[llvm] [lit] Run builtin cat / diff in-process instead of spawning (PR #208024)

[Aiden Grossman via llvm-commits]

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@@ -212,6 +217,76 @@ def _replaceReadFile(match):
     return arguments
 
 
+class InProcessPipe:
+    """Popen-compatible shim that runs an in-process builtin (cat/diff) inline.
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boomanaiden154 wrote:

We can't move off the `_launch_with_limit` wrapper. We need to spawn a subprocess that then lowers the limits, because we cannot reraise them without elevated permissions. It's also not really handled like a built-in. The performance of `ulimit` doesn't really matter given it's used in maybe 10 tests.

For this patch, `cat` and `diff` should be the only ones you need to be concerned with. All the other ones are just ideas that are future problems for now.

https://github.com/llvm/llvm-project/pull/208024