Disable exit-on-SIGPIPE in lldb

Occasionally, during test teardown, LLDB writes to a closed pipe.
Sometimes the communication is inherently unreliable, so LLDB tries to
avoid being killed due to SIGPIPE (it calls `signal(SIGPIPE, SIG_IGN)`).
However, LLVM's default SIGPIPE behavior overrides LLDB's, causing it to
exit with IO_ERR.

Opt LLDB out of the default SIGPIPE behavior. I expect that this will
resolve some LLDB test suite flakiness (tests randomly failing with
IO_ERR) that we've seen since r344372.

rdar://55750240

Differential Revision: https://reviews.llvm.org/D69148

llvm-svn: 375288

lldb/tools/driver/Driver.cpp

@@ -853,6 +853,16 @@ int main(int argc, char const *argv[])
   signal(SIGCONT, sigcont_handler);
 #endif
 
+  // Occasionally, during test teardown, LLDB writes to a closed pipe.
+  // Sometimes the communication is inherently unreliable, so LLDB tries to
+  // avoid being killed due to SIGPIPE. However, LLVM's default SIGPIPE behavior
+  // is to exit with IO_ERR. Opt LLDB out of that.
+  //
+  // We don't disable LLVM's signal handling entirely because we still want
+  // pretty stack traces, and file cleanup (for when, say, the clang embedded
+  // in LLDB leaves behind temporary objects).
+  llvm::sys::SetPipeSignalFunction(nullptr);
+
   int exit_code = 0;
   // Create a scope for driver so that the driver object will destroy itself
   // before SBDebugger::Terminate() is called.

llvm/include/llvm/Support/Signals.h

@@ -84,6 +84,17 @@ namespace sys {
   /// function.  Note also that the handler may be executed on a different
   /// thread on some platforms.
   void SetInfoSignalFunction(void (*Handler)());
+
+  /// Registers a function to be called when a "pipe" signal is delivered to
+  /// the process.
+  ///
+  /// The "pipe" signal typically indicates a failed write to a pipe (SIGPIPE).
+  /// The default installed handler calls `exit(EX_IOERR)`, causing the process
+  /// to immediately exit with an IO error exit code.
+  ///
+  /// This function is only applicable on POSIX systems.
+  void SetPipeSignalFunction(void (*Handler)());
+
 } // End sys namespace
 } // End llvm namespace
 

llvm/lib/Support/Unix/Signals.inc

@@ -82,12 +82,18 @@ using namespace llvm;
 static RETSIGTYPE SignalHandler(int Sig);  // defined below.
 static RETSIGTYPE InfoSignalHandler(int Sig);  // defined below.
 
+static void DefaultPipeSignalFunction() {
+  exit(EX_IOERR);
+}
+
 using SignalHandlerFunctionType = void (*)();
 /// The function to call if ctrl-c is pressed.
 static std::atomic<SignalHandlerFunctionType> InterruptFunction =
     ATOMIC_VAR_INIT(nullptr);
 static std::atomic<SignalHandlerFunctionType> InfoSignalFunction =
     ATOMIC_VAR_INIT(nullptr);
+static std::atomic<SignalHandlerFunctionType> PipeSignalFunction =
+    ATOMIC_VAR_INIT(DefaultPipeSignalFunction);
 
 namespace {
 /// Signal-safe removal of files.
@@ -363,7 +369,8 @@ static RETSIGTYPE SignalHandler(int Sig) {
 
       // Send a special return code that drivers can check for, from sysexits.h.
       if (Sig == SIGPIPE)
-        exit(EX_IOERR);
+        if (SignalHandlerFunctionType CurrentPipeFunction = PipeSignalFunction)
+          CurrentPipeFunction();
 
       raise(Sig);   // Execute the default handler.
       return;
@@ -403,6 +410,11 @@ void llvm::sys::SetInfoSignalFunction(void (*Handler)()) {
   RegisterHandlers();
 }
 
+void llvm::sys::SetPipeSignalFunction(void (*Handler)()) {
+  PipeSignalFunction.exchange(Handler);
+  RegisterHandlers();
+}
+
 // The public API
 bool llvm::sys::RemoveFileOnSignal(StringRef Filename,
                                    std::string* ErrMsg) {

llvm/lib/Support/Windows/Signals.inc

@@ -560,6 +560,9 @@ void llvm::sys::SetInfoSignalFunction(void (*Handler)()) {
   // Unimplemented.
 }
 
+void llvm::sys::SetPipeSignalFunction(void (*Handler)()) {
+  // Unimplemented.
+}
 
 /// Add a function to be called when a signal is delivered to the process. The
 /// handler can have a cookie passed to it to identify what instance of the

llvm/unittests/Support/CMakeLists.txt

@@ -58,6 +58,7 @@ add_llvm_unittest(SupportTests
   ReverseIterationTest.cpp
   ReplaceFileTest.cpp
   ScaledNumberTest.cpp
+  SignalsTest.cpp
   SourceMgrTest.cpp
   SpecialCaseListTest.cpp
   StringPool.cpp

llvm/unittests/Support/SignalsTest.cpp

@@ -0,0 +1,53 @@
+//========- unittests/Support/SignalsTest.cpp - Signal handling test =========//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#if !defined(_WIN32)
+#include <unistd.h>
+#include <sysexits.h>
+#endif // !defined(_WIN32)
+
+#include "llvm/Support/Signals.h"
+
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+#if !defined(_WIN32)
+TEST(SignalTest, IgnoreMultipleSIGPIPEs) {
+  // Ignore SIGPIPE.
+  signal(SIGPIPE, SIG_IGN);
+
+  // 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. 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);
+}
+#endif // !defined(_WIN32)