[InitLLVM] Ensure SIGPIPE handler installed before sigaction()

[vsk]

Summary

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.

[aganea]

Do you think it would be possible to add test coverage in Clang by using Nick's test?

[vsk]

Add a clang driver test.

[vsk]

Fix a typo in the test.

[nickdesaulniers]

Much thanks for the patch!

clang/test/Driver/sigpipe-handling.c

3

Looks like

clang -E -fno-integrated-cc1 %s | head

is even enough to repro (no tee needed).

[aganea]

What are the exact conditions to repro Nick's failure? On Fedora 32, I ran py llvm-lit sigpipe-handling.c before this patch and before rGbf401256edd00e921a5d3a0bf4cf6ee66ae51cd6 , the test suceeds, it should not, should it?

[vsk]

Reduce the test a bit further.

[vsk]

In D94324#2487572, @aganea wrote:

What are the exact conditions to repro Nick's failure? On Fedora 32, I ran py llvm-lit sigpipe-handling.c before this patch and before rGbf401256edd00e921a5d3a0bf4cf6ee66ae51cd6 , the test suceeds, it should not, should it?

In order for the test to fail pre-patch, the OS would need to send a SIGPIPE to clang when it writes to stdout after head exits. It's possible that certain OSes don't do this (perhaps they are patched to SIGPIPE less aggressively? it's the sort of patch an OS maintainer might plausibly ship as a stopgap to appease users).

clang/test/Driver/sigpipe-handling.c

+// REQUIRES: shell
+// RUN: %clang -E -fno-integrated-cc1 %s | head | FileCheck %s
+
+// Test that the parent clang driver process doesn't crash when the child cc1
+// process receives a SIGPIPE (Unix-only).
+//
+// The child should exit with IO_ERR, and the parent should exit cleanly.
+
+// CHECK: sigpipe-handling.c

llvm/include/llvm/Support/InitLLVM.h

 //===- InitLLVM.h -----------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #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"
 
 // The main() functions in typical LLVM tools start with InitLLVM which does
 // the following one-time initializations:
 //
 //  1. Setting up a signal handler so that pretty stack trace is printed out
InitLLVM(int &Argc, char **&Argv, bool InstallPipeSignalExitHandler = true)
       : InitLLVM(Argc, const_cast<const char **&>(Argv),
                  InstallPipeSignalExitHandler) {}
 
   ~InitLLVM();
 
 private:
   BumpPtrAllocator Alloc;
   SmallVector<const char *, 0> Args;
-  PrettyStackTraceProgram StackPrinter;
+  Optional<PrettyStackTraceProgram> StackPrinter;
 };
 } // namespace llvm
 
 #endif

llvm/lib/Support/InitLLVM.cpp

 #ifdef _WIN32
 #include "llvm/Support/Windows/WindowsSupport.h"
 #endif
 
 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();
 
 #ifdef _WIN32
   // We use UTF-8 as the internal character encoding. On Windows,
   // arguments passed to main() may not be encoded in UTF-8. In order
   // to reliably detect encoding of command line arguments, we use an
   // Windows API to obtain arguments, convert them to UTF-8, and then