#398

backend/app/services/preview/http_fetcher.rb

@@ -1,4 +1,5 @@
 require 'net/http'
+require 'json'
 
 module Preview
   class HttpFetcher
@@ -16,9 +17,17 @@ module Preview
       response = request(uri, addresses.first, max_bytes)
 
       if response.is_a?(Net::HTTPRedirection)
-        raise FetchFailed, 'redirect が多すぎます．' if redirects.zero?
-
         location = response['location']
+        if redirects.zero?
+          log_failure(:redirect_limit,
+                      url: uri.to_s,
+                      redirects:,
+                      location:,
+                      content_type: response['content-type'],
+                      content_length: response['content-length'])
+          raise FetchFailed, 'redirect が多すぎます．'
+        end
+
         raise FetchFailed, 'redirect 先が不正です．' if location.blank?
 
         return fetch(URI.join(uri, location).to_s,
@@ -27,13 +36,20 @@ module Preview
       end
 
       unless response.is_a?(Net::HTTPSuccess)
+        log_failure(:http_status,
+                    url: uri.to_s,
+                    code: response.code,
+                    content_type: response['content-type'],
+                    content_length: response['content-length'])
         raise FetchFailed, "外部サーバーが HTTP #{ response.code } を返しました．"
       end
 
       Response.new(response.body, response['content-type'].to_s, uri.to_s)
     rescue Net::OpenTimeout, Net::ReadTimeout, Timeout::Error => e
+      log_failure(:timeout, url: uri&.to_s || raw_url, error: e.class.name, message: e.message)
       raise FetchTimeout, e.message
     rescue SocketError, SystemCallError, OpenSSL::SSL::SSLError, EOFError => e
+      log_failure(:network_error, url: uri&.to_s || raw_url, error: e.class.name, message: e.message)
       raise FetchFailed, e.message
     end
 
@@ -51,12 +67,27 @@ module Preview
 
       http.request(request) do |response|
         length = response['content-length'].to_i
-        raise ResponseTooLarge, '外部データが大きすぎます．' if length > max_bytes
+        if length > max_bytes
+          log_failure(:response_too_large,
+                      url: uri.to_s,
+                      content_type: response['content-type'],
+                      content_length: response['content-length'],
+                      max_bytes:)
+          raise ResponseTooLarge, '外部データが大きすぎます．'
+        end
 
         body = +''
         response.read_body do |chunk|
           body << chunk
-          raise ResponseTooLarge, '外部データが大きすぎます．' if body.bytesize > max_bytes
+          next unless body.bytesize > max_bytes
+
+          log_failure(:response_too_large,
+                      url: uri.to_s,
+                      content_type: response['content-type'],
+                      content_length: response['content-length'],
+                      bytes_read: body.bytesize,
+                      max_bytes:)
+          raise ResponseTooLarge, '外部データが大きすぎます．'
         end
         response.instance_variable_set(:@body, body)
         response.instance_variable_set(:@read, true)
@@ -64,6 +95,10 @@ module Preview
       end
     end
 
-    private_class_method :request
+    def self.log_failure(reason, **payload)
+      Rails.logger.warn("preview_http_fetcher_failure #{ { reason:, **payload }.to_json }")
+    end
+
+    private_class_method :request, :log_failure
   end
 end