Jar resources issues with IO#seek #pos or #pread #9727

uri:classloader: resources: IO#pos, IO#seek and IO#pread silently return wrong results

• applies to any entry read through uri:classloader: / classpath: (any gem or app file in a bundled .jar or .war)
• came out with latest Psych (5.5.0) update, see bellow, where Rails is no longer able to read .yml files inside the archive.

Reproduction

For simplicity, let's just attempt to read a resource out of jruby.jar itself:

path = "uri:classloader:/jruby/kernel.rb"   # 1124 bytes
File.size(path)                             # => 1124  (correct)

File.open(path, "r") do |f|
  f.read(3)      # => "# T"
  f.pos          # => 0       EXPECTED 3
  f.seek(0)      # => 0
  f.read(3)      # => "his"   EXPECTED "# T"
end

File.open(path, "r") do |f|
  f.pread(10, 0)   # => "# This fil"
  f.pread(10, 50)  # => "e boots th"   EXPECTED "module JRu"
  f.pread(10, 0)   # => "e Ruby-bas"   EXPECTED "# This fil"
end

File.open(path, "r:bom|utf-8") do |f|
  f.pos          # => 0
  f.read         # => ""      EXPECTED the whole file
end

1. IO#pos always reports 0, whatever has been read.
2. IO#seek and IO#pos= return 0 and raise nothing, but position does not move.
3. IO#pread ignores offset argument and reads sequentially from wherever the handle happens to be.

Nothing raises in any of these cases. Callers get wrong data and cannot tell.

psych >= 5.5.0: Rails will not boot from a jar

Recent psych 5.5.0 added a BOM pre-pass to Psych::Parser#parse (lib/psych/parser.rb, for Bug #13615):

def parse yaml, path = ...
  _native_parse @handler, strip_bom(yaml), path
end

def skip_io_bom io, bom
  begin
    pos = io.pos
  rescue SystemCallError, IOError
    return                        # guard never fires: pos does not raise
  end
  head = io.read(bom.bytesize)
  io.seek(pos, IO::SEEK_SET) if head && head.b != bom
end

Psych.unsafe_load_file opens with File.open(filename, 'r:bom|utf-8'), stream goes empty, Psych.parse returns false for "no documents" and every YAML inside the jar reads as empty.

For a Rails app this is fatal at boot, because i18n rejects the non-Hash:

ArgumentError: (InvalidLocaleData) can not load translations from
uri:classloader:/gems/activesupport-8.1.3.1/lib/active_support/locale/en.yml:
expects it to return a hash, but does not
  at RUBY.load_file(uri:classloader:/gems/i18n-1.14.8/lib/i18n/backend/base.rb:245)
  ...
  at RUBY.eager_load!(uri:classloader:/gems/i18n-1.14.8/lib/i18n.rb:93)
  at RUBY.initialize!(uri:classloader:/gems/railties-8.1.3.1/lib/rails/application.rb:442)

mini_mime: reported in 2019, closed, still broken

discourse/mini_mime#37 is the same bug, hit through seek + read in MiniMime::Db::RandomAccessDb.
The reporter diagnosed it correctly at the time and pointed at jruby/jruby#3399.

It was closed by PR #50, which replaced seek + read with pread for fork safety and noted that this "also happens to fix an outstanding JRuby issue". On JRuby it does not, because of symptom 3: pread ignores its offset. The crash is gone, but mini_mime now reads from the wrong place in the database and returns a wrong MIME type instead. That is harder to notice than the original crash.

History

jruby/jruby#3399 (2015) reported seek raising Errno::EPIPE on these paths - got closed by making seek stop raising. The position never started moving, so the failure became silent.