feat(kvp): add store trait and Hyper-V KVP storage crate

Cargo.toml

@@ -50,6 +50,7 @@ path = "tests/functional_tests.rs"
 [workspace]
 members = [
     "libazureinit",
+    "libazureinit-kvp",
 ]
 
 [features]

doc/kvp_pr_description.md

@@ -0,0 +1,74 @@
+# PR: Add `libazureinit-kvp` with unified KVP pool store
+
+## Summary
+
+- Adds workspace member `libazureinit-kvp` in root `Cargo.toml`
+- Adds `libazureinit-kvp/Cargo.toml`
+- Adds `libazureinit-kvp/src/lib.rs` and `libazureinit-kvp/src/kvp_pool.rs`
+
+## Crate Design
+
+- One trait: `KvpStore`
+- One implementation: `KvpPoolStore`
+
+`KvpStore` splits each operation into a `backend_*` method (raw I/O,
+provided by the implementor) and a public method (`write`, `read`,
+`clear`) that validates inputs then delegates to the backend.
+
+Public API:
+
+- `write`, `read` (validate then delegate to `backend_write`/`backend_read`)
+- `entries`, `entries_raw`
+- `delete`, `clear`
+- `is_stale`
+
+Validation is centralized in trait default methods and policy-aware via:
+
+- `max_key_size(&self)`
+- `max_value_size(&self)`
+
+`KvpPoolStore` is file-backed using Hyper-V KVP wire format
+(fixed-size 512-byte key + 2048-byte value records), with lock-based concurrency.
+
+## Policy and Limits
+
+Constructor:
+
+- `new(path: Option<PathBuf>, mode: PoolMode, truncate_on_stale: bool)`
+
+`PoolMode`:
+
+- `Restricted` (default): key <= 254 bytes, value <= 1022 bytes
+- `Full`: key <= 512 bytes, value <= 2048 bytes
+
+Behavior:
+
+- Default path when `None`: `/var/lib/hyperv/.kvp_pool_1`
+- Unique key cap: 1024
+  - new key beyond cap is rejected
+  - overwrite of existing key at cap is allowed
+- `clear()` truncates the store
+- `truncate_on_stale` keeps truncation caller-controlled
+
+## Errors
+
+`KvpError` includes explicit variants:
+
+- `EmptyKey`
+- `KeyContainsNull`
+- `KeyTooLarge { max, actual }`
+- `ValueTooLarge { max, actual }`
+- `MaxUniqueKeysExceeded { max }`
+- `Io(io::Error)`
+
+## Testing
+
+17 tests covering:
+
+- restricted/full key/value boundary checks
+- default and explicit path behavior
+- mode getter
+- unique-key cap behavior (including overwrite-at-cap and add-after-delete)
+- `entries` last-write-wins and `entries_raw` duplicate preservation
+- `delete`, `clear`, and stale checks
+- key validation (empty and null-byte)

libazureinit-kvp/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "libazureinit-kvp"
+version = "0.1.0"
+edition = "2021"
+rust-version = "1.88"
+repository = "https://github.com/Azure/azure-init/"
+homepage = "https://github.com/Azure/azure-init/"
+license = "MIT"
+description = "Hyper-V KVP (Key-Value Pair) storage library for azure-init."
+
+[dependencies]
+libc = "0.2"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+sysinfo = "0.38"
+
+[dev-dependencies]
+tempfile = "3"
+
+[lib]
+name = "libazureinit_kvp"
+path = "src/lib.rs"