Report I/O errors from rmeta encoding with emit_fatal
https://github.com/rust-lang/rust/issues/119456 reminded me that I never did systematic testing to provoke the out-of-disk ICEs so I grepped through a recent crater run (https://github.com/rust-lang/rust/pull/119440#issuecomment-1873393963) for more out-of-disk ICEs on current master and yep there's 2 in there.
So I finally cooked up a way to provoke for these crashes. I wrote a little `cdylib` crate that has a `#[no_mangle] pub extern "C" fn write` which occasionally reports `ENOSPC`, and prints a backtrace when it does.
<details><summary><strong>code for the dylib</strong></summary>
<p>
```rust
// cargo add libc rand backtrace
use rand::Rng;
#[no_mangle]
pub extern "C" fn write(
fd: libc::c_int,
buf: *const libc::c_void,
count: libc::size_t,
) -> libc::ssize_t {
if fd > 2 && rand::thread_rng().gen::<u8>() == 0 {
let mut count = 0;
backtrace::trace(|frame| {
backtrace::resolve_frame(frame, |symbol| {
if let Some(name) = symbol.name() {
if count > 3 {
eprintln!("{}", name);
}
}
count += 1;
});
true
});
unsafe {
*libc::__errno_location() = libc::ENOSPC;
}
return -1;
} else {
unsafe {
let res =
libc::syscall(libc::SYS_write, fd as usize, buf as usize, count as usize) as isize;
if res < 0 {
*libc::__errno_location() = -res as i32;
-1
} else {
res
}
}
}
}
```
</p>
</details>
Then `LD_PRELOAD` that dylib and repeatedly build a big project until it ICEs, such as with this:
```bash
while true; do
cargo clean
LD_PRELOAD=/home/ben/evil/target/release/libevil.so cargo +stage1 check 2> errors
if grep "thread 'rustc' panicked" errors; then
break
fi
done
```
My "big project" for testing was an otherwise-empty project with `cargo add axum`.
Before this PR, the above procedure finds a crash in between 1 and 15 minutes. With this PR, I have not found a crash in 30 minutes, and I'll be leaving this to run overnight (starting now). (A night has now passed, no crashes were found)
I believe the problem is that even though since https://github.com/rust-lang/rust/pull/117301 we correctly check `FileEncoder` for errors on all paths, we use `emit_err`, so there is a window of time between the call to `emit_err` and the full error reporting where rustc believes it has emitted a valid rmeta file and will permit Cargo to launch a build for a dependent crate. Changing these calls to `emit_fatal` closes that window.
I think there are a number of other cases where `emit_err` has been used instead of the more-correct `emit_fatal` such as e51e98dde6/compiler/rustc_codegen_ssa/src/back/write.rs (L542) but unlike rmeta encoding I am not aware of those cases of those causing problems.
r? ``@WaffleLapkin``
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