Struct RetryingStderr 

pub struct RetryingStderr;

A Write for the supervisor’s own diagnostics on fd 2 that survives a NONBLOCKING description.

⚠️ THE GUEST CAN SET O_NONBLOCK ON HERMIT’S STDERR AND SILENTLY CUT THE SUPERVISOR’S ERROR CHANNEL. fd 2 is an INHERITED open file description shared with the guest, so fcntl(2, F_SETFL, O_NONBLOCK) in the guest changes the behaviour of hermit’s OWN later writes. Measured 2026-08-26 on a 4096-byte pipe under back-pressure, hermit --log info run -- <guest>:

  control guest (touches nothing)   rc=0    138 lines, ends on the run summary
  guest sets O_NONBLOCK on fd 2     rc=101  134 lines, ends mid-log

The summary was emitted by eprint!, which calls write_all; write_all does NOT retry EAGAIN, so it returns an error and the print macro PANICS. The panic message then went to the same full pipe and was lost as well, so the delivered output contained no panic text, no error, and no marker of any kind – it simply stopped on a plausible-looking line. Exit 101 was the only surviving evidence, and a caller that reads output rather than status sees a short report, not a truncated one.

⚠️ WAIT, DO NOT SPIN, AND DO NOT CLEAR THE FLAG. Clearing O_NONBLOCK would change what the guest observes on a descriptor it legitimately shares; this leaves the flag exactly as the guest set it and simply waits for the pipe to drain, which is what a blocking write would have done. A bare retry loop would busy-spin against a full pipe, so it blocks in poll(POLLOUT).

Trait Implementations

impl Write for RetryingStderr

fn write(&mut self, buf: &[u8]) -> Result<usize>

Writes a buffer into this writer, returning how many bytes were written.

fn flush(&mut self) -> Result<()>

Flushes this output stream, ensuring that all intermediately buffered contents reach their destination.

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize, Error>

Like write, except that it writes from a slice of buffers.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Writer has an efficient write_vectored implementation.

fn write_all(&mut self, buf: &[u8]) -> Result<(), Error>

Attempts to write an entire buffer into this writer.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<(), Error>

🔬This is a nightly-only experimental API. (write_all_vectored)

Attempts to write multiple buffers into this writer.

fn write_fmt(&mut self, args: Arguments<'_>) -> Result<(), Error>

Writes a formatted string into this writer, returning any error encountered.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adapter for this instance of Write.