Add basic stacktrace handler using libunwind.

tools/CrashHandler.cpp

+#include "CrashHandler.h"
+
+#include "SkTypes.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+
+#if defined(SK_BUILD_FOR_MAC)
+
+// We only use local unwinding, so we can define this to select a faster implementation.
+#define UNW_LOCAL_ONLY
+#include <libunwind.h>
+#include <cxxabi.h>
+
+static void handler(int sig) {
+    unw_context_t context;
+    unw_getcontext(&context);
+
+    unw_cursor_t cursor;
+    unw_init_local(&cursor, &context);
+
+    fprintf(stderr, "\nSignal %d:\n", sig);
+    while (unw_step(&cursor) > 0) {
+        static const size_t kMax = 256;
+        char mangled[kMax], demangled[kMax];
+        unw_word_t offset;
+        unw_get_proc_name(&cursor, mangled, kMax, &offset);
+
+        int ok;
+        size_t len = kMax;
+        abi::__cxa_demangle(mangled, demangled, &len, &ok);
+
+        fprintf(stderr, "%s (+0x%zx)\n", ok == 0 ? demangled : mangled, (size_t)offset);
+    }
+    fprintf(stderr, "\n");
+
+    // Exit NOW.  Don't notify other threads, don't call anything registered with atexit().
+    _Exit(sig);
+}
+
+#elif defined(SK_BUILD_FOR_UNIX)
+
+// We'd use libunwind here too, but it's a pain to get installed for both 32 and 64 bit on bots.
+// Doesn't matter much: catchsegv is best anyway.
+#include <execinfo.h>
+
+static void handler(int sig) {
+    static const int kMax = 64;
+    void* stack[kMax];
+    const int count = backtrace(stack, kMax);
+
+    fprintf(stderr, "\nSignal %d:\n", sig);
+    backtrace_symbols_fd(stack, count, 2/*stderr*/);
+
+    // Exit NOW.  Don't notify other threads, don't call anything registered with atexit().
+    _Exit(sig);
+}
+
+#endif
+
+#if defined(SK_BUILD_FOR_UNIX) || defined(SK_BUILD_FOR_MAC)
+
+void SetupCrashHandler() {
+    static const int kSignals[] = {
+        SIGABRT,
+        SIGBUS,
+        SIGFPE,
+        SIGILL,
+        SIGSEGV,
+    };
+
+    for (size_t i = 0; i < sizeof(kSignals) / sizeof(kSignals[0]); i++) {
+        // Register our signal handler unless something's already done so (e.g. catchsegv).
+        void (*prev)(int) = signal(kSignals[i], handler);
+        if (prev != SIG_DFL) {
+            signal(kSignals[i], prev);
+        }
+    }
+}
+
+// TODO: #elif defined(SK_BUILD_FOR_WIN) when I find a Windows machine to work from.
+
+#else
+
+void SetupCrashHandler() { }
+
+#endif