feat: lazy initialization for log_memory_store to avoid unnecessary log-memory charges

rs/replicated_state/src/canister_state/system_state/log_memory_store/header.rs

@@ -1,8 +1,14 @@
 use crate::canister_state::system_state::log_memory_store::{
     memory::{MemoryAddress, MemoryPosition, MemorySize},
-    ring_buffer::{DATA_REGION_OFFSET, INDEX_TABLE_PAGES, MAGIC},
+    ring_buffer::{DATA_REGION_OFFSET, INDEX_TABLE_PAGES},
 };
 
+/// Indicates a valid canister log buffer.
+pub const MAGIC_VALID: &[u8; 3] = b"CLB";
+
+/// Indicates an invalid canister log buffer.
+pub const MAGIC_INVALID: &[u8; 3] = b"---";
+
 /// Header structure for the log memory store (version 1).
 /// This is the in-memory representation of the header.
 #[derive(Debug, PartialEq, Clone, Copy)]
@@ -31,7 +37,7 @@ impl Header {
     pub fn new(data_capacity: MemorySize) -> Self {
         Self {
             version: 1,
-            magic: *MAGIC,
+            magic: *MAGIC_VALID,
 
             index_table_pages: INDEX_TABLE_PAGES as u16,
             index_entries_count: 0,
@@ -47,6 +53,26 @@ impl Header {
         }
     }
 
+    /// Creates an invalid header.
+    pub fn invalid() -> Self {
+        Self {
+            version: 0,
+            magic: *MAGIC_INVALID, // This is important in order not to charge uninstalled canister.
+
+            index_table_pages: 0,
+            index_entries_count: 0,
+
+            data_offset: MemoryAddress::new(0),
+            data_capacity: MemorySize::new(0),
+            data_head: MemoryPosition::new(0),
+            data_tail: MemoryPosition::new(0),
+            data_size: MemorySize::new(0),
+
+            next_idx: 0,
+            max_timestamp: 0,
+        }
+    }
+
     pub fn advance_position(
         &self,
         position: MemoryPosition,