Fix segmentation fault in lldb_private::Symbols::LocateExecutableSymbolFile()

Summary:
When `module_spec.GetFileSpec().GetDirectory().AsCString()` returned a `nullptr` this line caused a segmentation fault:

`std::string module_directory = module_spec.GetFileSpec().GetDirectory().AsCString()`

Some context:
I was remote debugging an executable built with Clang in an Ubuntu VM on my Windows machine using lldb-mi. I copied the executable and nothing else from the Ubuntu VM to the Windows machine.

Then started lldb-server in the Ubuntu VM:

```
./bin/lldb-server gdbserver *:8888 -- /home/enlight/Projects/dbgmits/build/Debug/data_tests_target
```

And ran `lldb-mi --interpreter` on Windows with the following commands:

```
-file-exec-and-symbols C:\Projects\data_tests_target
-target-select remote 192.168.56.101:8888
-exec-continue

```

After which the segmentation fault occurred at the aforementioned line. Inside this method `module_spec.GetFileSpec()` returns an empty `FileSpec` (no dir, no filename), while `module_spec.GetSymbolFileSpec().GetFilename()` returns `"libc-2.19.so"`.

Patch thanks to Vadim Macagon.

Reviewers: brucem, zturner, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D13201

llvm-svn: 249387

lldb/source/Host/common/Symbols.cpp

@@ -249,10 +249,6 @@ Symbols::LocateExecutableSymbolFile (const ModuleSpec &module_spec)
             uuid_str = uuid_str + ".debug";
         }
 
-        // Get directory of our module. Needed to check debug files like this:
-        //   /usr/lib/debug/usr/lib/library.so.debug
-        std::string module_directory = module_spec.GetFileSpec().GetDirectory().AsCString();
-
         size_t num_directories = debug_file_search_paths.GetSize();
         for (size_t idx = 0; idx < num_directories; ++idx)
         {
@@ -267,7 +263,11 @@ Symbols::LocateExecutableSymbolFile (const ModuleSpec &module_spec)
             files.push_back (dirname + "/" + symbol_filename);
             files.push_back (dirname + "/.debug/" + symbol_filename);
             files.push_back (dirname + "/.build-id/" + uuid_str);
-            files.push_back (dirname + module_directory + "/" + symbol_filename);
+
+            // Some debug files may stored in the module directory like this:
+            //   /usr/lib/debug/usr/lib/library.so.debug
+            if (!file_dir.IsEmpty())
+                files.push_back (dirname + file_dir.AsCString() + "/" + symbol_filename);
 
             const uint32_t num_files = files.size();
             for (size_t idx_file = 0; idx_file < num_files; ++idx_file)

lldb/unittests/Host/CMakeLists.txt

@@ -1,4 +1,5 @@
 add_lldb_unittest(HostTests
   SocketAddressTest.cpp
   SocketTest.cpp
+  SymbolsTest.cpp
   )

lldb/unittests/Host/SymbolsTest.cpp

@@ -0,0 +1,30 @@
+//===-- SymbolsTest.cpp -----------------------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "gtest/gtest.h"
+#include "lldb/Host/Symbols.h"
+#include "lldb/Core/ModuleSpec.h"
+
+using namespace lldb_private;
+
+TEST(SymbolsTest, LocateExecutableSymbolFileForUnknownExecutableAndUnknownSymbolFile)
+{
+    ModuleSpec module_spec;
+    FileSpec symbol_file_spec = Symbols::LocateExecutableSymbolFile(module_spec);
+    EXPECT_TRUE(symbol_file_spec.GetFilename().IsEmpty());
+}
+
+TEST(SymbolsTest, LocateExecutableSymbolFileForUnknownExecutableAndMissingSymbolFile)
+{
+    ModuleSpec module_spec;
+    // using a GUID here because the symbol file shouldn't actually exist on disk
+    module_spec.GetSymbolFileSpec().SetFile("4A524676-B24B-4F4E-968A-551D465EBAF1.so", false);
+    FileSpec symbol_file_spec = Symbols::LocateExecutableSymbolFile(module_spec);
+    EXPECT_TRUE(symbol_file_spec.GetFilename().IsEmpty());
+}