[llvm] [lit] Avoid multiprocessing for -j1 runs (PR #175587)

[Aiden Grossman via llvm-commits]

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@@ -0,0 +1,42 @@
+# Test that -j1 runs in single-process mode (no multiprocessing).
+#
+# This ensures lit can run in sandboxed environments where multiprocessing
+# primitives (semaphores, process creation) are blocked.
+
+# REQUIRES: strace
+
+# Use strace to trace file operations. With -j1, lit should NOT create
+# POSIX semaphores in /dev/shm which are used by multiprocessing.Pool.
+#
+# Python's multiprocessing.BoundedSemaphore creates files like:
+#   openat(AT_FDCWD, "/dev/shm/sem.XXXXXX", O_RDWR|O_CREAT|O_EXCL|...)
+#
+# These are the semaphores that fail in sandboxed environments, so verifying
+# they don't appear with -j1 confirms the fix works.
+#
+# Note: We use /usr/bin/env explicitly because lit's test config puts
+# fake-externals (which use #!/usr/bin/env python) at the start of PATH.
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boomanaiden154 wrote:

Why exactly is `env` problematic?

https://github.com/llvm/llvm-project/pull/175587