ok, backtrace support on Android

This adds a fallback backtracer for use on Android where <execinfo.h>
ins't present, instead using <unwind.h> to unwind and <dlfcn.h> to
lookup function names and addresses.

lockf() wasn't available until NDK API 24, so I've just no-op'd file
locking on older targets.  I tried switching from lockf() to flock(),
but flock() didn't see to _do_ anything, neither on Android nor on my
Mac laptop.  I think I should be able to use the lower-level fcntl()
APIs to restore file locking uniformly in a follow-up.  The upshot is
until then, we'll have interlaced logs and stack traces on Android
devices unless you set ndk_api=24 in GN.

We need to add a couple build flags to make backtraces useful:

   * -funwind-tables makes the call to _Unwind_Backtrace() actually
     traverse the call stack.  This is a small extra binary size cost.
   * -rdynamic makes symbols linked into the main executable visible
     to dladdr().  We do this on Linux already for the same reason.

Here's an example where I made aaxfermodes call SK_ABORT():

    650 ok, 1 crashed
    caught signal SIGABRT while running 'aaxfermodes'
        0x76ed936288 [unknown]+308
        0x76eec014e0 [unknown]+510811706592
        0x76ed367b2c tgkill+8
        0x76ed364f50 pthread_kill+68
        0x76ed31ff5c raise+28
        0x76ed318814 abort+56
        0x76edebd070 sk_out_of_memory()+12
        0x76ed99f664 AAXfermodesGM::draw_pass(SkCanvas*, AAXfermodesGM::DrawingPass)+96
        0x76ed99f4e4 AAXfermodesGM::onDraw(SkCanvas*)+36
        0x76ed9e8550 skiagm::GM::drawContent(SkCanvas*)+224
        0x76ed9e82ac skiagm::GM::draw(SkCanvas*)+288
        0x76ed93b10c GMStream::GMSrc::draw(SkCanvas*)+96
        0x76ed937b08 SWDst::draw(Src*)+284
        0x76ed936ca0 [unknown]+112
        0x76ed939b4c ForkEngine::spawn(std::function<Status ()>)+88
        0x76ed934d00 main+2200
        0x76ed316598 __libc_init+92
        0x76ed93434c [unknown]+510791992140

Change-Id: Ica4849d99a3b97f48d778f4c15a7fa36275b8133
Reviewed-on: https://skia-review.googlesource.com/40802
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 7be1617d23..b2013c571b 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -387,7 +387,10 @@ config("debug_symbols") {
   # It's annoying to wait for full debug symbols to push over
   # to Android devices.  -gline-tables-only is a lot slimmer.
   if (is_android) {
-    cflags = [ "-gline-tables-only" ]
+    cflags = [
+      "-gline-tables-only",
+      "-funwind-tables",  # Helps make in-process backtraces fuller.
+    ]
   } else if (is_win) {
     cflags = [ "/Zi" ]
     ldflags = [ "/DEBUG" ]
@@ -434,7 +437,10 @@ config("release") {
 
 config("executable") {
   if (is_android) {
-    ldflags = [ "-pie" ]
+    ldflags = [
+      "-pie",
+      "-rdynamic",
+    ]
   } else if (is_mac) {
     ldflags = [ "-Wl,-rpath,@loader_path/." ]
   } else if (is_linux) {