core/rawdb: add logging and fix comments around AncientRange function. (#28379)

This adds warning logs when the read does not match the expected count.
We can also remove the size limit since the function documentation explicitly states
that callers should limit the count.

Author: jakub-freebit

core/rawdb/accessors_chain.go

@@ -334,13 +334,18 @@ func ReadHeaderRange(db ethdb.Reader, number uint64, count uint64) []rlp.RawValu
 		return rlpHeaders
 	}
 	// read remaining from ancients
-	max := count * 700
-	data, err := db.AncientRange(ChainFreezerHeaderTable, i+1-count, count, max)
-	if err == nil && uint64(len(data)) == count {
-		// the data is on the order [h, h+1, .., n] -- reordering needed
-		for i := range data {
-			rlpHeaders = append(rlpHeaders, data[len(data)-1-i])
-		}
+	data, err := db.AncientRange(ChainFreezerHeaderTable, i+1-count, count, 0)
+	if err != nil {
+		log.Error("Failed to read headers from freezer", "err", err)
+		return rlpHeaders
+	}
+	if uint64(len(data)) != count {
+		log.Warn("Incomplete read of headers from freezer", "wanted", count, "read", len(data))
+		return rlpHeaders
+	}
+	// The data is on the order [h, h+1, .., n] -- reordering needed
+	for i := range data {
+		rlpHeaders = append(rlpHeaders, data[len(data)-1-i])
 	}
 	return rlpHeaders
 }

core/rawdb/freezer_resettable.go

@@ -119,9 +119,10 @@ func (f *ResettableFreezer) Ancient(kind string, number uint64) ([]byte, error)
 
 // AncientRange retrieves multiple items in sequence, starting from the index 'start'.
 // It will return
-//   - at most 'max' items,
-//   - at least 1 item (even if exceeding the maxByteSize), but will otherwise
-//     return as many items as fit into maxByteSize
+//   - at most 'count' items,
+//   - if maxBytes is specified: at least 1 item (even if exceeding the maxByteSize),
+//     but will otherwise return as many items as fit into maxByteSize.
+//   - if maxBytes is not specified, 'count' items will be returned if they are present.
 func (f *ResettableFreezer) AncientRange(kind string, start, count, maxBytes uint64) ([][]byte, error) {
 	f.lock.RLock()
 	defer f.lock.RUnlock()