[llvm-dev] Swallowing of input in FileCheck

[George Karpenkov via llvm-dev]

Hi,

Debugging tests which make use of FileCheck can be a frustrating experience, as all input will be swallowed (even with -v flag passed to lit),
and one would often need to copy-and-paste and rerun the failing command manually without piping into FileCheck.
Initially I’ve assumed that this is done due to stream processing, but looking at FileCheck source code I can see that
it actually gets the entire input into RAM anyway:

```
  do {
    Buffer.reserve(Buffer.size() + ChunkSize);
    ReadBytes = read(FD, Buffer.end(), ChunkSize);
    if (ReadBytes == -1) {
      if (errno == EINTR) continue;
      return std::error_code(errno, std::generic_category());
    }
    Buffer.set_size(Buffer.size() + ReadBytes);
  } while (ReadBytes != 0);
```

Thus, I propose modifying FileCheck default behavior to dump all swallowed output on stderr when the test has failed.
Would there be any objections to such a change?
I understand the concern that log files might become unnecessarily large, but since it would only be done for failed
test I think the added readability would be worth it.

Regards,
George