[compiler-rt] [sanitizer] Print diagnostic if ptrace syscall fails (PR #151406)

[Thurston Dang via llvm-commits]

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@@ -403,7 +403,51 @@ struct ScopedSetTracerPID {
   }
 };
 
+// This detects whether ptrace is blocked (e.g., by seccomp), by forking and
+// then attempting ptrace.
+// This separate check is necessary because StopTheWorld() creates a *thread*,
+// and therefore cannot use waitpid() due to the shared errno.
+static void TestPTrace() {
+  // Only check the first time this is called.
+  static bool checked = false;
+  if (checked)
+    return;
+  checked = true;
+
+  // fork() should be cheap because of copy-on-write. Besides, this is only
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thurstond wrote:

Reworded

https://github.com/llvm/llvm-project/pull/151406