lib: modify storage monitor to return more up-to-date information (#380)

Instead of checking the file size, it now locally queries
the dbstat table and aggregates the result.
A new connection is established on each iteration on purpose:
it's not a heavy operation, and at the same time it makes the fiber
resilient to any issues with connections that broke.
The update is also performed once per minute now, instead of previous
15 minutes.

Author: psarna

sqld/src/lib.rs

@@ -402,11 +402,21 @@ async fn start_primary(
 // TODO: Once we have a separate fiber that does WAL checkpoints, running this routine
 // right after checkpointing is exactly where it should be done.
 async fn run_storage_monitor(mut db_path: PathBuf, stats: Stats) -> anyhow::Result<()> {
-    let duration = tokio::time::Duration::from_secs(60 * 15);
+    let duration = tokio::time::Duration::from_secs(60);
     db_path.push("data");
     loop {
-        let attr = tokio::fs::metadata(&db_path).await;
-        stats.set_storage_bytes_used(attr.map_or(0, |stats| stats.len()));
+        if let Ok(conn) = rusqlite::Connection::open_with_flags(
+            &db_path,
+            rusqlite::OpenFlags::SQLITE_OPEN_READ_ONLY,
+        ) {
+            if let Ok(storage_bytes_used) =
+                conn.query_row("select sum(pgsize) from dbstat;", [], |row| {
+                    row.get::<usize, u64>(0)
+                })
+            {
+                stats.set_storage_bytes_used(storage_bytes_used);
+            }
+        }
         tokio::time::sleep(duration).await;
     }
 }