[Support] Fix Process::PreventCoreFiles() when coredumps are piped

On many current Linux systems, coredumps are no longer dumped in the CWD
but instead piped to a utility such as systemd-coredumpd that stores
them in a deterministic location. This can be done by setting the
kernel.core_pattern sysctl to start with a '|'. However, when using such
a setup the kernel ignores a coredump limit of 0 (since there is no file
being written) and we can end up piping many gigabytes of data to
systemd-coredumpd which causes the test suite to freeze for a long time.
While most piped coredump handlers do respect the crashing processes'
RLIMIT_CORE, this is notable not the case for Debian's systemd-coredump
due to a local patch that changes sysctl.d/50-coredump.conf to ignore
the specified limit and instead use RLIM_INFINITY
(https://salsa.debian.org/systemd-team/systemd/-/commit/64599ffe44f0d).

Fortunately there is a workaround: the kernel recognizes the magic value
of 1 for RLIMIT_CORE to disable coredumps when piping. One byte is also
too small to generate any coredump, so it effectively behaves as if we
had set the value to zero.

The alternative to using RLIMIT_CORE=1 would be to use prctl() with the
PR_SET_DUMPABLE flag, however that also prevents ptrace(), so makes it
impossible to attach a debugger.

See #83701 and
#45797

Reviewed By: MaskRay

Pull Request: #83703

Author: arichardson

llvm/lib/Support/Unix/Process.inc

@@ -143,7 +143,26 @@ void Process::GetTimeUsage(TimePoint<> &elapsed,
 void Process::PreventCoreFiles() {
 #if HAVE_SETRLIMIT
   struct rlimit rlim;
-  rlim.rlim_cur = rlim.rlim_max = 0;
+  getrlimit(RLIMIT_CORE, &rlim);
+#ifdef __linux__
+  // On Linux, if the kernel.core_pattern sysctl starts with a '|' (i.e. it
+  // is being piped to a coredump handler such as systemd-coredumpd), the
+  // kernel ignores RLIMIT_CORE (since we aren't creating a file in the file
+  // system) except for the magic value of 1, which disables coredumps when
+  // piping. 1 byte is too small for any kind of valid core dump, so it
+  // also disables coredumps if kernel.core_pattern creates files directly.
+  // While most piped coredump handlers do respect the crashing processes'
+  // RLIMIT_CORE, this is notable not the case for Debian's systemd-coredump
+  // due to a local patch that changes sysctl.d/50-coredump.conf to ignore
+  // the specified limit and instead use RLIM_INFINITY.
+  //
+  // The alternative to using RLIMIT_CORE=1 would be to use prctl() with the
+  // PR_SET_DUMPABLE flag, however that also prevents ptrace(), so makes it
+  // impossible to attach a debugger.
+  rlim.rlim_cur = std::min<rlim_t>(1, rlim.rlim_max);
+#else
+  rlim.rlim_cur = 0;
+#endif
   setrlimit(RLIMIT_CORE, &rlim);
 #endif