Ensure that we don't crash when a table is started twice

coveralls.json

@@ -10,7 +10,9 @@
     "def.+(.+\\\\.+).+do",
     "^\\s+use\\s+"
   ],
-  "custom_stop_words": [ ],
+  "custom_stop_words": [
+    "with"
+  ],
   "coverage_options": {
     "treat_no_relevant_lines_as_covered": true
   },

lib/eternal/supervisor.ex

@@ -27,14 +27,16 @@ defmodule Eternal.Supervisor do
         { :ok, pid, Table.t } | :ignore |
         { :error, { :already_started, pid } | { :shutdown, term } | term }
   def start_link(name, ets_opts \\ [], opts \\ []) when is_opts(name, ets_opts, opts) do
-    super_tab  = :ets.new(name, [ :public ] ++ ets_opts)
-    super_args = { super_tab, opts, self() }
-    super_opts = [ name: Table.to_name(super_tab, true) ]
-    super_proc = Supervisor.start_link(__MODULE__, super_args, super_opts)
+    detect_clash(name, ets_opts, fn ->
+      super_tab  = :ets.new(name, [ :public ] ++ ets_opts)
+      super_args = { super_tab, opts, self() }
+      super_opts = [ name: Table.to_name(super_tab, true) ]
+      super_proc = Supervisor.start_link(__MODULE__, super_args, super_opts)
 
-    with { :ok, pid } <- super_proc do
-      { :ok, pid, super_tab }
-    end
+      with { :ok, pid } <- super_proc do
+        { :ok, pid, super_tab }
+      end
+    end)
   end
 
   @doc false
@@ -58,4 +60,25 @@ defmodule Eternal.Supervisor do
     supervise(children, strategy: :one_for_one)
   end
 
+  # Detects a potential name clash inside ETS. If we have a named table and the
+  # table is already in use, we return a link to the existing Supervisor. This
+  # means we can be transparent to any crashes caused by starting the same ETS
+  # table twice. Otherwise, we execute the callback which will create the table.
+  defp detect_clash(name, ets_opts, fun) do
+    if exists?(name, ets_opts) do
+      { :ok, Process.whereis(name), name }
+    else
+      fun.()
+    end
+  end
+
+  # Shorthand function to determine if an ETS table exists or not. We calculate
+  # this by looking for the name inside the list of ETS tables, but only if the
+  # options specify that we should name the ETS table. If it's not named, there
+  # won't be a table clash when starting a new table, so we're safe to continue.
+  defp exists?(name, ets_opts) do
+    Enum.member?(ets_opts, :named_table) and
+    Enum.member?(:ets.all(), name)
+  end
+
 end

test/eternal_test.exs

@@ -71,6 +71,13 @@ defmodule EternalTest do
     assert(Regex.match?(~r/\[debug\] \[eternal\] Table 'logging_output' gifted to #PID<\d\.\d+\.\d> via #PID<\d\.\d+\.\d>/, msg))
   end
 
+  test "starting a table twice finds the previous owner" do
+    { :ok, pid1 } = Eternal.start_link(:existing_table, [], [ quiet: true ])
+    { :ok, pid2 } = Eternal.start_link(:existing_table, [], [ quiet: true ])
+
+    assert(pid1 == pid2)
+  end
+
   test "deprecation warnings when using new/3" do
     msg = capture_log(fn ->
       tab = Eternal.new(:deprecation_new, [], [ quiet: true ])