[linux] Fix FileUtils::getContents with folder (cocos2d#19157)

* fix FileUtils::getContents on linux/mac

* use stat.st_mode

* simplify

Author: PatriceJiang

cocos/platform/CCFileUtils.cpp

@@ -669,20 +669,21 @@ FileUtils::Status FileUtils::getContents(const std::string& filename, ResizableB
     if (fullPath.empty())
         return Status::NotExists;
 
-    FILE *fp = fopen(fs->getSuitableFOpen(fullPath).c_str(), "rb");
-    if (!fp)
-        return Status::OpenFailed;
+    std::string suitableFullPath = fs->getSuitableFOpen(fullPath);
 
-#if defined(_MSC_VER)
-    auto descriptor = _fileno(fp);
-#else
-    auto descriptor = fileno(fp);
-#endif
     struct stat statBuf;
-    if (fstat(descriptor, &statBuf) == -1) {
-        fclose(fp);
+    if (stat(suitableFullPath.c_str(), &statBuf) == -1) {
         return Status::ReadFailed;
     }
+
+    if (!(statBuf.st_mode & S_IFREG)) { 
+        return Status::NotRegularFileType;
+    }
+
+    FILE *fp = fopen(suitableFullPath.c_str(), "rb");
+    if (!fp)
+        return Status::OpenFailed;
+
     size_t size = statBuf.st_size;
 
     buffer->resize(size);

cocos/platform/CCFileUtils.h

@@ -197,7 +197,8 @@ class CC_DLL FileUtils
         ReadFailed = 3, // Read failed
         NotInitialized = 4, // FileUtils is not initializes
         TooLarge = 5, // The file is too large (great than 2^32-1)
-        ObtainSizeFailed = 6 // Failed to obtain the file size.
+        ObtainSizeFailed = 6, // Failed to obtain the file size.
+        NotRegularFileType = 7 // File type is not S_IFREG
     };
 
     /**