Gem fails to initialize when mounted in a read-only filesystem

[ben]

I'm running a Rails app in Kubernetes, and the root filesystem is mounted as read-only. On startup using 0.13.0, I get an exception with this as the first few lines of the stacktrace:

Read-only file system @ rb_sysopen - /app/.const_generator20210415-16-174j4gr
/usr/lib/ruby/2.7.0/tempfile.rb:129:in `initialize'
/usr/lib/ruby/2.7.0/tempfile.rb:129:in `open'
/usr/lib/ruby/2.7.0/tempfile.rb:129:in `block in initialize'
/usr/lib/ruby/2.7.0/tmpdir.rb:129:in `create'
/usr/lib/ruby/2.7.0/tempfile.rb:127:in `initialize'
/usr/lib/ruby/2.7.0/tempfile.rb:287:in `new'
/usr/lib/ruby/2.7.0/tempfile.rb:287:in `open'
/usr/bundle/ruby/2.7.0/gems/ffi-1.15.0/lib/ffi/tools/const_generator.rb:110:in `calculate'
/usr/bundle/ruby/2.7.0/gems/ffi-1.15.0/lib/ffi/tools/const_generator.rb:49:in `initialize'
/usr/bundle/ruby/2.7.0/gems/ethon-0.13.0/lib/ethon/libc.rb:17:in `new'
/usr/bundle/ruby/2.7.0/gems/ethon-0.13.0/lib/ethon/libc.rb:17:in `<module:Libc>'
/usr/bundle/ruby/2.7.0/gems/ethon-0.13.0/lib/ethon/libc.rb:7:in `<module:Ethon>'
/usr/bundle/ruby/2.7.0/gems/ethon-0.13.0/lib/ethon/libc.rb:2:in `<top (required)>'

Looks like between 0.12.0 and 0.13.0 there was a change that introduced the use of FFI::ConstGenerator, whose default initializer sets it up to write to the current directory.

Could ethon initialize the ConstGenerator to use ENV['TMPDIR'] || '/tmp' instead of the default?

[Kjarrigan]

Hm. Not sure if this is possible. Because the line that crashed is https://github.com/ffi/ffi/blob/master/lib/ffi/tools/const_generator.rb#L110 and Tempfile.opens second param (which is not set in FFI) is used to change the temp_dir

But in https://github.com/ffi/ffi/blob/master/lib/ffi/tools/const_generator.rb#L108 they already use Dir.tmpdir for the binary so I think you could open a FFI-Pullrequest to use it for Tempfile.open too

[ben]

The line that opens the file uses "#{@prefix}.const_generator" as the filename. @prefix is set in the initializer. So if ethon were to do this at the callsite:

fcg = FFI::ConstGenerator.new(ENV['TMPDIR'] || '/tmp') do |gen|

…I believe that would solve this issue.

[tagliala]

Hi, I had a similar issue because FFI was attempting to execute a file in the tmp dir

Any chance to TL;DR what that binary does?

edit TL;DR: gets the value of _SC_OPEN_MAX

[Kjarrigan]

@ben I don't think so¹ but feel free to try it out. After all this is Ruby and you can change all the code you want.

¹The prefix of a tempfile only influences the filename as you can see in the docs I linked.

[Kjarrigan]

Should be solved in the next version. See #185