1 files changed, 17 insertions, 3 deletions

diff --git a/src/google/protobuf/compiler/importer.h b/src/google/protobuf/compiler/importer.h
index 7a2efc2..0171d2f 100644
--- a/src/google/protobuf/compiler/importer.h
+++ b/src/google/protobuf/compiler/importer.h
@@ -166,6 +166,9 @@ class LIBPROTOBUF_EXPORT Importer {
     return &pool_;
   }
 
+  void AddUnusedImportTrackFile(const string& file_name);
+  void ClearUnusedImportTrackFiles();
+
  private:
   SourceTreeDescriptorDatabase database_;
   DescriptorPool pool_;
@@ -204,6 +207,13 @@ class LIBPROTOBUF_EXPORT SourceTree {
   // contain "." or ".." components.
   virtual io::ZeroCopyInputStream* Open(const string& filename) = 0;
 
+  // If Open() returns NULL, calling this method immediately will return an
+  // description of the error.
+  // Subclasses should implement this method and return a meaningful value for
+  // better error reporting.
+  // TODO(xiaofeng): change this to a pure virtual function.
+  virtual string GetLastErrorMessage();
+
  private:
   GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(SourceTree);
 };
@@ -273,17 +283,21 @@ class LIBPROTOBUF_EXPORT DiskSourceTree : public SourceTree {
   bool VirtualFileToDiskFile(const string& virtual_file, string* disk_file);
 
   // implements SourceTree -------------------------------------------
-  io::ZeroCopyInputStream* Open(const string& filename);
+  virtual io::ZeroCopyInputStream* Open(const string& filename);
+
+  virtual string GetLastErrorMessage();
 
  private:
   struct Mapping {
     string virtual_path;
     string disk_path;
 
-    inline Mapping(const string& virtual_path, const string& disk_path)
-      : virtual_path(virtual_path), disk_path(disk_path) {}
+    inline Mapping(const string& virtual_path_param,
+                   const string& disk_path_param)
+      : virtual_path(virtual_path_param), disk_path(disk_path_param) {}
   };
   vector<Mapping> mappings_;
+  string last_error_message_;
 
   // Like Open(), but returns the on-disk path in disk_file if disk_file is
   // non-NULL and the file could be successfully opened.