Nondeterministic compiletest failure on long compiler output

[ashtneoi]

If I write a UI test that produces a lot of compiler errors, the test fails with a message like this:

running 1 test
F
failures:

---- [ui] ui/dont-commit.rs stdout ----

error: failed to decode compiler output as json: `EOF while parsing a string at line 1 column 302`
output: {"message":"snipped for brevity"}
line: {"message":"snipped for brevity"}

I can reliably trigger the bug with a UI test like this one:

fn main() {
    let e = 1;
    e = 2;
    e = 2;
    e = 2; // repeated a thousand times or so
}

However, the exact column at which the JSON parse fails is apparently nondeterministic. I ran the test multiple times under varying system load and got 237, 302, and 678. Seems like a race condition to me, although I don't know enough about compiletest to guess where the race might be.

version info

The bug is present on commit 3e05f80.

$ build/x86_64-unknown-linux-gnu/stage1/bin/rustc --version --verbose
rustc 1.30.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.30.0-dev
LLVM version: 7.0

[oli-obk]

cc @Zoxc maybe related to parallel rustc things?

[Zoxc]

It is probably disabled here. A lot of tests would fail otherwise.

[ashtneoi]

Found the problem:

rust/src/tools/compiletest/src/runtest.rs

Lines 3259 to 3273 in 7c13eb4

	fn into_bytes(self) -> Vec<u8> {
	match self {
	ProcOutput::Full(bytes) => bytes,
	ProcOutput::Abbreviated {
	mut head,
	skipped,
	tail,
	} => {
	write!(&mut head, "\n\n<<<<<< SKIPPED {} BYTES >>>>>>\n\n", skipped).unwrap();
	head.extend_from_slice(&tail);
	head
	}
	}
	}
	}

So what's happening is compiletest reads the compiler output from a pipe using read2_abbreviated, which throws away the middle of the output if it's too long and inserts a helpful error in its place. Fortunately for us, since the head of the output is almost always truncated in the middle of a line, we get a JSON parse error, rather than a bunch of incorrect "expected error not found" messages (which would have made this harder to debug).

[ashtneoi]

I'll see what happens if I change the caller to use read2 directly instead of read2_abbreviated. Fingers crossed that it's that simple.

[ashtneoi]

It works! I'll submit a PR and we can work through the finer details there.