Allow trailing slashes in --proto_path mappings.

Patch by Kevin Ko <kevin.s.ko@gmail.com>.

CONTRIBUTORS.txt

@@ -33,3 +33,6 @@ Documentation:
 
 Maven packaging:
   Gregory Kick <gak@google.com>
+
+Non-Google patch contributors:
+  Kevin Ko <kevin.s.ko@gmail.com>

src/google/protobuf/compiler/importer.cc

@@ -280,10 +280,18 @@ static bool ApplyMapping(const string& filename,
       // Not an exact match.  Is the next character a '/'?  Otherwise,
       // this isn't actually a match at all.  E.g. the prefix "foo/bar"
       // does not match the filename "foo/barbaz".
+      int after_prefix_start = -1;
       if (filename[old_prefix.size()] == '/') {
+        after_prefix_start = old_prefix.size() + 1;
+      } else if (filename[old_prefix.size() - 1] == '/') {
+        // old_prefix is never empty, and canonicalized paths never have
+        // consecutive '/' characters.
+        after_prefix_start = old_prefix.size();
+      }
+      if (after_prefix_start != -1) {
         // Yep.  So the prefixes are directories and the filename is a file
         // inside them.
-        string after_prefix = filename.substr(old_prefix.size() + 1);
+        string after_prefix = filename.substr(after_prefix_start);
         if (ContainsParentReference(after_prefix)) {
           // We do not allow the file name to use "..".
           return false;

src/google/protobuf/compiler/importer_unittest.cc

@@ -492,6 +492,7 @@ TEST_F(DiskSourceTreeTest, DiskFileToVirtualFileCanonicalization) {
   source_tree_.MapPath("dir3", "./foo/bar/.");
   source_tree_.MapPath("dir4", ".");
   source_tree_.MapPath("", "/qux");
+  source_tree_.MapPath("dir5", "/quux/");
 
   string virtual_file;
   string shadowing_disk_file;
@@ -530,6 +531,12 @@ TEST_F(DiskSourceTreeTest, DiskFileToVirtualFileCanonicalization) {
     source_tree_.DiskFileToVirtualFile(
       "/qux/baz", &virtual_file, &shadowing_disk_file));
   EXPECT_EQ("baz", virtual_file);
+
+  // "/quux/bar" is under "/quux".
+  EXPECT_EQ(DiskSourceTree::CANNOT_OPEN,
+    source_tree_.DiskFileToVirtualFile(
+      "/quux/bar", &virtual_file, &shadowing_disk_file));
+  EXPECT_EQ("dir5/bar", virtual_file);
 }
 
 }  // namespace