agent v1.4: MySQL/MariaDB live metrics (_mysql_stats)

Same fields as the postgres variant: sizes/tables/rows/top tables from
information_schema.TABLES, connections/uptime from SHOW GLOBAL STATUS,
InnoDB buffer-pool hit ratio as cache_hit_pct. Best-effort like the
rest — any failure returns None and never disturbs the heartbeat.

backhaul-agent.py

@@ -22,7 +22,7 @@ import subprocess
 import sys
 import urllib.request
 
-VERSION = "1.3"
+VERSION = "1.4"
 RESULT_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)), ".dump-result.json")
 
 
@@ -81,9 +81,11 @@ def db_version(t):
 def db_stats(t):
     """Read-only live metrics for the dashboard's 'nerd stats' — size, tables, rows, index
     footprint, connections, cache-hit ratio, the biggest tables, server version + uptime.
-    Best-effort: any failure returns None and never disturbs the heartbeat. Postgres only."""
+    Best-effort: any failure returns None and never disturbs the heartbeat."""
     if t["engine"] == "sqlite":
         return _sqlite_stats(t)
+    if t["engine"] in ("mysql", "mariadb"):
+        return _mysql_stats(t)
     if t["engine"] != "postgres":
         return None
     env = dict(os.environ, PGPASSWORD=t["pw"])
@@ -120,6 +122,45 @@ def db_stats(t):
         return None
 
 
+def _mysql_stats(t):
+    """Live metrics for MySQL/MariaDB, same fields as the postgres variant. Sizes and row
+    counts come from information_schema.TABLES (InnoDB estimates — same spirit as
+    n_live_tup); cache hit is the InnoDB buffer pool hit ratio."""
+    def q(sql):
+        r = subprocess.run(("mysql", "-h", t["host"], "-P", t["port"], "-u", t["user"],
+                            f"-p{t['pw']}", "-N", "-B", t["db"], "-e", sql),
+                           capture_output=True, text=True, timeout=30)
+        return r.stdout.strip()
+
+    def status(var):
+        out = q(f"SHOW GLOBAL STATUS LIKE '{var}'")
+        return int(out.split("\t")[1]) if "\t" in out else 0
+
+    try:
+        size, idx, tables, rows = (int(float(v or 0)) for v in q(
+            "SELECT COALESCE(SUM(data_length+index_length),0),"
+            " COALESCE(SUM(index_length),0), COUNT(*), COALESCE(SUM(table_rows),0)"
+            " FROM information_schema.TABLES"
+            " WHERE table_schema=DATABASE() AND table_type='BASE TABLE'").split("\t"))
+        top = [{"name": f[0], "bytes": int(f[1]), "rows": int(f[2])}
+               for f in (line.split("\t") for line in q(
+                   "SELECT table_name, data_length+index_length, COALESCE(table_rows,0)"
+                   " FROM information_schema.TABLES"
+                   " WHERE table_schema=DATABASE() AND table_type='BASE TABLE'"
+                   " ORDER BY data_length+index_length DESC LIMIT 10").splitlines())
+               if len(f) == 3]
+        req = status("Innodb_buffer_pool_read_requests")
+        miss = status("Innodb_buffer_pool_reads")
+        return {"size_bytes": size, "tables": tables, "rows_est": rows,
+                "index_bytes": idx, "connections": status("Threads_connected"),
+                "cache_hit_pct": round(100.0 * (req - miss) / req, 2) if req else None,
+                "server_version": q("SELECT VERSION()"),
+                "uptime_s": status("Uptime") or None,
+                "top_tables": top}
+    except Exception:
+        return None
+
+
 def _sqlite_stats(t):
     """Live metrics for a SQLite file: file size, table count, row totals, largest tables."""
     def q(sql):