Improve user detection in lock and pubsub implementation

lib/mix/lib/mix/sync/lock.ex

@@ -72,7 +72,6 @@ defmodule Mix.Sync.Lock do
   @probe_data "mixlock"
   @probe_data_size byte_size(@probe_data)
   @probe_timeout_ms 5_000
-  @version 2
 
   @typedoc """
   Options for `with_lock/3`.
@@ -132,12 +131,7 @@ defmodule Mix.Sync.Lock do
 
   defp base_path do
     # We include user in the dir to avoid permission conflicts across users
-    user = System.get_env("USER", "default")
-
-    Path.join(
-      System.tmp_dir!(),
-      "mix_lock_#{@version}_#{Base.url_encode64(user, padding: false)}"
-    )
+    Path.join(System.tmp_dir!(), "mix_lock_user#{Mix.Utils.detect_user_id!()}")
   end
 
   defp lock_disabled?(), do: System.get_env("MIX_OS_CONCURRENCY_LOCK") in ~w(0 false)

lib/mix/lib/mix/sync/pubsub.ex

@@ -282,8 +282,7 @@ defmodule Mix.Sync.PubSub do
 
   defp base_path do
     # We include user in the dir to avoid permission conflicts across users
-    user = System.get_env("USER", "default")
-    Path.join(System.tmp_dir!(), "mix_pubsub_#{Base.url_encode64(user, padding: false)}")
+    Path.join(System.tmp_dir!(), "mix_pubsub_user#{Mix.Utils.detect_user_id!()}")
   end
 
   defp recv(socket, size, timeout \\ :infinity) do

lib/mix/lib/mix/utils.ex

@@ -900,4 +900,37 @@ defmodule Mix.Utils do
 
     [proxy_auth: {user, pass}]
   end
+
+  @doc """
+  Returns the user id of the currently running process.
+
+  The user id is obtained by creating a temporary file and checking
+  it's UID. Note that the UID may be `nil` on non-Unix systems.
+  """
+  def detect_user_id!() do
+    case Mix.State.fetch(:user_id) do
+      {:ok, uid} ->
+        uid
+
+      :error ->
+        dir = System.tmp_dir!()
+        File.mkdir_p!(dir)
+        rand = :crypto.strong_rand_bytes(3) |> Base.url_encode64()
+        path = Path.join(dir, "mix_user_check_#{System.os_time()}_#{rand}")
+
+        uid =
+          try do
+            File.touch!(path)
+            File.stat!(path)
+          else
+            %{uid: :undefined} -> nil
+            %{uid: uid} -> uid
+          after
+            File.rm(path)
+          end
+
+        Mix.State.put(:user_id, uid)
+        uid
+    end
+  end
 end