feat(c-sdk): optional version/size fast-path and concurrent block reads

[somfornot]

What

Two changes to the C SDK read path (talon_read_async), aimed at removing avoidable overhead when the caller already knows an object's identity.

1. Optional caller-supplied version + size (skip StatObject)

talon_read_async gains two new, independently-nullable parameters:

int talon_read_async(
    talon_client *client, const char *uri, uint64_t offset,
    uint8_t *dst, size_t dst_len,
    const char *version,          /* NULL = unknown */
    const uint64_t *object_size,  /* NULL = unknown */
    talon_callback callback, void *user_data, uint64_t *request_id_out);

• Both non-NULL → fast path: the read uses the caller's version + size and skips the StatObject control round trip entirely. The caller owns keeping them current for the object generation being read.
• Either NULL → the SDK resolves both with a StatObject first (the historical behavior); a lone supplied value is ignored, since a read cannot skip the stat without both halves.

object_size is a pointer specifically so "no valid size" (NULL) is distinct from "a genuinely empty object" (*object_size == 0, an unambiguous zero-byte read). EOF semantics are preserved: a size smaller than offset + dst_len yields a POSIX short read, and a size larger than the object surfaces as a read error rather than fabricated bytes.

2. Concurrent block fetches

A read spanning multiple blocks previously fetched them in a serial await loop. It now fans the per-block fetches out concurrently (a JoinSet over disjoint 'static sub-slices of the caller buffer), so a large read no longer pays one worker RTT per block in series.

Note

This is a breaking change to the C ABI of talon_read_async (two new parameters). Existing callers that want the previous behavior pass NULL, NULL. The in-repo header, smoke test, example, and MinIO e2e are all updated; the e2e adds a fast-path case that reuses the version + size from its stat.

Testing

• cargo test -p talon-c: 13 unit tests pass, including new coverage for the fast path skipping the stat, version-without-size falling back to the stat, the empty-object (size == 0) fast path, and concurrent multi-block reassembly.
• cargo clippy -p talon-c --all-targets clean; cargo fmt clean.
• All C sources compile under -std=c11 -Wall -Wextra -Werror.

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