This patch is motivated by wasm-ld's concatenation of custom sections with the same name. wit-component and related stores component binaries in a module's custom section, and if these sections are unintentionally duplicated and then concatenated by wasm-ld, we get a strange error when parsing the corrupted binary.

There is no way to detect that a wasm module or component contains more information than its original length, because the wasm binary format is a fixed header and then any number of sections.

If two valid wasm modules or components are concatenated into a single binary, the start of the second module will be parsed as another section. The wasm magic number \0asm where \0 is parsed as the marker for a custom section, a will be parsed as an leb section length of 97, s will parse as an leb length 115 for the custom section's string name. Because that string does not fit in the section, this "section" will always fail to parse. Currently, it gives the error message unexpected end-of-file, which is somewhat misleading - it wasn't actually the end-of-file encountered, that ends up just being an artifact of how wasmparser represents sections to readers.

So, we now do a 4-byte lookahead at the start of parsing each section and give this error case a more useful message.