[shoumikhin]

What is wrong today

external_engine_data= names the .ptd file that holds the TensorRT engines. Its check refuses directories, drive prefixes and a trailing .ptd, but it accepts names that cannot become a normal file:

• Windows device names, such as NUL, CON, COM1 or LPT1, in any case. On Windows these open a device, not a file, so the engines may be lost with no error.
• Characters Windows does not allow in a file name: ? * | < > and the double quote, plus control characters.
• Names too long for the file system. A 252 character name fails with "file name too long".

save() only fails when it writes the .ptd. By then the .pte is already on disk, and it cannot load without its engines.

Also, one error message covers every rule, so engines:v2 is told it has "a directory, a drive or the .ptd extension". And the code comment gives a reason for refusing . and .. that does not apply.

What this change does

The name must now:

• use only ASCII letters, digits, ., - and _,
• start with a letter, a digit or _,
• be at most 251 characters, so <name>.ptd fits the usual 255 byte limit,
• not end with .ptd, which ExecuTorch adds,
• not be a Windows device name, with or without an extension.

Each kind of problem gets its own error message. The check still runs before anything is compiled or written, in export(), save(), the partitioner and the backend, so a bad name fails at once.

torch_tensorrt.save(trt_gm, "model.pte", output_format="executorch", external_engine_data="NUL")
# ValueError: external_engine_data must not be a Windows device name such as NUL or CON, got 'NUL'.

Names like model, my-model_v2.1 and aoti_cuda_blob (to share the CUDA backend's file) still work. Some names that worked on Linux before are now refused, such as ones with a space, a + or a non-ASCII letter. That is on purpose: a short portable name keeps a saved program usable on every system.

What was tested

On Linux x86-64 with the pinned ExecuTorch nightly:

• test_external_engine_data.py, test_backend.py and test_partitioner.py: 101 passed.
• With the old check and the new tests, the 14 new rejected names and the 2 updated message checks fail, as expected.
• Nothing was run on Windows. The device name and character rules follow Microsoft's file naming rules.