docs: clarify application of HOMEBREW_ARTIFACT_DOMAIN

bottle URLs are treated differently than any other URL.
this change clarifies the resulting URLs in the manpage.

closes #13222

Library/Homebrew/env_config.rb

@@ -23,7 +23,12 @@ module Homebrew
         description: "Prefix all download URLs, including those for bottles, with this value. " \
                      "For example, `HOMEBREW_ARTIFACT_DOMAIN=http://localhost:8080` will cause a " \
                      "formula with the URL `https://example.com/foo.tar.gz` to instead download from " \
-                     "`http://localhost:8080/example.com/foo.tar.gz`.",
+                     "`http://localhost:8080/https://example.com/foo.tar.gz`. " \
+                     "Bottle URLs however, have their domain replaced with this prefix. " \
+                     "Using the same value for example, would cause data hosted under " \
+                     "`https://ghcr.io/v2/homebrew/core/gettext/manifests/0.21` " \
+                     "to be instead downloaded from " \
+                     "`http://localhost:8080/v2/homebrew/core/gettext/manifests/0.21`",
       },
       HOMEBREW_AUTO_UPDATE_SECS:                 {
         description: "Run `brew update` once every `HOMEBREW_AUTO_UPDATE_SECS` seconds before some commands, " \

Library/Homebrew/test/download_strategies/curl_spec.rb

@@ -10,6 +10,7 @@ describe CurlDownloadStrategy do
   let(:url) { "https://example.com/foo.tar.gz" }
   let(:version) { "1.2.3" }
   let(:specs) { { user: "download:123456" } }
+  let(:artifact_domain) { nil }
 
   it "parses the opts and sets the corresponding args" do
     expect(strategy.send(:_curl_args)).to eq(["--user", "download:123456"])
@@ -17,6 +18,8 @@ describe CurlDownloadStrategy do
 
   describe "#fetch" do
     before do
+      allow(Homebrew::EnvConfig).to receive(:artifact_domain).and_return(artifact_domain)
+
       strategy.temporary_path.dirname.mkpath
       FileUtils.touch strategy.temporary_path
     end
@@ -123,6 +126,59 @@ describe CurlDownloadStrategy do
         strategy.fetch
       end
     end
+
+    context "with artifact_domain set" do
+      let(:artifact_domain) { "https://mirror.example.com/oci" }
+
+      context "with an asset hosted under example.com" do
+        let(:status) { instance_double(Process::Status, success?: true, exitstatus: 0) }
+
+        it "prefixes the URL unchanged" do
+          expect(strategy).to receive(:system_command).with(
+            /curl/,
+            hash_including(args: array_including_cons("#{artifact_domain}/#{url}")),
+          )
+          .at_least(:once)
+          .and_return(SystemCommand::Result.new(["curl"], [""], status, secrets: []))
+
+          strategy.fetch
+        end
+      end
+
+      context "with an asset hosted under #{GitHubPackages::URL_DOMAIN} (HTTP)" do
+        let(:resource_path) { "v2/homebrew/core/spec/manifests/0.0" }
+        let(:url) { "http://#{GitHubPackages::URL_DOMAIN}/#{resource_path}" }
+        let(:status) { instance_double(Process::Status, success?: true, exitstatus: 0) }
+
+        it "rewrites the URL correctly" do
+          expect(strategy).to receive(:system_command).with(
+            /curl/,
+            hash_including(args: array_including_cons("#{artifact_domain}/#{resource_path}")),
+          )
+          .at_least(:once)
+          .and_return(SystemCommand::Result.new(["curl"], [""], status, secrets: []))
+
+          strategy.fetch
+        end
+      end
+
+      context "with an asset hosted under #{GitHubPackages::URL_DOMAIN} (HTTPS)" do
+        let(:resource_path) { "v2/homebrew/core/spec/manifests/0.0" }
+        let(:url) { "https://#{GitHubPackages::URL_DOMAIN}/#{resource_path}" }
+        let(:status) { instance_double(Process::Status, success?: true, exitstatus: 0) }
+
+        it "rewrites the URL correctly" do
+          expect(strategy).to receive(:system_command).with(
+            /curl/,
+            hash_including(args: array_including_cons("#{artifact_domain}/#{resource_path}")),
+          )
+          .at_least(:once)
+          .and_return(SystemCommand::Result.new(["curl"], [""], status, secrets: []))
+
+          strategy.fetch
+        end
+      end
+    end
   end
 
   describe "#cached_location" do