Band-aid fix to stop race conditions in llvm errors

[aidanhs]

This is a big hammer, but should be effective at completely removing a
few issues, including inconsistent error messages and segfaults when
LLVM workers race to report results

LLVM_THREAD_LOCAL has been present in LLVM since 8 months before 3.7
(the earliest supported LLVM version that Rust can use)

Maybe fixes #43402 (third time lucky?)

r? @alexcrichton

---

You can see that in

rust/src/librustc_trans/back/write.rs

Lines 75 to 100 in 5f578df

	pub fn llvm_err(handler: &errors::Handler, msg: String) -> FatalError {
	match llvm::last_error() {
	Some(err) => handler.fatal(&format!("{}: {}", msg, err)),
	None => handler.fatal(&msg),
	}
	}
	pub fn write_output_file(
	handler: &errors::Handler,
	target: llvm::TargetMachineRef,
	pm: llvm::PassManagerRef,
	m: ModuleRef,
	output: &Path,
	file_type: llvm::FileType) -> Result<(), FatalError> {
	unsafe {
	let output_c = path2cstr(output);
	let result = llvm::LLVMRustWriteOutputFile(
	target, pm, m, output_c.as_ptr(), file_type);
	if result.into_result().is_err() {
	let msg = format!("could not write output to {}", output.display());
	Err(llvm_err(handler, msg))
	} else {
	Ok(())
	}
	}
	}

there's a small window where the static global error message (made thread local in this PR) could be altered by another thread.

Note that we can't use thread_local because gcc 4.7 (permitted according to the readme) does not support it.

Maybe ideally all the functions should be modified to not use a global, but this PR makes things deterministic at least. My only hesitation is whether errors are checked in different threads to where they occur, but I figure that's probably unlikely (and is less bad than racing code).

As an aside, segfault evidence before this patch when I was doing some debugging:

$ while grep 'No such file or directory' log2; do RUST_LOG=debug ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc -o "" y.rs >log2 2>&1; done
error: could not write output to : No such file or directory
error: could not write output to : No such file or directory
error: could not write output to : No such file or directory
error: could not write output to : No such file or directory
error: could not write output to : No such file or directory
error: could not write output to : No such file or directory
error: could not write output to : No such file or directory
Segmentation fault (core dumped)
error: could not write output to : No such file or directory
error: could not write output to : No such file or directory

[alexcrichton]

@bors: r+

Oh dear, that should definitely be a thread local!

[bors]

📌 Commit 8628d51 has been approved by alexcrichton

[alexcrichton]

@bors: p=1

[bors]

⌛ Testing commit 8628d51 with merge ec016f8...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing ec016f8 to master...