/test/support/helper/spec/shared_context/integration_test.rb

@@ -134,10 +134,12 @@ RSpec.shared_context "integration test" do # rubocop:disable RSpec/ContextWordin
                          bottle_block: nil, tab_attributes: nil)
     case name
     when /^testball/
+      # Use a different tarball for testball2 to avoid lock errors when writing concurrency tests
+      prefix = (name == "testball2") ? "testball2" : "testball"
       tarball = if OS.linux?
-        TEST_FIXTURE_DIR/"tarballs/testball-0.1-linux.tbz"
+        TEST_FIXTURE_DIR/"tarballs/#{prefix}-0.1-linux.tbz"
       else
-        TEST_FIXTURE_DIR/"tarballs/testball-0.1.tbz"
+        TEST_FIXTURE_DIR/"tarballs/#{prefix}-0.1.tbz"
       end
       content = <<~RUBY
         desc "Some test"