Inconsistent behavior in `get_many` for non-existent paths

[Palkovsky]

Hello, first of all I want to say that I'm getting a staggering performance using the get_many_unchecked in a field extraction pipeline. However, I've encountered some quirky behaviors, that I feel are likely bugs. get_by_schema doesn't come with similar issues, but based on my benchmarks it is much slower.

Describe the bug

get_many/get_many_unchecked has inconsistent behavior when querying paths that don't exist. Querying a missing key in an empty object returns Err, but querying a missing key in a non-empty object returns Ok([None]). This causes the entire multi-path query to fail when any path encounters an empty object/array.

To Reproduce

use sonic_rs::{get_many, pointer, PointerTree};

fn main() {
    // Case 1: Non-empty object with missing key -> Ok([None])
    let json = r#"{"a": {"d": 1}, "b": {"c": "foo"}}"#;
    let mut tree = PointerTree::new();
    tree.add_path(&pointer!["a", "c"]); // "a" exists but has no key "c"

    let result = get_many(json, &tree);
    assert!(result.is_ok());
    assert!(result.unwrap()[0].is_none()); // Works as documented

    // Case 2: Empty object with missing key -> Err
    let json = r#"{"a": {}, "b": {"c": "foo"}}"#;
    let mut tree = PointerTree::new();
    tree.add_path(&pointer!["a", "c"]); // "a" is empty

    let result = get_many(json, &tree);
    assert!(result.is_err()); // Unexpected. My intuition: should be Ok([None])
}

Both cases query a path that doesn't exist (a.c), but they behave differently based on whether the intermediate object is empty or not.

Expected behavior

According to the documentation: "If a key is unknown, the value at the corresponding position in Vec will be None."

Both cases should return Ok([None]) since the path a.c doesn't exist in either case.

Following this logic, I feel that out-of-bounds array accesses shouldn't fail the entire get_many call. Just return None for a missing item. This would prevent a single missing path from failing an entire multi-path query:

let json = r#"{"a": [1, 2], "b": {"c": "found"}, "d": {}}"#;
let mut tree = PointerTree::new();
tree.add_path(&pointer!["a", 99]);  // out of bounds
tree.add_path(&pointer!["b", "c"]); // valid path
tree.add_path(&pointer!["b", "d"]); // invalid path
tree.add_path(&pointer!["d", "e"]); // invalid path, non-existent object

let result = get_many(json, &tree);
// Currently: Err (entire query fails)
// Expected: Ok([None, Some("found"), None, None])

sonic-rs version:

Both 0.5.6 and master exhibit same behavior

[liuq19]

recommend to use get_by_schema API

[Palkovsky]

The problem with get_by_schema is that it is much slower than get_many_unchecked. When processing large events ~10kB and extracting couple of properties from them, throughput measured on get_by_schema sits at ~200k eps. With get_many_unchecked and SIMD enabled it reaches up to 900 eps.