Dump memory going from unreadable to readable.

If the first read when dumping memory returns no data, skip ahead to
the next page boundary and try and read from there. This fixes a case
where the address at which to start dumping memory is unreadable, but
crosses back into readable memory.

Bug: 22234753
Change-Id: Ie28d5c027013577ca06f5396aba498366a3b6749
diff --git a/debuggerd/test/BacktraceMock.h b/debuggerd/test/BacktraceMock.h
index 5c252ab..f75534e 100644
--- a/debuggerd/test/BacktraceMock.h
+++ b/debuggerd/test/BacktraceMock.h
@@ -60,12 +60,14 @@
     }
     size_t bytes_available = buffer_.size() - offset;
 
-    if (bytes_partial_read_ > 0) {
+    if (do_partial_read_) {
       // Do a partial read.
       if (bytes > bytes_partial_read_) {
         bytes = bytes_partial_read_;
       }
       bytes_partial_read_ -= bytes;
+      // Only support a single partial read.
+      do_partial_read_ = false;
     } else if (bytes > bytes_available) {
       bytes = bytes_available;
     }
@@ -82,6 +84,7 @@
     buffer_.resize(bytes);
     memcpy(buffer_.data(), buffer, bytes);
     bytes_partial_read_ = 0;
+    do_partial_read_ = false;
     last_read_addr_ = 0;
   }
 
@@ -90,12 +93,14 @@
       abort();
     }
     bytes_partial_read_ = bytes;
+    do_partial_read_ = true;
   }
 
  private:
   std::vector<uint8_t> buffer_;
   size_t bytes_partial_read_ = 0;
   uintptr_t last_read_addr_ = 0;
+  bool do_partial_read_ = false;
 };
 
 #endif //  _DEBUGGERD_TEST_BACKTRACE_MOCK_H