Index: llvm/unittests/Support/SignalsTest.cpp
===================================================================
--- llvm/unittests/Support/SignalsTest.cpp
+++ llvm/unittests/Support/SignalsTest.cpp
@@ -19,7 +19,69 @@
 using namespace llvm;
 
 #if !defined(_WIN32)
-TEST(SignalTest, IgnoreMultipleSIGPIPEs) {
+struct SignalDeathTest : ::testing::Test {
+  void SetUp() {
+    // Note: SetUp runs after llvm registers its default signal handlers. Also,
+    // the static initializers from Signals.inc only run once, so the order of
+    // test execution matters a lot. Swapping two tests around can break them,
+    // as they may rely on llvm's handlers being in a specific state.
+
+    // The unit test process may inherit a mask for SIGPIPE from its parent.
+    // Unblock SIGPIPE.
+    sigset_t UnblockSet;
+    sigemptyset(&UnblockSet);
+    sigaddset(&UnblockSet, SIGPIPE);
+    sigprocmask(SIG_UNBLOCK, &UnblockSet, nullptr);
+  }
+};
+
+void NoOpSigHandler(void *) {}
+
+// This test must run first, as it assumes llvm's default signal handler state.
+TEST_F(SignalDeathTest, EnableExitOnSIGPIPE) {
+  // Register default signal handlers.
+  sys::AddSignalHandler(NoOpSigHandler, nullptr);
+
+  // Create unidirectional read/write pipes.
+  int fds[2];
+  int err = pipe(fds);
+  if (err != 0)
+    return; // If we can't make pipes, this isn't testing anything.
+
+  // Close the read pipe.
+  close(fds[0]);
+
+  // Attempt to write to the write pipe. Expect exit(IOERR).
+  const void *buf = (const void *)&fds;
+
+  // LLVM's default handler should catch SIGPIPE and exit with IOERR.
+  EXPECT_EXIT(write(fds[1], buf, 1), ::testing::ExitedWithCode(EX_IOERR), "");
+}
+
+TEST_F(SignalDeathTest, DisableExitOnSIGPIPE) {
+  // Reset SIGPIPE's default behavior.
+  signal(SIGPIPE, SIG_DFL);
+
+  // Disable exit-on-SIGPIPE.
+  sys::SetPipeSignalFunction(nullptr);
+
+  // Create unidirectional read/write pipes.
+  int fds[2];
+  int err = pipe(fds);
+  if (err != 0)
+    return; // If we can't make pipes, this isn't testing anything.
+
+  // Close the read pipe.
+  close(fds[0]);
+
+  // Attempt to write to the write pipe.
+  const void *buf = (const void *)&fds;
+
+  // No handler should be installed: we expect kill-by-SIGPIPE.
+  EXPECT_EXIT(write(fds[1], buf, 1), ::testing::KilledBySignal(SIGPIPE), "");
+}
+
+TEST_F(SignalDeathTest, IgnoreMultipleSIGPIPEs) {
   // Ignore SIGPIPE.
   signal(SIGPIPE, SIG_IGN);
 
@@ -35,20 +97,16 @@
   // Close the read pipe.
   close(fds[0]);
 
-  // Attempt to write to the write pipe. Currently we're asserting that the
-  // write fails, which isn't great.
-  //
-  // What we really want is a death test that checks that this block exits
-  // with a special exit "success" code, as opposed to unexpectedly exiting due
-  // to a kill-by-SIGNAL or due to the default SIGPIPE handler.
-  //
-  // Unfortunately llvm's unit tests aren't set up to support death tests well.
-  // For one, death tests are flaky in a multithreaded context. And sigactions
-  // inherited from llvm-lit interfere with what's being tested.
-  const void *buf = (const void *)&fds;
-  err = write(fds[1], buf, 1);
-  ASSERT_EQ(err, -1);
-  err = write(fds[1], buf, 1);
-  ASSERT_EQ(err, -1);
+  // Attempt to write to the write pipe. Assert that the writes both fail.
+  EXPECT_EXIT(
+      {
+        const void *buf = (const void *)&fds;
+        err = write(fds[1], buf, 1);
+        ASSERT_EQ(err, -1);
+        err = write(fds[1], buf, 1);
+        ASSERT_EQ(err, -1);
+        exit(123);
+      },
+      ::testing::ExitedWithCode(123), "");
 }
 #endif // !defined(_WIN32)