Fixed the problem of pathjoin function splicing paths when basepath is the root directory. (#2118)

* fix path join

Author: linrunqi08

Diff for: core/common/FileSystemUtil.h

@@ -41,15 +41,10 @@ extern const std::string PATH_SEPARATOR;
 // PathJoin concats base and sub (by adding necessary path separator).
 // NOTE: the implementation is not elegant for better performance (backward).
 inline std::string PathJoin(const std::string& base, const std::string& sub) {
-    // Only Windows can collect root path, so linux do as old way.
-#if defined(__linux__)
-    return base + PATH_SEPARATOR + sub;
-#elif defined(_MSC_VER)
-    if (!BOOL_FLAG(enable_root_path_collection) || base.back() != PATH_SEPARATOR[0]) {
+    if (!base.empty() && base.back() != PATH_SEPARATOR[0]) {
         return base + PATH_SEPARATOR + sub;
     }
     return base + sub;
-#endif
 }
 
 std::string ParentPath(const std::string& path);

Diff for: core/unittest/common/FileSystemUtilUnittest.h

@@ -453,4 +453,27 @@ TEST_F(FileSystemUtilUnittest, TestGetFdPath) {
     fclose(file);
 }
 
+TEST_F(FileSystemUtilUnittest, TestPathJoin) {
+    std::string filePath;
+#if defined(_MSC_VER)
+    filePath = PathJoin("D:\\", "dataA");
+    EXPECT_EQ(filePath, "D:\\dataA");
+
+    filePath = PathJoin("D:\\xx", "dataA");
+    EXPECT_EQ(filePath, "D:\\xx\\dataA");
+
+    filePath = PathJoin("D:\\xx\\", "dataA");
+    EXPECT_EQ(filePath, "D:\\xx\\dataA");
+#else
+    filePath = PathJoin("/", "dataA");
+    EXPECT_EQ(filePath, "/dataA");
+
+    filePath = PathJoin("/xx", "dataA");
+    EXPECT_EQ(filePath, "/xx/dataA");
+
+    filePath = PathJoin("/xx/", "dataA");
+    EXPECT_EQ(filePath, "/xx/dataA");
+#endif
+}
+
 } // namespace logtail