[Signal] Allow llvm clients to opt into one-shot SIGPIPE handling

Allow clients of the llvm library to opt-in to one-shot SIGPIPE
handling, instead of forcing them to undo llvm's SIGPIPE handler
registration (which is brittle).

The current behavior is preserved for all llvm-derived tools (except
lldb) by means of a default-`true` flag in the InitLLVM constructor.

This prevents "IO error" crashes in long-lived processes (lldb is the
motivating example) which both a) load llvm as a dynamic library and b)
*really* need to ignore SIGPIPE.

As llvm signal handlers can be installed when calling into libclang
(say, via RemoveFileOnSignal), thereby overriding a previous SIG_IGN for
SIGPIPE, there is no clean way to opt-out of "exit-on-SIGPIPE" in the
current model.

Differential Revision: https://reviews.llvm.org/D70277

3 files changed, 32 insertions, 5 deletions

diff --git a/llvm/lib/Support/InitLLVM.cpp b/llvm/lib/Support/InitLLVM.cpp
index 0d7d7fcc8cb..bb9b569d2de 100644
--- a/llvm/lib/Support/InitLLVM.cpp
+++ b/llvm/lib/Support/InitLLVM.cpp
@@ -21,7 +21,11 @@
 using namespace llvm;
 using namespace llvm::sys;
 
-InitLLVM::InitLLVM(int &Argc, const char **&Argv) : StackPrinter(Argc, Argv) {
+InitLLVM::InitLLVM(int &Argc, const char **&Argv,
+                   bool InstallPipeSignalExitHandler)
+    : StackPrinter(Argc, Argv) {
+  if (InstallPipeSignalExitHandler)
+    sys::SetOneShotPipeSignalFunction(sys::DefaultOneShotPipeSignalHandler);
   sys::PrintStackTraceOnErrorSignal(Argv[0]);
   install_out_of_memory_new_handler();
 
diff --git a/llvm/lib/Support/Unix/Signals.inc b/llvm/lib/Support/Unix/Signals.inc
index be05eabfb2e..8c26fa9b8f2 100644
--- a/llvm/lib/Support/Unix/Signals.inc
+++ b/llvm/lib/Support/Unix/Signals.inc
@@ -88,6 +88,9 @@ static std::atomic<SignalHandlerFunctionType> InterruptFunction =
     ATOMIC_VAR_INIT(nullptr);
 static std::atomic<SignalHandlerFunctionType> InfoSignalFunction =
     ATOMIC_VAR_INIT(nullptr);
+/// The function to call on SIGPIPE (one-time use only).
+static std::atomic<SignalHandlerFunctionType> OneShotPipeSignalFunction =
+    ATOMIC_VAR_INIT(nullptr);
 
 namespace {
 /// Signal-safe removal of files.
@@ -206,7 +209,7 @@ static StringRef Argv0;
 /// if there is, it's not our direct responsibility. For whatever reason, our
 /// continued execution is no longer desirable.
 static const int IntSigs[] = {
-  SIGHUP, SIGINT, SIGPIPE, SIGTERM, SIGUSR2
+  SIGHUP, SIGINT, SIGTERM, SIGUSR2
 };
 
 /// Signals that represent that we have a bug, and our prompt termination has
@@ -237,7 +240,7 @@ static const int InfoSigs[] = {
 
 static const size_t NumSigs =
     array_lengthof(IntSigs) + array_lengthof(KillSigs) +
-    array_lengthof(InfoSigs);
+    array_lengthof(InfoSigs) + 1 /* SIGPIPE */;
 
 
 static std::atomic<unsigned> NumRegisteredSignals = ATOMIC_VAR_INIT(0);
@@ -322,6 +325,8 @@ static void RegisterHandlers() { // Not signal-safe.
     registerHandler(S, SignalKind::IsKill);
   for (auto S : KillSigs)
     registerHandler(S, SignalKind::IsKill);
+  if (OneShotPipeSignalFunction)
+    registerHandler(SIGPIPE, SignalKind::IsKill);
   for (auto S : InfoSigs)
     registerHandler(S, SignalKind::IsInfo);
 }
@@ -361,9 +366,10 @@ static RETSIGTYPE SignalHandler(int Sig) {
       if (auto OldInterruptFunction = InterruptFunction.exchange(nullptr))
         return OldInterruptFunction();
 
-      // Send a special return code that drivers can check for, from sysexits.h.
       if (Sig == SIGPIPE)
-        exit(EX_IOERR);
+        if (auto OldOneShotPipeFunction =
+                OneShotPipeSignalFunction.exchange(nullptr))
+          return OldOneShotPipeFunction();
 
       raise(Sig);   // Execute the default handler.
       return;
@@ -403,6 +409,16 @@ void llvm::sys::SetInfoSignalFunction(void (*Handler)()) {
   RegisterHandlers();
 }
 
+void llvm::sys::SetOneShotPipeSignalFunction(void (*Handler)()) {
+  OneShotPipeSignalFunction.exchange(Handler);
+  RegisterHandlers();
+}
+
+void llvm::sys::DefaultOneShotPipeSignalHandler() {
+  // Send a special return code that drivers can check for, from sysexits.h.
+  exit(EX_IOERR);
+}
+
 // The public API
 bool llvm::sys::RemoveFileOnSignal(StringRef Filename,
                                    std::string* ErrMsg) {
diff --git a/llvm/lib/Support/Windows/Signals.inc b/llvm/lib/Support/Windows/Signals.inc
index 6a820ef22b1..08a3616427b 100644
--- a/llvm/lib/Support/Windows/Signals.inc
+++ b/llvm/lib/Support/Windows/Signals.inc
@@ -560,6 +560,13 @@ void llvm::sys::SetInfoSignalFunction(void (*Handler)()) {
   // Unimplemented.
 }
 
+void llvm::sys::SetOneShotPipeSignalFunction(void (*Handler)()) {
+  // Unimplemented.
+}
+
+void llvm::sys::DefaultOneShotPipeSignalHandler() {
+  // 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