Safety check before running File.rm_rf! in doc gen

lib/ex_doc/formatter/epub.ex

@@ -10,9 +10,14 @@ defmodule ExDoc.Formatter.EPUB do
   """
   @spec run(list, ExDoc.Config.t()) :: String.t()
   def run(project_nodes, config) when is_map(config) do
+    parent = config.output
     config = normalize_config(config)
-    File.rm_rf!(config.output)
-    File.mkdir_p!(Path.join(config.output, "OEBPS"))
+
+    HTML.setup_output(
+      parent,
+      &cleanup_output_dir(&1, config),
+      &create_output_dir(&1, config)
+    )
 
     project_nodes = HTML.render_all(project_nodes, ".xhtml", config, highlight_tag: "samp")
 
@@ -44,6 +49,16 @@ defmodule ExDoc.Formatter.EPUB do
     Path.relative_to_cwd(epub)
   end
 
+  defp create_output_dir(root, config) do
+    File.mkdir_p!(Path.join(config.output, "OEBPS"))
+    File.touch!(Path.join(root, ".ex_doc"))
+  end
+
+  defp cleanup_output_dir(docs_root, config) do
+    File.rm_rf!(config.output)
+    create_output_dir(docs_root, config)
+  end
+
   defp normalize_config(config) do
     output =
       config.output

lib/ex_doc/formatter/html.ex

@@ -16,7 +16,7 @@ defmodule ExDoc.Formatter.HTML do
     config = %{config | output: Path.expand(config.output)}
 
     build = Path.join(config.output, ".build")
-    output_setup(build, config)
+    setup_output(config.output, &cleanup_output_dir(&1, config), &create_output_dir(&1, config))
 
     project_nodes = render_all(project_nodes, ".html", config, [])
     extras = build_extras(config, ".html")
@@ -146,18 +146,51 @@ defmodule ExDoc.Formatter.HTML do
     |> ExDoc.DocAST.highlight(language, opts)
   end
 
-  defp output_setup(build, config) do
+  def setup_output(root, cleanup, create) do
+    safety_path = Path.join(root, ".ex_doc")
+
+    cond do
+      File.exists?(safety_path) and File.exists?(root) ->
+        cleanup.(root)
+
+      not File.exists?(root) ->
+        create.(root)
+
+      File.ls!(root) == [] ->
+        add_safety_file(root)
+        :ok
+
+      true ->
+        IO.warn(
+          "ExDoc is outputting to an existing directory. " <>
+            "Beware documentation output may be mixed with other entries"
+        )
+    end
+  end
+
+  defp create_output_dir(root, _config) do
+    File.mkdir_p!(root)
+    add_safety_file(root)
+  end
+
+  defp add_safety_file(root) do
+    File.touch!(Path.join(root, ".ex_doc"))
+  end
+
+  defp cleanup_output_dir(docs_root, config) do
+    build = Path.join(docs_root, ".build")
+
     if File.exists?(build) do
       build
       |> File.read!()
       |> String.split("\n", trim: true)
-      |> Enum.map(&Path.join(config.output, &1))
+      |> Enum.map(&Path.join(docs_root, &1))
       |> Enum.each(&File.rm/1)
 
       File.rm(build)
     else
-      File.rm_rf!(config.output)
-      File.mkdir_p!(config.output)
+      File.rm_rf!(docs_root)
+      create_output_dir(docs_root, config)
     end
   end
 

test/ex_doc/formatter/epub_test.exs

@@ -103,6 +103,33 @@ defmodule ExDoc.Formatter.EPUBTest do
     assert File.regular?(tmp_dir <> "/epub/another_dir/#{doc_config(context)[:project]}.epub")
   end
 
+  test "succeeds if trying to write into an empty existing directory", context do
+    config = doc_config(context)
+
+    new_output = config[:output] <> "/new-dir"
+    File.mkdir_p!(new_output)
+
+    new_config = Keyword.put(config, :output, new_output)
+
+    refute ExUnit.CaptureIO.capture_io(:stderr, fn ->
+             generate_docs(new_config)
+           end) =~ "ExDoc is outputting to an existing directory"
+  end
+
+  test "warns if trying to write into existing directory with files", context do
+    config = doc_config(context)
+    new_output = config[:output] <> "/new-dir"
+
+    File.mkdir_p!(new_output)
+    File.touch!(Path.join(new_output, "dummy-file"))
+
+    new_config = Keyword.put(config, :output, new_output)
+
+    assert ExUnit.CaptureIO.capture_io(:stderr, fn ->
+             generate_docs(new_config)
+           end) =~ "ExDoc is outputting to an existing directory"
+  end
+
   test "generates an EPUB file with a standardized structure", %{tmp_dir: tmp_dir} = context do
     generate_docs_and_unzip(context, doc_config(context))
 

test/ex_doc/formatter/html/erlang_test.exs

@@ -5,6 +5,12 @@ defmodule ExDoc.Formatter.HTML.ErlangTest do
   @moduletag :otp_eep48
   @moduletag :tmp_dir
 
+  setup %{tmp_dir: tmp_dir} do
+    output = tmp_dir <> "/doc"
+    File.mkdir!(output)
+    File.touch!("#{output}/.ex_doc")
+  end
+
   test "smoke test", c do
     erlc(c, :foo, ~S"""
     %% @doc

test/ex_doc/formatter/html/search_items_test.exs

@@ -4,6 +4,12 @@ defmodule ExDoc.Formatter.HTML.SearchItemsTest do
 
   @moduletag :tmp_dir
 
+  setup %{tmp_dir: tmp_dir} do
+    output = tmp_dir <> "/doc"
+    File.mkdir!(output)
+    File.touch!("#{output}/.ex_doc")
+  end
+
   test "Elixir module", c do
     modules =
       elixirc(c, ~S'''

test/ex_doc/formatter/html_test.exs

@@ -6,6 +6,12 @@ defmodule ExDoc.Formatter.HTMLTest do
 
   @moduletag :tmp_dir
 
+  setup %{tmp_dir: tmp_dir} do
+    output = tmp_dir <> "/html"
+    File.mkdir_p!(output)
+    File.touch!(output <> "/.ex_doc")
+  end
+
   defp read_wildcard!(path) do
     [file] = Path.wildcard(path)
     File.read!(file)
@@ -146,6 +152,33 @@ defmodule ExDoc.Formatter.HTMLTest do
     refute content_module =~ re[:index][:refresh]
   end
 
+  test "succeeds if trying to write into an empty existing directory", context do
+    config = doc_config(context)
+
+    new_output = config[:output] <> "/new-dir"
+    File.mkdir_p!(new_output)
+
+    new_config = Keyword.put(config, :output, new_output)
+
+    refute ExUnit.CaptureIO.capture_io(:stderr, fn ->
+             generate_docs(new_config)
+           end) =~ "ExDoc is outputting to an existing directory"
+  end
+
+  test "warns if trying to write into existing directory with files", context do
+    config = doc_config(context)
+    new_output = config[:output] <> "/new-dir"
+
+    File.mkdir_p!(new_output)
+    File.touch!(Path.join(new_output, "dummy-file"))
+
+    new_config = Keyword.put(config, :output, new_output)
+
+    assert ExUnit.CaptureIO.capture_io(:stderr, fn ->
+             generate_docs(new_config)
+           end) =~ "ExDoc is outputting to an existing directory"
+  end
+
   test "allows to set the authors of the document", %{tmp_dir: tmp_dir} = context do
     generate_docs(doc_config(context, authors: ["John Doe", "Jane Doe"]))
     content_index = File.read!(tmp_dir <> "/html/api-reference.html")

test/test_helper.exs

@@ -22,6 +22,10 @@ defmodule TestHelper do
   def elixirc(context, filename \\ "nofile", code) do
     dir = context.tmp_dir
 
+    output_dir = context.tmp_dir <> "/html"
+    File.mkdir_p!(output_dir)
+    File.write!(output_dir <> "/.ex_doc", "")
+
     src_path = Path.join([dir, filename])
     src_path |> Path.dirname() |> File.mkdir_p!()
     File.write!(src_path, code)