A rewrite read the header of the file it replaces but not whether the file was
a store: C# ReadStoreHeader skipped one value and ignored bytes after it, and
took a non-array value or an array whose first slot is a string that is not a
store header for a headerless legacy file (so did the C# Mesh raw write, which
uses it); TypeScript's pushAll took any non-array value or non-header first slot
for v1; Rust ignored bytes after the array.
The rule, one per runtime: a file is a store on rewrite only when it is exactly
one complete MessagePack array. Bytes after it, a value that is not an array,
and a first slot that is a string but not a header this runtime reads are
refused, and the file is left untouched. An empty file and a legacy envelope
array (first slot not a string, empty included) carry no header and are still
rewritten.
- C#: ReadStoreHeader refuses non-array values, trailing bytes and a non-header
first string, with a NotSupportedException naming which.
- TypeScript: storeHeader (store-format.ts) decides the rewrite header;
@msgpack/msgpack's decode already refuses trailing bytes.
- Rust: store_header(bytes, whole) walks with a cursor; the rewrite path asks
for the whole file, the read paths keep reading as before.
- Python: push_all already reads the whole file (unpackb refuses trailing
bytes, the legacy decoder refuses anything but a list of maps); pinned by a
test.
Tests in each runtime run the same file matrix (truncated, trailing, scalar,
map, string first; empty, empty array, legacy), plus a C# unconditional commit
over a store with trailing bytes and the C# Mesh raw write.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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