[InitLLVM] Ensure SIGPIPE handler installed before sigaction()

The pipe signal handler must be installed before any other handlers are
registered. This is because the Unix RegisterHandlers function does not
perform a sigaction() for SIGPIPE unless a one-shot handler is present,
to allow long-lived processes (like lldb) to fully opt-out of llvm's
SIGPIPE handling and ignore the signal safely.

Fixes a bug introduced in D70277.

Tested by running Nick's test case:

% xcrun ./bin/clang -E -fno-integrated-cc1 x.c | tee foo.txt | head

I verified that child cc1 process exits with IO_ERR, and that the parent
recognizes the error code, exiting cleanly.

Differential Revision: https://reviews.llvm.org/D94324

include/llvm/Support/InitLLVM.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_SUPPORT_LLVM_H
 #define LLVM_SUPPORT_LLVM_H
 
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/PrettyStackTrace.h"
@@ -44,7 +45,7 @@ public:
 private:
   BumpPtrAllocator Alloc;
   SmallVector<const char *, 0> Args;
-  PrettyStackTraceProgram StackPrinter;
+  Optional<PrettyStackTraceProgram> StackPrinter;
 };
 } // namespace llvm
 

lib/Support/InitLLVM.cpp

@@ -22,10 +22,17 @@ using namespace llvm;
 using namespace llvm::sys;
 
 InitLLVM::InitLLVM(int &Argc, const char **&Argv,
-                   bool InstallPipeSignalExitHandler)
-    : StackPrinter(Argc, Argv) {
+                   bool InstallPipeSignalExitHandler) {
   if (InstallPipeSignalExitHandler)
+    // The pipe signal handler must be installed before any other handlers are
+    // registered. This is because the Unix \ref RegisterHandlers function does
+    // not perform a sigaction() for SIGPIPE unless a one-shot handler is
+    // present, to allow long-lived processes (like lldb) to fully opt-out of
+    // llvm's SIGPIPE handling and ignore the signal safely.
     sys::SetOneShotPipeSignalFunction(sys::DefaultOneShotPipeSignalHandler);
+  // Initialize the stack printer after installing the one-shot pipe signal
+  // handler, so we can perform a sigaction() for SIGPIPE on Unix if requested.
+  StackPrinter.emplace(Argc, Argv);
   sys::PrintStackTraceOnErrorSignal(Argv[0]);
   install_out_of_memory_new_handler();