When printing a stack trace, demangle it if possible. Since we are potentially

in a signal handler, allocating memory or doing other unsafe things is bad,
which means we should do it in a different process.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11689 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Signals.cpp

@@ -23,6 +23,8 @@
 #endif
 #include <signal.h>
 #include <unistd.h>
+#include <sys/wait.h>
+#include <cerrno>
 using namespace llvm;
 
 static std::vector<std::string> FilesToRemove;
@@ -43,7 +45,65 @@ static const int KillSigs[] = {
 };
 static const int *KillSigsEnd = KillSigs + sizeof(KillSigs)/sizeof(KillSigs[0]);
 
+#ifdef HAVE_BACKTRACE
 static void* StackTrace[256];
+#endif
+
+
+// PrintStackTrace - In the case of a program crash or fault, print out a stack
+// trace so that the user has an indication of why and where we died.
+//
+// On glibc systems we have the 'backtrace' function, which works nicely, but
+// doesn't demangle symbols.  In order to backtrace symbols, we fork and exec a
+// 'c++filt' process to do the demangling.  This seems like the simplest and
+// most robust solution when we can't allocate memory (such as in a signal
+// handler).  If we can't find 'c++filt', we fallback to printing mangled names.
+//
+static void PrintStackTrace() {
+#ifdef HAVE_BACKTRACE
+  // Use backtrace() to output a backtrace on Linux systems with glibc.
+  int depth = backtrace(StackTrace, sizeof(StackTrace)/sizeof(StackTrace[0]));
+  
+  // Create a one-way unix pipe.  The backtracing process writes to PipeFDs[1],
+  // the c++filt process reads from PipeFDs[0].
+  int PipeFDs[2];
+  if (pipe(PipeFDs)) {
+    backtrace_symbols_fd(StackTrace, depth, STDERR_FILENO);
+    return;
+  }
+
+  switch (pid_t ChildPID = fork()) {
+  case -1:        // Error forking, print mangled stack trace
+    close(PipeFDs[0]);
+    close(PipeFDs[1]);
+    backtrace_symbols_fd(StackTrace, depth, STDERR_FILENO);
+    return;
+  default:        // backtracing process
+    close(PipeFDs[0]);  // Close the reader side.
+
+    // Print the mangled backtrace into the pipe.
+    backtrace_symbols_fd(StackTrace, depth, PipeFDs[1]);
+    close(PipeFDs[1]);   // We are done writing.
+    while (waitpid(ChildPID, 0, 0) == -1)
+      if (errno != EINTR) break;
+    return;
+
+  case 0:         // c++filt process
+    close(PipeFDs[1]);    // Close the writer side.
+    dup2(PipeFDs[0], 0);  // Read from standard input
+    close(PipeFDs[0]);    // Close the old descriptor
+    dup2(2, 1);           // Revector stdout -> stderr
+
+    // Try to run c++filt or gc++filt.  If neither is found, call back on 'cat'
+    // to print the mangled stack trace.  If we can't find cat, just exit.
+    execlp("c++filt", "c++filt", 0);
+    execlp("gc++filt", "gc++filt", 0);
+    execlp("cat", "cat", 0);
+    execlp("/usr/bin/cat", "cat", 0);
+    exit(0);
+  }
+#endif
+}
 
 // SignalHandler - The signal handler that runs...
 static RETSIGTYPE SignalHandler(int Sig) {
@@ -57,11 +117,7 @@ static RETSIGTYPE SignalHandler(int Sig) {
 
   // Otherwise if it is a fault (like SEGV) output the stacktrace to
   // STDERR (if we can) and reissue the signal to die...
-#ifdef HAVE_BACKTRACE
+  PrintStackTrace();
-  // Use backtrace() to output a backtrace on Linux systems with glibc.
-  int depth = backtrace(StackTrace, sizeof(StackTrace)/sizeof(StackTrace[0]));
-  backtrace_symbols_fd(StackTrace, depth, STDERR_FILENO);
-#endif
   signal(Sig, SIG_DFL);
 }