Fuzzing for K2VItem Crdt (#1438)

Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/1438
Reviewed-by: Alex <lx@deuxfleurs.fr>

fuzz/fuzz_targets/k2v_item_crdt.rs

@@ -0,0 +1,36 @@
+#![no_main]
+
+use std::collections::BTreeMap;
+
+use garage_fuzz::check_crdt_laws;
+use garage_model::k2v::item_table::{DvvsEntry, DvvsValue, K2VItem};
+use libfuzzer_sys::fuzz_target;
+
+// Timestamps are encoded as `(ts << 32) | shift` so that items built with different
+// shifts (0, 1, 2) have disjoint timestamp spaces that still interleave in the sorted merge.
+fn make(raw: BTreeMap<u64, (u32, BTreeMap<u32, DvvsValue>)>, shift: u32) -> K2VItem {
+	let shift = shift as u64;
+	let items = raw
+		.into_iter()
+		.map(|(node, (t_discard, values))| {
+			let entry = DvvsEntry::from_raw(
+				(t_discard as u64) << 32 | shift,
+				values
+					.into_iter()
+					.map(|(ts, v)| ((ts as u64) << 32 | shift, v))
+					.collect(),
+			);
+			(node, entry)
+		})
+		.collect();
+	K2VItem::with_raw_items(items)
+}
+
+fuzz_target!(|inputs: (
+	BTreeMap<u64, (u32, BTreeMap<u32, DvvsValue>)>,
+	BTreeMap<u64, (u32, BTreeMap<u32, DvvsValue>)>,
+	BTreeMap<u64, (u32, BTreeMap<u32, DvvsValue>)>,
+)| {
+	let (a, b, c) = inputs;
+	check_crdt_laws(make(a, 0), make(b, 1), make(c, 2));
+});