Fallback to pure the pure ruby path scanner on unexpected error

Fix: #516
Fix: #517
Fix: #521

When we run into what appears to be a libc bug, we fallback to
the pure Ruby implementation.

That should unblock users, but people willing to help debug this
further can set BOOTSNAP_DEBUG=1 to fail on error.

The error should indicate which directory failed to open, and
perhaps closer inspection will allow to narrow the issue.

Author: byroot

Gemfile

@@ -17,5 +17,7 @@ gem "mocha"
 
 group :development do
   gem "rubocop", "~> 1.50.2" # Ruby 2.6 support
+  gem "benchmark"
+  gem "ostruct"
   gem "base64"
 end

lib/bootsnap.rb

@@ -2,8 +2,6 @@
 
 require_relative "bootsnap/version"
 require_relative "bootsnap/bundler"
-require_relative "bootsnap/compile_cache"
-require_relative "bootsnap/load_path_cache"
 
 module Bootsnap
   InvalidConfiguration = Class.new(StandardError)
@@ -164,3 +162,6 @@ def bool_env(key, default: false)
     end
   end
 end
+
+require_relative "bootsnap/compile_cache"
+require_relative "bootsnap/load_path_cache"

lib/bootsnap/load_path_cache/path_scanner.rb

@@ -102,6 +102,13 @@ def native_call(root_path)
             end
 
             all_requirables
+          rescue SystemCallError => error
+            if ENV["BOOTSNAP_DEBUG"]
+              raise
+            else
+              Bootsnap.logger&.call("Unexpected error: #{error.class}: #{error.message}")
+              ruby_call(root_path)
+            end
           end
           alias_method :call, :native_call
         else

test/test_helper.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+ENV["BOOTSNAP_DEBUG"] = "1"
+
 $LOAD_PATH.unshift(File.expand_path("../lib", __dir__))
 
 if Warning.respond_to?(:[]=)