Improve preflight file permissions check (#114)

* Added startup permissions check with more helpful message

* Updated README

* Fixes faulty test

lib/pinchflat/release.ex

@@ -5,6 +5,10 @@ defmodule Pinchflat.Release do
   """
   @app :pinchflat
 
+  require Logger
+
+  alias Pinchflat.Filesystem.FilesystemHelpers
+
   def migrate do
     load_app()
 
@@ -18,6 +22,36 @@ defmodule Pinchflat.Release do
     {:ok, _, _} = Ecto.Migrator.with_repo(repo, &Ecto.Migrator.run(&1, :down, to: version))
   end
 
+  def check_file_permissions do
+    load_app()
+
+    directories = [
+      "/config",
+      "/downloads",
+      Application.get_env(:pinchflat, :media_directory),
+      Application.get_env(:pinchflat, :tmpfile_directory),
+      Application.get_env(:pinchflat, :metadata_directory)
+    ]
+
+    Enum.each(directories, fn dir ->
+      Logger.info("Checking permissions for #{dir}")
+      filepath = Path.join([dir, ".keep"])
+
+      case FilesystemHelpers.write_p(filepath, "") do
+        :ok ->
+          Logger.info("Permissions OK")
+
+        {:error, :eacces} ->
+          Logger.error(permission_denied_screed(dir))
+          raise "Permission denied"
+
+        err ->
+          Logger.error("Permissions check failed: #{inspect(err)}")
+          raise "Unknown error"
+      end
+    end)
+  end
+
   defp repos do
     Application.fetch_env!(@app, :ecto_repos)
   end
@@ -25,4 +59,31 @@ defmodule Pinchflat.Release do
   defp load_app do
     Application.load(@app)
   end
+
+  defp permission_denied_screed(dir) do
+    """
+    The directory "#{dir}" is not writeable by the Docker container.
+
+    Please ensure that the directory exists and is writeable by the Docker
+    container. All setups are different, but you may be able to run something
+    like this on the *host*:
+
+      chown nobody -R <host path that maps to #{dir}>
+      chmod 755 -R <host path that maps to #{dir}>
+
+    Swapping in your real host path. Then, you should set the user running
+    this container by editing your `docker run` command like so:
+
+        docker run --user 99:100 <rest of the command>
+
+    Or adding `user: '99:100'` to the Pinchflat service of your Docker Compose
+    file. Again, there are many ways to do this depending on your setup and
+    this is just one example. See issue #106 in the Pinchflat Github for more.
+
+    No matter the case, this _is_ a permissions error and allowing the container
+    to write to the directory is the only way to fix it. It is not recommended
+    to run the container as `root` because files created by Pinchflat may not
+    be accessible to other apps that want to modify them.
+    """
+  end
 end