Rewrite hexdocs.pm canonical links to per-package subdomains

lib/hexdocs/file_rewriter.ex

@@ -16,9 +16,25 @@ defmodule Hexdocs.FileRewriter do
     |> add_elixir_org_link(path)
     |> add_analytics(path)
     |> remove_noindex(path)
+    |> rewrite_canonical_links(path)
     |> add_nofollow(path)
   end
 
+  @canonical_tag_re ~r{<link[^>]*\brel=["']canonical["'][^>]*>}i
+  @hexdocs_link_re ~r{https?://hexdocs\.pm/([a-z][a-z0-9_]*)(?![a-zA-Z0-9_.-])}
+
+  defp rewrite_canonical_links(content, path) do
+    if String.ends_with?(path, ".html") do
+      Regex.replace(@canonical_tag_re, content, fn tag ->
+        Regex.replace(@hexdocs_link_re, tag, fn _match, package ->
+          "https://#{Hexdocs.Utils.package_to_subdomain(package)}.hexdocs.pm"
+        end)
+      end)
+    else
+      content
+    end
+  end
+
   defp add_elixir_org_link(content, path) do
     if String.ends_with?(path, ".html") and not String.contains?(content, @link_addition) do
       String.replace(content, @link_hooks, &(&1 <> " for the " <> @link_addition))

test/hexdocs/file_rewriter_test.exs

@@ -17,6 +17,108 @@ defmodule Hexdocs.FileRewriterTest do
     assert FileRewriter.run("index.html", ~s|<meta name="robots" content="noindex">|) == ""
   end
 
+  describe "rewrite hexdocs.pm canonical links to subdomains" do
+    test "rewrites a canonical link" do
+      assert FileRewriter.run(
+               "index.html",
+               ~s|<link rel="canonical" href="https://hexdocs.pm/jason/Jason.html"/>|
+             ) ==
+               ~s|<link rel="canonical" href="https://jason.hexdocs.pm/Jason.html"/>|
+    end
+
+    test "preserves version, query and fragment in the tail" do
+      assert FileRewriter.run(
+               "index.html",
+               ~s|<link rel="canonical" href="https://hexdocs.pm/jason/1.4.0/Jason.html?foo=bar#decode/2"/>|
+             ) ==
+               ~s|<link rel="canonical" href="https://jason.hexdocs.pm/1.4.0/Jason.html?foo=bar#decode/2"/>|
+    end
+
+    test "maps underscores in the package name to hyphens" do
+      assert FileRewriter.run(
+               "index.html",
+               ~s|<link rel="canonical" href="https://hexdocs.pm/phoenix_html/Phoenix.HTML.html"/>|
+             ) ==
+               ~s|<link rel="canonical" href="https://phoenix-html.hexdocs.pm/Phoenix.HTML.html"/>|
+    end
+
+    test "rewrites the hex package" do
+      assert FileRewriter.run(
+               "index.html",
+               ~s|<link rel="canonical" href="https://hexdocs.pm/hex/usage.html"/>|
+             ) ==
+               ~s|<link rel="canonical" href="https://hex.hexdocs.pm/usage.html"/>|
+    end
+
+    test "rewrites http links to https subdomains" do
+      assert FileRewriter.run(
+               "index.html",
+               ~s|<link rel="canonical" href="http://hexdocs.pm/jason/Jason.html"/>|
+             ) ==
+               ~s|<link rel="canonical" href="https://jason.hexdocs.pm/Jason.html"/>|
+    end
+
+    test "handles href before rel in the canonical tag" do
+      assert FileRewriter.run(
+               "index.html",
+               ~s|<link href="https://hexdocs.pm/jason/Jason.html" rel="canonical">|
+             ) ==
+               ~s|<link href="https://jason.hexdocs.pm/Jason.html" rel="canonical">|
+    end
+
+    test "does not rewrite body links or text" do
+      for input <- [
+            ~s|<a href="https://hexdocs.pm/jason/Jason.html">Jason</a>|,
+            ~s|<pre><code>visit https://hexdocs.pm/jason/readme.html</code></pre>|
+          ] do
+        assert FileRewriter.run("index.html", input) == input
+      end
+    end
+
+    test "does not rewrite other link tags" do
+      input = ~s|<link rel="stylesheet" href="https://hexdocs.pm/jason/app.css">|
+      assert FileRewriter.run("index.html", input) == input
+    end
+
+    test "leaves the bare apex untouched" do
+      for input <- [
+            ~s|<link rel="canonical" href="https://hexdocs.pm"/>|,
+            ~s|<link rel="canonical" href="https://hexdocs.pm/"/>|
+          ] do
+        assert FileRewriter.run("index.html", input) == input
+      end
+    end
+
+    test "leaves apex files untouched" do
+      for input <- [
+            ~s|<link rel="canonical" href="https://hexdocs.pm/sitemap.xml"/>|,
+            ~s|<link rel="canonical" href="https://hexdocs.pm/foo.html"/>|
+          ] do
+        assert FileRewriter.run("index.html", input) == input
+      end
+    end
+
+    test "does not touch canonical links that already use a subdomain" do
+      for input <- [
+            ~s|<link rel="canonical" href="https://jason.hexdocs.pm/Jason.html"/>|,
+            ~s|<link rel="canonical" href="https://preview.hexdocs.pm/foo/Foo.html"/>|
+          ] do
+        assert FileRewriter.run("index.html", input) == input
+      end
+    end
+
+    test "is idempotent" do
+      input = ~s|<link rel="canonical" href="https://hexdocs.pm/jason/Jason.html"/>|
+      once = FileRewriter.run("index.html", input)
+      assert FileRewriter.run("index.html", once) == once
+    end
+
+    test "does not modify non-html files" do
+      input = ~s|<link rel="canonical" href="https://hexdocs.pm/jason/Jason.html"/>|
+      assert FileRewriter.run("index.js", input) == input
+    end
+  end
+
   describe "add_nofollow" do
     test "adds rel=nofollow to external links" do
       assert FileRewriter.run("index.html", ~s|<a href="https://example.com">example</a>|) ==
@@ -42,7 +144,7 @@ defmodule Hexdocs.FileRewriterTest do
     test "does not add nofollow to official ecosystem links" do
       for url <- [
             "https://hex.pm/packages/foo",
-            "https://hexdocs.pm/foo",
+            "https://hexdocs.pm",
             "https://elixir-lang.org",
             "https://www.erlang.org",
             "https://preview.hexdocs.pm/foo"