feat(gc): FOREST_SNAPSHOT_GC_KEEP_STATE_TREE_EPOCHS env var

Author: hanabi1224

docs/docs/users/reference/env_variables.md

@@ -48,6 +48,7 @@ process.
 | `FOREST_STATE_MIGRATION_DB_WRITE_BUFFER`                  | non-negative integer             | 10000                                          | 100000                                                        | The size of db write buffer for state migration (`~10MB` RAM per `10k` buffer)                                        |
 | `FOREST_SNAPSHOT_GC_INTERVAL_EPOCHS`                      | non-negative integer             | 20160                                          | 8000                                                          | The interval in epochs for scheduling snapshot GC                                                                     |
 | `FOREST_SNAPSHOT_GC_CHECK_INTERVAL_SECONDS`               | non-negative integer             | 300                                            | 60                                                            | The interval in seconds for checking if snapshot GC should run                                                        |
+| `FOREST_SNAPSHOT_GC_KEEP_STATE_TREE_EPOCHS`               | non-negative integer             | 2000                                           | 20160                                                         | The number of most recent epochs of state trees to keep after GC                                                      |
 | `FOREST_DISABLE_BAD_BLOCK_CACHE`                          | 1 or true                        | empty                                          | 1                                                             | Whether or not to disable bad block cache                                                                             |
 | `FOREST_ZSTD_FRAME_CACHE_DEFAULT_MAX_SIZE`                | positive integer                 | 268435456                                      | 536870912                                                     | The default zstd frame cache max size in bytes                                                                        |
 | `FOREST_JWT_DISABLE_EXP_VALIDATION`                       | 1 or true                        | empty                                          | 1                                                             | Whether or not to disable JWT expiration validation                                                                   |

src/db/gc/snapshot.rs

@@ -100,7 +100,20 @@ where
         let chain_data_path = chain_path(config);
         let db_root_dir = db_root(&chain_data_path)?;
         let car_db_dir = db_root_dir.join(CAR_DB_DIR_NAME);
-        let recent_state_roots = config.sync.recent_state_roots;
+        let recent_state_roots = std::env::var("FOREST_SNAPSHOT_GC_KEEP_STATE_TREE_EPOCHS")
+            .ok()
+            .and_then(|i| {
+                i.parse().ok().and_then(|i| {
+                    if i >= config.sync.recent_state_roots {
+                        tracing::info!("Snapshot GC is set to keep {i} epochs of state trees");
+                        Some(i)
+                    } else {
+                        tracing::warn!("Snapshot GC cannot to be set to keep {i} epochs of state trees, at least {} is required for snapshot export", config.sync.recent_state_roots);
+                        None
+                    }
+                })
+            })
+            .unwrap_or(config.sync.recent_state_roots);
         let (reboot_tx, reboot_rx) = flume::bounded(1);
         let (trigger_tx, trigger_rx) = flume::bounded(1);
         Ok((