chore: limit Vec preallocation to follow serde convention (#6616)

Author: LesnyRumcajs

src/chain/store/chain_store.rs

@@ -537,12 +537,11 @@ where
 {
     let amt = Amt::<Cid, _>::load(root, db)?;
 
-    let mut cids = Vec::new();
-    for i in 0..amt.count() {
-        if let Some(c) = amt.get(i)? {
-            cids.push(*c);
-        }
-    }
+    let mut cids = Vec::with_capacity(amt.count() as usize);
+    amt.for_each_cacheless(|_, c| {
+        cids.push(*c);
+        Ok(())
+    })?;
 
     Ok(cids)
 }

src/utils/encoding/cid_de_cbor.rs

@@ -63,7 +63,8 @@ impl<'de> DeserializeSeed<'de> for FilterCids<'_> {
             where
                 V: de::MapAccess<'de>,
             {
-                self.0.reserve(visitor.size_hint().unwrap_or(0));
+                let capacity = super::size_hint_cautious_cid(visitor.size_hint().unwrap_or(0));
+                self.0.reserve(capacity);
                 // This is where recursion happens, we unravel each [`Ipld`] till we reach all
                 // the nodes.
                 while visitor
@@ -83,7 +84,8 @@ impl<'de> DeserializeSeed<'de> for FilterCids<'_> {
             where
                 A: SeqAccess<'de>,
             {
-                self.0.reserve(seq.size_hint().unwrap_or(0));
+                let capacity = super::size_hint_cautious_cid(seq.size_hint().unwrap_or(0));
+                self.0.reserve(capacity);
                 // This is where recursion happens, we unravel each [`Ipld`] till we reach all
                 // the nodes.
                 while seq.next_element_seed(FilterCids(self.0))?.is_some() {

src/utils/encoding/mod.rs

@@ -9,6 +9,14 @@ use serde::{Deserializer, Serializer, de, ser};
 
 mod fallback_de_ipld_dagcbor;
 
+/// Limit the the number of bytes that are used for pre-allocating `Vec<Cid>`s. This follows what `serde` is
+/// doing internally with `serde::private::size_hint::cautious()`.
+/// The limit is set to 1 MiB, which is a reasonable upper bound for most use cases.
+fn size_hint_cautious_cid(size_hint: usize) -> usize {
+    const MAX_PREALLOC_BYTES: usize = 1024 * 1024;
+    size_hint.min(MAX_PREALLOC_BYTES / std::mem::size_of::<cid::Cid>())
+}
+
 /// This method will attempt to de-serialize given bytes using the regular
 /// `serde_ipld_dagcbor::from_slice`. Due to a historical issue in Lotus (see more in
 /// [FIP-0027](https://github.com/filecoin-project/FIPs/blob/master/FIPS/fip-0027.md), we must still
@@ -258,4 +266,13 @@ mod tests {
             matches!(from_slice_with_fallback::<Ipld>(&corrupted).unwrap(), Ipld::Bytes(bytes) if bytes == [0x63, 0x74, 0x68, 0x75, 0x6c, 0xa0, 0xa1])
         )
     }
+
+    #[test]
+    fn size_hint_cautious_test() {
+        assert_eq!(size_hint_cautious_cid(0), 0);
+        assert_eq!(
+            size_hint_cautious_cid(1024 * 1024),
+            1024 * 1024 / std::mem::size_of::<cid::Cid>()
+        );
+    }
 }