check robots.txt for permission to crawl

backend/lib/backend/crawler/api_crawler.ex

@@ -38,6 +38,12 @@ defmodule Backend.Crawler.ApiCrawler do
   Check whether the instance at the given domain is of the type that this ApiCrawler implements.
   """
   @callback is_instance_type?(String.t()) :: boolean()
+
+  @doc """
+  Check whether the instance allows crawling according to its robots.txt or otherwise.
+  """
+  @callback allows_crawling?(String.t()) :: boolean()
+
   @doc """
   Crawl the instance at the given domain.
   """

backend/lib/backend/crawler/crawler.ex

@@ -17,6 +17,7 @@ defmodule Backend.Crawler do
     # a list of ApiCrawlers that will be attempted
     :api_crawlers,
     :found_api?,
+    :allows_crawling?,
     :result,
     :error
   ]
@@ -25,6 +26,7 @@ defmodule Backend.Crawler do
           domain: String.t(),
           api_crawlers: [ApiCrawler.t()],
           found_api?: boolean,
+          allows_crawling?: boolean,
           result: ApiCrawler.t() | nil,
           error: String.t() | nil
         }
@@ -32,7 +34,15 @@ defmodule Backend.Crawler do
   def run(domain) do
     Logger.info("Crawling #{domain}...")
     HTTPoison.start()
-    state = %Crawler{domain: domain, api_crawlers: [], found_api?: false, result: nil, error: nil}
+
+    state = %Crawler{
+      domain: domain,
+      api_crawlers: [],
+      found_api?: false,
+      allows_crawling?: true,
+      result: nil,
+      error: nil
+    }
 
     state
     # register APICrawlers here
@@ -59,6 +69,7 @@ defmodule Backend.Crawler do
       Logger.debug("Found #{curr} instance")
       state = Map.put(state, :found_api?, true)
 
+      if curr.allows_crawling?(domain) do
         try do
           %Crawler{state | result: curr.crawl(domain), api_crawlers: []}
         rescue
@@ -71,6 +82,10 @@ defmodule Backend.Crawler do
           e in _ ->
             Map.put(state, :error, "Unknown error: " <> inspect(e))
         end
+      else
+        Logger.debug("#{domain} does not allow crawling.")
+        Map.put(state, :allows_crawling?, false)
+      end
     else
       # Nothing found so check the next APICrawler
       Logger.debug("#{domain} is not an instance of #{curr}")
@@ -79,7 +94,13 @@ defmodule Backend.Crawler do
   end
 
   # Save the state (after crawling) to the database.
-  defp save(%Crawler{domain: domain, result: result, found_api?: true, error: nil}) do
+  defp save(%Crawler{
+         domain: domain,
+         result: result,
+         found_api?: true,
+         error: nil,
+         allows_crawling?: true
+       }) do
     now = get_now()
 
     ## Update the instance we crawled ##
@@ -187,9 +208,12 @@ defmodule Backend.Crawler do
     |> Repo.insert_all(interactions)
   end
 
-  defp save(%{domain: domain, error: error}) do
-    if error == nil do
-      error = "no api found"
+  defp save(%{domain: domain, error: error, allows_crawling?: allows_crawling}) do
+    error =
+      cond do
+        not allows_crawling -> "robots.txt"
+        error == nil -> "no api found"
+        true -> "unknown error"
       end
 
     Repo.insert!(%Crawl{

backend/lib/backend/crawler/crawlers/mastodon.ex

@@ -14,6 +14,21 @@ defmodule Backend.Crawler.Crawlers.Mastodon do
     end
   end
 
+  @impl ApiCrawler
+  def allows_crawling?(domain) do
+    endpoints = [
+      "/api/v1/instance",
+      "/api/v1/instance/peers",
+      "/api/v1/timelines/public"
+    ]
+
+    user_agent = get_config(:user_agent)
+
+    endpoints
+    |> Enum.map(fn endpoint -> "https://#{domain}#{endpoint}" end)
+    |> Enum.all?(fn endpoint -> Gollum.crawlable?(user_agent, endpoint) != :uncrawlable end)
+  end
+
   @impl ApiCrawler
   # sobelow_skip ["DOS.StringToAtom"]
   def crawl(domain) do

backend/mix.exs

@@ -20,7 +20,7 @@ defmodule Backend.MixProject do
   def application do
     [
       mod: {Backend.Application, []},
-      extra_applications: [:logger, :runtime_tools, :mnesia]
+      extra_applications: [:logger, :runtime_tools, :mnesia, :gollum]
     ]
   end
 
@@ -46,7 +46,8 @@ defmodule Backend.MixProject do
       {:honeydew, "~> 1.4.3"},
       {:quantum, "~> 2.3"},
       {:corsica, "~> 1.1.2"},
-      {:sobelow, "~> 0.8", only: :dev}
+      {:sobelow, "~> 0.8", only: :dev},
+      {:gollum, "~> 0.3.2"}
     ]
   end
 

backend/mix.lock

@@ -15,6 +15,7 @@
   "gen_stage": {:hex, :gen_stage, "0.14.2", "6a2a578a510c5bfca8a45e6b27552f613b41cf584b58210f017088d3d17d0b14", [:mix], [], "hexpm"},
   "gen_state_machine": {:hex, :gen_state_machine, "2.0.5", "9ac15ec6e66acac994cc442dcc2c6f9796cf380ec4b08267223014be1c728a95", [:mix], [], "hexpm"},
   "gettext": {:hex, :gettext, "0.17.0", "abe21542c831887a2b16f4c94556db9c421ab301aee417b7c4fbde7fbdbe01ec", [:mix], [], "hexpm"},
+  "gollum": {:hex, :gollum, "0.3.2", "bd6a17febb98f5c362b5c5c08d9b102738fb875016e1eefec626f5e87c23f20e", [:mix], [{:httpoison, "~> 1.5.1", [hex: :httpoison, repo: "hexpm", optional: false]}], "hexpm"},
   "hackney": {:hex, :hackney, "1.15.1", "9f8f471c844b8ce395f7b6d8398139e26ddca9ebc171a8b91342ee15a19963f4", [:rebar3], [{:certifi, "2.5.1", [hex: :certifi, repo: "hexpm", optional: false]}, {:idna, "6.0.0", [hex: :idna, repo: "hexpm", optional: false]}, {:metrics, "1.0.1", [hex: :metrics, repo: "hexpm", optional: false]}, {:mimerl, "~>1.1", [hex: :mimerl, repo: "hexpm", optional: false]}, {:ssl_verify_fun, "1.1.4", [hex: :ssl_verify_fun, repo: "hexpm", optional: false]}], "hexpm"},
   "honeydew": {:hex, :honeydew, "1.4.3", "f2d976aaf8b9b914a635d2d483f1a71d2f6d8651809474dd5db581953cbebb30", [:mix], [{:ecto, "~> 3.0", [hex: :ecto, repo: "hexpm", optional: true]}], "hexpm"},
   "httpoison": {:hex, :httpoison, "1.5.1", "0f55b5b673b03c5c327dac7015a67cb571b99b631acc0bc1b0b98dcd6b9f2104", [:mix], [{:hackney, "~> 1.8", [hex: :hackney, repo: "hexpm", optional: false]}], "hexpm"},