[Support] On Unix, let the CrashRecoveryContext return the signal code

Before this patch, the CrashRecoveryContext was returning -2 upon a signal, like ExecuteAndWait does. This didn't match the behavior on Windows, where the the exception code was returned.

We now return the signal's code, which optionally allows for re-throwing the signal later. Doing so requires all custom handlers to be removed first, through llvm::sys::unregisterHandlers() which we made a public API.

This is part of https://reviews.llvm.org/D70378

include/llvm/Support/Signals.h

@@ -117,6 +117,8 @@ namespace sys {
   /// Context is a system-specific failure context: it is the signal type on
   /// Unix; the ExceptionContext on Windows.
   void CleanupOnSignal(uintptr_t Context);
+
+  void unregisterHandlers();
 } // End sys namespace
 } // End llvm namespace
 

lib/Support/CrashRecoveryContext.cpp

@@ -375,9 +375,10 @@ static void CrashRecoverySignalHandler(int Signal) {
   sigaddset(&SigMask, Signal);
   sigprocmask(SIG_UNBLOCK, &SigMask, nullptr);
 
-  // As per convention, -2 indicates a crash or timeout as opposed to failure to
-  // execute (see llvm/include/llvm/Support/Program.h)
-  int RetCode = -2;
+  // Return the same error code as if the program crashed, as mentioned in the
+  // section "Exit Status for Commands":
+  // https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu_chap02.html
+  int RetCode = 128 + Signal;
 
   // Don't consider a broken pipe as a crash (see clang/lib/Driver/Driver.cpp)
   if (Signal == SIGPIPE)

lib/Support/Unix/Signals.inc

@@ -331,7 +331,7 @@ static void RegisterHandlers() { // Not signal-safe.
     registerHandler(S, SignalKind::IsInfo);
 }
 
-static void UnregisterHandlers() {
+void sys::unregisterHandlers() {
   // Restore all of the signal handlers to how they were before we showed up.
   for (unsigned i = 0, e = NumRegisteredSignals.load(); i != e; ++i) {
     sigaction(RegisteredSignalInfo[i].SigNo,
@@ -367,7 +367,7 @@ static RETSIGTYPE SignalHandler(int Sig) {
   // crashes when we return and the signal reissues.  This also ensures that if
   // we crash in our signal handler that the program will terminate immediately
   // instead of recursing in the signal handler.
-  UnregisterHandlers();
+  sys::unregisterHandlers();
 
   // Unmask all potentially blocked kill signals.
   sigset_t SigMask;

lib/Support/Windows/Signals.inc

@@ -869,3 +869,5 @@ static BOOL WINAPI LLVMConsoleCtrlHandler(DWORD dwCtrlType) {
  #pragma GCC diagnostic warning "-Wformat"
  #pragma GCC diagnostic warning "-Wformat-extra-args"
 #endif
+
+void sys::unregisterHandlers() {}

unittests/Support/CrashRecoveryTest.cpp

@@ -7,10 +7,12 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/Triple.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/CrashRecoveryContext.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Host.h"
+#include "llvm/Support/Program.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/raw_ostream.h"
 #include "gtest/gtest.h"
@@ -131,3 +133,43 @@ TEST(CrashRecoveryTest, Abort) {
   EXPECT_FALSE(CrashRecoveryContext().RunSafely(A));
 }
 #endif
+
+// Specifically ensure that programs that signal() or abort() through the
+// CrashRecoveryContext can re-throw again their signal, so that `not --crash`
+// succeeds.
+#ifdef LLVM_ON_UNIX
+// See llvm/utils/unittest/UnitTestMain/TestMain.cpp
+extern const char *TestMainArgv0;
+
+// Just a reachable symbol to ease resolving of the executable's path.
+static cl::opt<std::string> CrashTestStringArg1("crash-test-string-arg1");
+
+TEST(CrashRecoveryTest, UnixCRCReturnCode) {
+  using namespace llvm::sys;
+  if (getenv("LLVM_CRC_UNIXCRCRETURNCODE")) {
+    llvm::CrashRecoveryContext::Enable();
+    CrashRecoveryContext CRC;
+    EXPECT_FALSE(CRC.RunSafely(abort));
+    EXPECT_EQ(CRC.RetCode, 128 + SIGABRT);
+    // re-throw signal
+    llvm::sys::unregisterHandlers();
+    raise(CRC.RetCode - 128);
+    llvm_unreachable("Should have exited already!");
+  }
+
+  std::string Executable =
+      sys::fs::getMainExecutable(TestMainArgv0, &CrashTestStringArg1);
+  StringRef argv[] = {
+      Executable, "--gtest_filter=CrashRecoveryTest.UnixCRCReturnCode"};
+
+  // Add LLVM_CRC_UNIXCRCRETURNCODE to the environment of the child process.
+  std::vector<StringRef> EnvTable;
+  EnvTable.push_back("LLVM_CRC_UNIXCRCRETURNCODE=1");
+
+  std::string Error;
+  bool ExecutionFailed;
+  int RetCode = ExecuteAndWait(Executable, argv, makeArrayRef(EnvTable), {}, 0, 0, &Error,
+                               &ExecutionFailed);
+  ASSERT_EQ(-2, RetCode);
+}
+#endif