Fix: Add cxxabi fallback to buffer-based demangle function

folly/Demangle.cpp

@@ -256,6 +256,19 @@ size_t demangle(const char* name, char* out, size_t outSize) {
     }
   }
 
+  // fallback to cxxabi if available
+  if (cxxabi_demangle) {
+    int status;
+    size_t len = outSize;
+    // Use the provided buffer instead of allocating
+    char* demangled = cxxabi_demangle(name, out, &len, &status);
+    if (status == 0 && demangled == out) {
+      // Successfully demangled into the provided buffer
+      return strlen(out);
+    }
+    // If demangling failed or allocated (shouldn't happen), fall through
+  }
+
   // fallback - just return original
   return folly::strlcpy(out, name, outSize);
 }

folly/Demangle.h

@@ -59,11 +59,10 @@ inline fbstring demangle(const std::type_info& type) {
  *
  * This function does not allocate memory and is async-signal-safe.
  *
- * Note that the underlying function for the fbstring-returning demangle is
- * somewhat standard (abi::__cxa_demangle, which uses malloc), the underlying
- * function for this version is less so (cplus_demangle_v3_callback from
- * libiberty), so it is possible for the fbstring version to work, while this
- * version returns the original, mangled name.
+ * This function tries libiberty first (cplus_demangle_v3_callback), then falls
+ * back to cxxabi (abi::__cxa_demangle with the provided buffer) if libiberty
+ * is not available. If neither is available, it returns the original mangled
+ * name.
  */
 size_t demangle(const char* name, char* out, size_t outSize);
 inline size_t demangle(const std::type_info& type, char* buf, size_t bufSize) {

folly/test/DemangleTest.cpp

@@ -41,8 +41,10 @@ TEST_F(DemangleTest, demangle_return_string) {
 TEST_F(DemangleTest, demangle_to_buffer) {
   using type = folly_test::ThisIsAVeryLongStructureName;
   auto const raw = typeid(type).name();
-  auto const expected =
-      demangle_build_has_liberty() ? pretty_name<type>() : raw;
+  // Buffer-based demangle now falls back to cxxabi if liberty is not available
+  auto const expected = (demangle_build_has_liberty() || demangle_build_has_cxxabi())
+      ? pretty_name<type>()
+      : raw;
 
   {
     std::vector<char> buf;
@@ -69,7 +71,10 @@ TEST_F(DemangleTest, demangle_long_symbol) {
 
   EXPECT_EQ(std::string(choice), demangle(raw).toStdString());
 
-  auto const expected = demangle_build_has_liberty() ? choice : raw;
+  // Buffer-based demangle now falls back to cxxabi if liberty is not available
+  auto const expected =
+      (demangle_build_has_liberty() || demangle_build_has_cxxabi()) ? choice
+                                                                     : raw;
   constexpr size_t size = 15;
   std::vector<char> buf;
   buf.resize(1 + size);