Add error reporting mechanism for failing Unwind.

Remove the logging of an error if a thread disappears before the unwind
can begin. This can happen, so allow the caller to determine if this
is really a problem worth logging.

Bug: 27449879
Change-Id: Ie81718d53fb0e519fa0a7db9fd5f314b72bfa431
diff --git a/libbacktrace/backtrace_test.cpp b/libbacktrace/backtrace_test.cpp
index ab09564..df6c6c1 100644
--- a/libbacktrace/backtrace_test.cpp
+++ b/libbacktrace/backtrace_test.cpp
@@ -162,6 +162,7 @@
       Backtrace::Create(BACKTRACE_CURRENT_PROCESS, BACKTRACE_CURRENT_THREAD));
   ASSERT_TRUE(backtrace.get() != nullptr);
   ASSERT_TRUE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
   VerifyLevelDump(backtrace.get());
 }
@@ -183,6 +184,7 @@
       Backtrace::Create(BACKTRACE_CURRENT_PROCESS, BACKTRACE_CURRENT_THREAD));
   ASSERT_TRUE(backtrace.get() != nullptr);
   ASSERT_TRUE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
   VerifyMaxDump(backtrace.get());
 }
@@ -200,6 +202,7 @@
   std::unique_ptr<Backtrace> backtrace(Backtrace::Create(getpid(), tid));
   ASSERT_TRUE(backtrace.get() != nullptr);
   ASSERT_TRUE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
   VerifyFunc(backtrace.get());
 }
@@ -220,6 +223,7 @@
   std::unique_ptr<Backtrace> backtrace(Backtrace::Create(getpid(), getpid()));
   ASSERT_TRUE(backtrace.get() != nullptr);
   ASSERT_TRUE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
   ASSERT_TRUE(backtrace->NumFrames() != 0);
   for (const auto& frame : *backtrace ) {
@@ -267,16 +271,19 @@
       Backtrace::Create(BACKTRACE_CURRENT_PROCESS, BACKTRACE_CURRENT_THREAD));
   ASSERT_TRUE(all.get() != nullptr);
   ASSERT_TRUE(all->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, all->GetError());
 
   std::unique_ptr<Backtrace> ign1(
       Backtrace::Create(BACKTRACE_CURRENT_PROCESS, BACKTRACE_CURRENT_THREAD));
   ASSERT_TRUE(ign1.get() != nullptr);
   ASSERT_TRUE(ign1->Unwind(1));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, ign1->GetError());
 
   std::unique_ptr<Backtrace> ign2(
       Backtrace::Create(BACKTRACE_CURRENT_PROCESS, BACKTRACE_CURRENT_THREAD));
   ASSERT_TRUE(ign2.get() != nullptr);
   ASSERT_TRUE(ign2->Unwind(2));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, ign2->GetError());
 
   VerifyIgnoreFrames(all.get(), ign1.get(), ign2.get(), "VerifyLevelIgnoreFrames");
 }
@@ -314,6 +321,7 @@
       std::unique_ptr<Backtrace> backtrace(Backtrace::Create(pid, tid, map.get()));
       ASSERT_TRUE(backtrace.get() != nullptr);
       ASSERT_TRUE(backtrace->Unwind(0));
+      ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
       if (ReadyFunc(backtrace.get())) {
         VerifyFunc(backtrace.get());
         verified = true;
@@ -372,10 +380,12 @@
   std::unique_ptr<Backtrace> ign1(Backtrace::Create(bt_all->Pid(), BACKTRACE_CURRENT_THREAD));
   ASSERT_TRUE(ign1.get() != nullptr);
   ASSERT_TRUE(ign1->Unwind(1));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, ign1->GetError());
 
   std::unique_ptr<Backtrace> ign2(Backtrace::Create(bt_all->Pid(), BACKTRACE_CURRENT_THREAD));
   ASSERT_TRUE(ign2.get() != nullptr);
   ASSERT_TRUE(ign2->Unwind(2));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, ign2->GetError());
 
   VerifyIgnoreFrames(bt_all, ign1.get(), ign2.get(), nullptr);
 }
@@ -462,6 +472,7 @@
   std::unique_ptr<Backtrace> backtrace(Backtrace::Create(getpid(), gettid()));
   ASSERT_TRUE(backtrace.get() != nullptr);
   ASSERT_TRUE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
   VerifyLevelDump(backtrace.get());
 }
@@ -474,6 +485,7 @@
   std::unique_ptr<Backtrace> backtrace(Backtrace::Create(getpid(), gettid()));
   ASSERT_TRUE(backtrace.get() != nullptr);
   ASSERT_TRUE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
   VerifyMaxDump(backtrace.get());
 }
@@ -515,6 +527,7 @@
   std::unique_ptr<Backtrace> backtrace(Backtrace::Create(getpid(), thread_data.tid));
   ASSERT_TRUE(backtrace.get() != nullptr);
   ASSERT_TRUE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
   VerifyLevelDump(backtrace.get());
 
@@ -554,14 +567,17 @@
   std::unique_ptr<Backtrace> all(Backtrace::Create(getpid(), thread_data.tid));
   ASSERT_TRUE(all.get() != nullptr);
   ASSERT_TRUE(all->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, all->GetError());
 
   std::unique_ptr<Backtrace> ign1(Backtrace::Create(getpid(), thread_data.tid));
   ASSERT_TRUE(ign1.get() != nullptr);
   ASSERT_TRUE(ign1->Unwind(1));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, ign1->GetError());
 
   std::unique_ptr<Backtrace> ign2(Backtrace::Create(getpid(), thread_data.tid));
   ASSERT_TRUE(ign2.get() != nullptr);
   ASSERT_TRUE(ign2->Unwind(2));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, ign2->GetError());
 
   VerifyIgnoreFrames(all.get(), ign1.get(), ign2.get(), nullptr);
 
@@ -592,6 +608,7 @@
   std::unique_ptr<Backtrace> backtrace(Backtrace::Create(getpid(), thread_data.tid));
   ASSERT_TRUE(backtrace.get() != nullptr);
   ASSERT_TRUE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
   VerifyMaxDump(backtrace.get());
 
@@ -718,18 +735,21 @@
   Backtrace* back1 = Backtrace::Create(getpid(), BACKTRACE_CURRENT_THREAD, map1);
   ASSERT_TRUE(back1 != nullptr);
   EXPECT_TRUE(back1->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, back1->GetError());
   delete back1;
   delete map1;
 
   Backtrace* back2 = Backtrace::Create(getpid(), BACKTRACE_CURRENT_THREAD, map2);
   ASSERT_TRUE(back2 != nullptr);
   EXPECT_TRUE(back2->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, back2->GetError());
   delete back2;
   delete map2;
 
   Backtrace* back3 = Backtrace::Create(getpid(), BACKTRACE_CURRENT_THREAD, map3);
   ASSERT_TRUE(back3 != nullptr);
   EXPECT_TRUE(back3->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, back3->GetError());
   delete back3;
   delete map3;
 }
@@ -1308,6 +1328,7 @@
                                                          BACKTRACE_CURRENT_THREAD));
   ASSERT_TRUE(backtrace.get() != nullptr);
   ASSERT_TRUE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
   size_t frame_num;
   ASSERT_TRUE(FindFuncFrameInBacktrace(backtrace.get(), test_func, &frame_num));
@@ -1364,6 +1385,7 @@
     std::unique_ptr<Backtrace> backtrace(Backtrace::Create(pid, BACKTRACE_CURRENT_THREAD));
     ASSERT_TRUE(backtrace.get() != nullptr);
     ASSERT_TRUE(backtrace->Unwind(0));
+    ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
 
     size_t frame_num;
     if (FindFuncFrameInBacktrace(backtrace.get(),
@@ -1387,6 +1409,14 @@
   ASSERT_TRUE(done) << "Test function never found in unwind.";
 }
 
+TEST(libbacktrace, unwind_thread_doesnt_exist) {
+  std::unique_ptr<Backtrace> backtrace(
+      Backtrace::Create(BACKTRACE_CURRENT_PROCESS, 99999999));
+  ASSERT_TRUE(backtrace.get() != nullptr);
+  ASSERT_FALSE(backtrace->Unwind(0));
+  ASSERT_EQ(BACKTRACE_UNWIND_ERROR_THREAD_DOESNT_EXIST, backtrace->GetError());
+}
+
 #if defined(ENABLE_PSS_TESTS)
 #include "GetPss.h"
 
@@ -1398,6 +1428,7 @@
     Backtrace* backtrace = Backtrace::Create(pid, tid);
     ASSERT_TRUE(backtrace != nullptr);
     ASSERT_TRUE(backtrace->Unwind(0));
+    ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
     delete backtrace;
   }
   size_t stable_pss = GetPssBytes();
@@ -1408,6 +1439,7 @@
     Backtrace* backtrace = Backtrace::Create(pid, tid);
     ASSERT_TRUE(backtrace != nullptr);
     ASSERT_TRUE(backtrace->Unwind(0));
+    ASSERT_EQ(BACKTRACE_UNWIND_NO_ERROR, backtrace->GetError());
     delete backtrace;
   }
   size_t new_pss = GetPssBytes();