Add an environment variable to optionally disable compilation locking

lib/mix/lib/mix.ex

@@ -352,6 +352,11 @@ defmodule Mix do
     * `MIX_INSTALL_DIR` *(since v1.12.0)* - specifies directory where `Mix.install/2` keeps
        install cache
 
+    * `MIX_OS_CONCURRENCY_LOCK` - when set to `0` or `false`, disables mix compilation locking.
+      While not recommended, this may be necessary in cases where hard links or TCP sockets are
+      not available. When opting for this behaviour, make sure to not start concurrent compilations
+      of the same project.
+
     * `MIX_PATH` - appends extra code paths
 
     * `MIX_PROFILE` - a list of comma-separated Mix tasks to profile the time spent on

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

@@ -79,6 +79,10 @@ defmodule Mix.Sync.Lock do
   This function can also be called if this process already has the
   lock. In such case the function is executed immediately.
 
+  When the `MIX_OS_CONCURRENCY_LOCK` environment variable is set to
+  a falsy value, the lock is ignored and the function is executed
+  immediately.
+
   ## Options
 
     * `:on_taken` - a one-arity function called if the lock is held
@@ -96,9 +100,9 @@ defmodule Mix.Sync.Lock do
     path = Path.join([System.tmp_dir!(), "mix_lock", hash])
 
     pdict_key = {__MODULE__, path}
-    has_lock? = Process.get(pdict_key)
+    has_lock? = Process.get(pdict_key, false)
 
-    if has_lock? do
+    if has_lock? or lock_disabled?() do
       fun.()
     else
       lock = lock(path, opts[:on_taken])
@@ -115,6 +119,8 @@ defmodule Mix.Sync.Lock do
     end
   end
 
+  defp lock_disabled?(), do: System.get_env("MIX_OS_CONCURRENCY_LOCK") in ~w(0 false)
+
   defp lock(path, on_taken) do
     File.mkdir_p!(path)
 
@@ -198,11 +204,12 @@ defmodule Mix.Sync.Lock do
         :invalidated
 
       {:error, reason} ->
-        raise File.LinkError,
-          reason: reason,
-          action: "create hard link",
-          existing: port_path,
-          new: lock_path
+        Mix.raise("""
+        could not create hard link from #{port_path} to "#{lock_path}: #{:file.format_error(reason)}.
+
+        Hard link support is required for Mix compilation locking. If your system \
+        does not support hard links, set MIX_OS_CONCURRENCY_LOCK=1\
+        """)
     end
   end