Add local caching to repo identifier lookup

Prevents fetching the same identifier on every client call by caching
the value in the current process dictionary. While the git command isn't
particularly slow, this should avoid spawning during repeated http
calls.

Author: sorentwo

lib/hex/utils.ex

@@ -331,18 +331,33 @@ defmodule Hex.Utils do
   This function finds the SHA of the first commit in the repository and hashes it once more for
   anonymization.
 
-  Returns `nil` if git is not available or the directory is not a git repository.
+  Returns `nil` when:
+
+  - The `HEX_REPO_IDENTIFIER` environment variable is set to anything other `1` or `true`
+  - The `git` executable isn't available 
+  - The current directory isn't within a git repository
   """
   def repo_identifier do
-    with flag when flag in ["1", "true"] <- System.get_env("HEX_REPO_IDENTIFIER", "1"),
-         path when is_binary(path) <- System.find_executable("git"),
-         {output, 0} <- System.cmd("git", ["rev-list", "--max-parents=0", "HEAD"]) do
-      output
-      |> String.trim()
-      |> then(&:crypto.hash(:sha256, &1))
-      |> Base.encode16(case: :lower)
-    else
-      _ -> nil
+    case Process.get(:hex_repo_identifier, :unset) do
+      cached when is_binary(cached) ->
+        cached
+
+      _ ->
+        with flag when flag in ["1", "true"] <- System.get_env("HEX_REPO_IDENTIFIER", "1"),
+             path when is_binary(path) <- System.find_executable("git"),
+             {output, 0} <- System.cmd("git", ["rev-list", "--max-parents=0", "HEAD"]) do
+          identifier =
+            output
+            |> String.trim()
+            |> then(&:crypto.hash(:sha256, &1))
+            |> Base.encode16(case: :lower)
+
+          Process.put(:hex_repo_identifier, identifier)
+
+          identifier
+        else
+          _other -> nil
+        end
     end
   end
 end

test/hex/utils_test.exs

@@ -1,6 +1,10 @@
 defmodule Hex.UtilsTest do
   use ExUnit.Case
 
+  setup do
+    on_exit(fn -> Process.delete(:hex_repo_identifier) end)
+  end
+
   describe "repo_identifier/0" do
     test "an identifier is included within a repository" do
       assert Hex.Utils.repo_identifier() =~ ~r/^[a-f0-9]{64}$/
@@ -33,5 +37,13 @@ defmodule Hex.UtilsTest do
     after
       System.delete_env("HEX_REPO_IDENTIFIER")
     end
+
+    test "the identifier is cached within the current process" do
+      value = "cached-identifier"
+
+      Process.put(:hex_repo_identifier, value)
+
+      assert value == Hex.Utils.repo_identifier()
+    end
   end
 end