`String::ScrubName` AssertionError when called with `String::null()`

[felangel]

Expected Behavior

I would expect the following test to pass in debug mode:

ISOLATE_UNIT_TEST_CASE(String_ScrubName_Null) {
  const auto& null_string = String::Handle(String::null());
  EXPECT(null_string.IsNull());
  EXPECT_STREQ("String: null", null_string.ToCString());
  EXPECT_STREQ("String: null", String::ScrubName(null_string));
}

Actual Behavior

The test fails due to an assertion error

./xcodebuild/DebugARM64/run_vm_tests String_ScrubName_Null   
Running test: String_ScrubName_Null
../../runtime/vm/object.cc: 307: error: expected: strlen(cname) == static_cast<size_t>(name.Length())
version=3.6.2 (stable) (Mon Feb 10 09:36:33 2025 -0600) on "macos_simarm64"
pid=25426, thread=259, isolate_group=isolate(0x118008200), isolate=isolate(0x11800fa00)
os=macos, arch=arm64, comp=no, sim=yes
isolate_instructions=102484d00, vm_instructions=102484d00
fp=16d97a110, sp=16d97a0e0, pc=102d98478
  pc 0x0000000102d98478 fp 0x000000016d97a110 dart::Profiler::DumpStackTrace(bool)+0x20
  pc 0x0000000102d983e8 fp 0x000000016d97a160 dart::Profiler::DumpStackTrace(void*)+0x28
  pc 0x00000001034f7410 fp 0x000000016d97a180 Dart_DumpNativeStackTrace+0x18
  pc 0x0000000102a6854c fp 0x000000016d97a1b0 dart::Assert::Fail(char const*, ...) const+0x40
  pc 0x0000000102c537b4 fp 0x000000016d97a380 dart::String::ScrubName(dart::String const&, bool)+0xf8
  pc 0x00000001026c6374 fp 0x000000016d97a410 dart::Dart_TestHelperString_ScrubName_Null(dart::Thread*)+0xd8
  pc 0x00000001026c624c fp 0x000000016d97aba0 dart::Dart_TestString_ScrubName_Null()+0xdc
  pc 0x00000001024b3388 fp 0x000000016d97abd0 dart::TestCase::Run()+0x3c
  pc 0x00000001024b34c4 fp 0x000000016d97ac20 dart::TestCaseBase::RunTest()+0x64
  pc 0x0000000102772260 fp 0x000000016d97ac40 dart::TestCaseBase::RunAll()+0x40
  pc 0x00000001024b3bcc fp 0x000000016d97ad60 dart::Main(int, char const**)+0x41c
  pc 0x00000001024b37ac fp 0x000000016d97ad80 main+0x24
  pc 0x00000001839b20e0 fp 0x000000016d97afb0 start+0x938
-- End of DumpStackTrace
[2]    25426 abort      ./xcodebuild/DebugARM64/run_vm_tests String_ScrubName_Null

Is this expected behavior? String::ScrubName has the following assert:

const char* cname = name.ToCString(); // "String: null"
ASSERT(strlen(cname) == static_cast<size_t>(name.Length()));

Which is incompatible with the behavior of ToCString() when IsNull() is true:

const char* String::ToCString() const {
  if (IsNull()) {
    return "String: null";
  }
  // ...
}

[dart-github-bot]

Summary: String::ScrubName asserts that a string's length matches its C-string representation. This fails for null strings because ToCString() returns a fixed string regardless of length.

[mraleph]

You are not supposed to call String::ScrubName on null - if you are scrubbing null something went wrong somewhere else.

[felangel]

Would be nice for there to be an assert or at least some documentation mentioning that. It’s definitely not obvious based on the API that you’re not allowed to scrub a potentially null String 🤷