Issue #20586 has been updated by ivoanjo (Ivo Anjo). File readdir-bug-repro.c added mame (Yusuke Endoh) wrote in #note-2:
How would you like to fix it?
IMO, it would be reasonable to have `Dir.glob` raise an exception when `readdir` fails. [The spec of readdir](https://pubs.opengroup.org/onlinepubs/009604599/functions/readdir.html) says:
Applications wishing to check for error situations should set errno to 0 before calling readdir(). If errno is set to non-zero on return, an error occurred.
Do you want Ruby to automatically retry when readdir returns `EINTR`? If so, I am unsure because neither [the manpage you linked](https://man7.org/linux/man-pages/man3/readdir.3.html) nor [the spec](https://pubs.opengroup.org/onlinepubs/009604599/functions/readdir.html) indicate that readdir may fail with `EINTR`.
I believe the correct thing to do is to raise an exception. This will mean profiling the application could lead to these exceptions showing up where they didn't before, but at least they're an indication that something happened, rather than an incorrect result. We should avoid retrying, or at least retrying forever, because if some operation always takes 100ms, and the profiler executes every 10ms, then it will keep interrupting the operation and the app will also get stuck.
Have you actually confirmed that readdir is returning `EINTR`?
I have! I have attached a pure-C reproducer, and when I run it with the gcsfuse folder I get: ``` $ gcc readdir-bug-repro.c -o readdir-bug-repro -lpthread && ./readdir-bug-repro fuse-testing/ Set up signal handler! Received 113 signals, calling readdir... Failed to read directory 'fuse-testing/': Interrupted system call ``` For reference, I'm running it on: * Linux rubyshade 6.5.0-35-generic #35~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue May 7 09:00:52 UTC 2 x86_64 x86_64 x86_64 GNU/Linux * Ubuntu 22.04.4 LTS * gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04)
If so, it could be a bug in the OS. However, if such a bug exists in major platforms, it may be unavoidable for Ruby to handle it.
This is a good question. I'll be honest that I could never reproduce with a local filesystem. I'm not sure if this just means that some filesystems (perhaps those implemented with [FUSE](https://www.kernel.org/doc/html/next/filesystems/fuse.html)?) allow this kind of failure, or if it's just really hard to hit it when there's kernel cache and all those things making sure these calls are very speedy. ---------------------------------------- Bug #20586: Some filesystem calls in dir.c are missing error handling and can return incorrect results if interrupted https://bugs.ruby-lang.org/issues/20586#change-108860 * Author: ivoanjo (Ivo Anjo) * Status: Open * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN ---------------------------------------- #### Background Hey! I work for Datadog on the Ruby profiler part of the [`datadog` (previously `ddtrace`)](https://github.com/datadog/dd-trace-rb) gem. A customer reached [out with an issue](https://github.com/DataDog/dd-trace-rb/issues/3450) where enabling the profiler made `Dir.glob` return no files for a given folder: Without profiler: ``` irb(main):002:0> Dir.glob('/gcsfuse/t*') => ["/gcsfuse/test.html", "/gcsfuse/test.txt"] ``` With profiler: ``` irb(main):002:0> Dir.glob('/gcsfuse/t*') => [] ``` It turns out the issue is related to missing error handling in `dir.c`. The Datadog Ruby profiler, like stackprof, pf2 and vernier, uses unix signals to interrupt the currently-active thread and take a sample (usually `SIGPROF`). When some system calls get interrupted by a signal, they return an [EINTR error code](https://man7.org/linux/man-pages/man7/signal.7.html#:~:text=Interruption%20o...) back to the caller. Consider for instance the implementation of `dir_each_entry` in `dir.c`: ```c static VALUE dir_each_entry(VALUE dir, VALUE (*each)(VALUE, VALUE), VALUE arg, int children_only) { struct dir_data *dirp; struct dirent *dp; IF_NORMALIZE_UTF8PATH(int norm_p); GetDIR(dir, dirp); rewinddir(dirp->dir); IF_NORMALIZE_UTF8PATH(norm_p = need_normalization(dirp->dir, RSTRING_PTR(dirp->path))); while ((dp = READDIR(dirp->dir, dirp->enc)) != NULL) { // ... do things } return dir; } ``` If `READDIR` returns `NULL`, then `dir_each_entry` assumes it has iterated the entire directory. But looking [at the man page for `readdir`](https://man7.org/linux/man-pages/man3/readdir.3.html) we see the following sharp edge (emphasis mine):
It returns NULL on reaching the end of the directory stream **or if an error occurred**.
So what's happening in this situation is: `readdir` gets interrupted, returns `NULL` + sets errno to `EINTR`. But `dir_each_entry` doesn't check `errno`, so rather than raising an exception to flag the issue, it treats it as if the end of the directory has been reached. #### How to reproduce Reproducing this is somewhat annoying, because it's dependent on timing: the signal must arrive at the exact time the dir API is getting executed. I was able to reproduce this every time by using the [google cloud `gcsfuse`](https://cloud.google.com/storage/docs/gcs-fuse) tool. This somewhat makes sense -- a remote filesystem is much slower than a local one, so there's a much bigger window of opportunity for a signal to arrive while the system call is blocked. Here's an example I included in https://github.com/DataDog/dd-trace-rb/pull/3720: ``` # Not shown: Set up a trial google cloud account, install gcsfuse, create a cloud storage bucket and put in some test files $ gcsfuse test_fs_dd_trace_rb fuse-testing/ $ ls fuse-testing/ hello.txt test.html test.txt # Not shown: Add `datadog` gem to `Gemfile` $ DD_PROFILING_ENABLED=true DD_PROFILING_DIR_INTERRUPTION_WORKAROUND_ENABLED=false bundle exec ddprofrb exec ruby -e "Datadog::Profiling.wait_until_running; pp Dir.children('fuse-testing/')" [] ``` Let me know if you'd like me to try to create a reproducer that does not depend on the `datadog` gem. #### Additional notes I've spent quite some time looking at the `dir.c` sources, and here's the full list of APIs that suffer from issues: * `dir_each_entry` does not check `errno`; all of its users have interruption bugs * `dir_tell` will return -1 instead of the correct position (which means that passing -1 to `dir_seek`/`dir_set_pos` will cause it to not list the directory properly) * `do_opendir` an error in system calls will only sometimes be turned into a raised exception * Indirect callers that pass in rb_glob_error as errfunc: rb_glob, Dir.[], Dir.glob * Indirect callers that pass in 0 as errfunc: ruby_glob, ruby_brace_glob * `glob_opendir` does not check errno; all of its users have interruption bugs * `glob_getent` does not check errno; all of its users have interruption bugs * `nogvl_dir_empty_p` does not check errno (of readdir! it actually checks for opendir); all of its users have interruption bugs Less sure about these: * `do_stat`/`do_lstat` will turn errors into warnings (unclear if enabled or disabled by default) * `need_normalization` calls `fgetattrlist` / `getattrlist` and all errors `(ret != 0)` are treated in the same way * `rb_glob_error` is and `rb_glob_caller` leave exceptions as pending and rely on callers to raise them properly * Error handling of `rb_home_dir_of` and `rb_default_home_dir` are a bit suspicious As a workaround in the Datadog Ruby profiler, in https://github.com/DataDog/dd-trace-rb/pull/3720 I've added monkey patches to all of the Ruby-level APIs that use the above functions and mask out `SIGPROF` so these calls are never interrupted. This solution is does successfully work around the issue, although it prevents the profiler from sampling during these system calls, which will mean less visibility if e.g. these calls are taking a long time. And well, maintaining monkey patches is always problematic for future Ruby compatibility. ---Files-------------------------------- readdir-bug-repro.c (2.11 KB) -- https://bugs.ruby-lang.org/