From 105f697037d00743f3071e931c8b47bf8f0952c2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jonatan=20K=C5=82osko?= <jonatanklosko@gmail.com>
Date: Mon, 29 Sep 2025 23:39:05 +0200
Subject: [PATCH 1/3] Improve user detection in lock and pubsub implementation

---
 lib/mix/lib/mix/sync/lock.ex   |  3 +--
 lib/mix/lib/mix/sync/pubsub.ex |  3 +--
 lib/mix/lib/mix/utils.ex       | 23 +++++++++++++++++++++++
 3 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/lib/mix/lib/mix/sync/lock.ex b/lib/mix/lib/mix/sync/lock.ex
index 9f13b853435..afe22f655d1 100644
--- a/lib/mix/lib/mix/sync/lock.ex
+++ b/lib/mix/lib/mix/sync/lock.ex
@@ -131,8 +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_#{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)
diff --git a/lib/mix/lib/mix/sync/pubsub.ex b/lib/mix/lib/mix/sync/pubsub.ex
index 00ad2ad4925..db6a23a99c7 100644
--- a/lib/mix/lib/mix/sync/pubsub.ex
+++ b/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
diff --git a/lib/mix/lib/mix/utils.ex b/lib/mix/lib/mix/utils.ex
index b1f9d561f35..bb85762a488 100644
--- a/lib/mix/lib/mix/utils.ex
+++ b/lib/mix/lib/mix/utils.ex
@@ -900,4 +900,27 @@ 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
+    dir = System.tmp_dir!()
+    File.mkdir_p!(dir)
+
+    path =
+      Path.join(dir, "mix_user_check_#{System.os_time()}_#{System.unique_integer([:positive])}")
+
+    File.touch!(path)
+    %{uid: uid} = File.stat!(path)
+    _ = File.rm(path)
+
+    case uid do
+      :undefined -> nil
+      uid -> uid
+    end
+  end
 end

From 7436f4672418b78feead9f98b6602b47c7d56e60 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Valim?= <jose.valim@gmail.com>
Date: Tue, 30 Sep 2025 13:01:56 +0200
Subject: [PATCH 2/3] Update lib/mix/lib/mix/utils.ex

---
 lib/mix/lib/mix/utils.ex | 34 ++++++++++++++++++++++------------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/lib/mix/lib/mix/utils.ex b/lib/mix/lib/mix/utils.ex
index bb85762a488..56374a8b3ea 100644
--- a/lib/mix/lib/mix/utils.ex
+++ b/lib/mix/lib/mix/utils.ex
@@ -908,19 +908,29 @@ defmodule Mix.Utils do
   it's UID. Note that the UID may be `nil` on non-Unix systems.
   """
   def detect_user_id!() do
-    dir = System.tmp_dir!()
-    File.mkdir_p!(dir)
-
-    path =
-      Path.join(dir, "mix_user_check_#{System.os_time()}_#{System.unique_integer([:positive])}")
-
-    File.touch!(path)
-    %{uid: uid} = File.stat!(path)
-    _ = File.rm(path)
+    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
 
-    case uid do
-      :undefined -> nil
-      uid -> uid
+        Mix.State.put(:user_id, uid)
+        uid
     end
   end
 end

From 22cfac142f2b7e9be3d248b3604c6ee74df2b7d7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Valim?= <jose.valim@gmail.com>
Date: Tue, 30 Sep 2025 13:14:56 +0200
Subject: [PATCH 3/3] Update lib/mix/lib/mix/utils.ex

---
 lib/mix/lib/mix/utils.ex | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/mix/lib/mix/utils.ex b/lib/mix/lib/mix/utils.ex
index 56374a8b3ea..c8a5cf6730f 100644
--- a/lib/mix/lib/mix/utils.ex
+++ b/lib/mix/lib/mix/utils.ex
@@ -917,7 +917,7 @@ defmodule Mix.Utils do
         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)