WasmFS Fetch backend corrupts file reads with custom chunk size when HTTP Range is unsupported #28001

Version of emscripten/emsdk: 6.0.12

Description:

When using wasmfs_create_fetch_backend() with an HTTP server that does not support Range requests (emrun --no_browser), only the first chunk of the file is read correctly. All subsequent bytes are zero.

fread() reports the correct number of bytes read, despite the data being incomplete.

This happens regardless of the configured chunk size:

• 64 * 1024 (64 KiB): files larger than 64 KiB are truncated to the first chunk.
• 0 (default 16 MiB): files larger than 16 MiB are truncated to the first chunk.

Minimal setup:

backend_t backend = wasmfs_create_fetch_backend(
    "http://localhost:8080/assets/",
    64 * 1024
);

wasmfs_create_directory("/assets", 0777, backend);
wasmfs_create_file("/assets/test.bin", 0444, backend);

Reading /assets/test.bin via fread() reproduces the issue when the file exceeds the chunk size.

Expected behavior: The entire file is read correctly, even when the server does not support HTTP Range requests.

Actual behavior: Only the first chunk contains valid data. The remaining bytes are zero, while fread() reports a successful full read.

Suspected cause: Incorrect handling of multi-chunk files when falling back to full-file downloads on servers without HTTP Range support.