db: add DebugString method

Add a `DB.DebugString()` method which can be used during debugging.
The meta test uses it to log the final state of the db (which after
reduction will be right after the problematic operation).

db.go

@@ -3082,3 +3082,10 @@ func (d *DB) checkVirtualBounds(m *fileMetadata) {
 		panic(err)
 	}
 }
+
+// DebugString returns a debugging string describing the LSM.
+func (d *DB) DebugString() string {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	return d.mu.versions.currentVersion().DebugString(base.DefaultFormatter)
+}

metamorphic/meta.go

@@ -527,6 +527,17 @@ func RunOnce(t TestingT, runDir string, seed uint64, historyPath string, rOpts .
 	if err := Execute(m); err != nil {
 		fmt.Fprintf(os.Stderr, "Seed: %d\n", seed)
 		fmt.Fprintln(os.Stderr, err)
+	}
+
+	// If we have unclosed databases, print LSM details. This will be the case
+	// when we use --try-to-reduce.
+	for i, db := range m.dbs {
+		if db != nil {
+			fmt.Fprintf(os.Stderr, "\ndb%d:\n%s", i+1, db.DebugString())
+		}
+	}
+
+	if err != nil {
 		m.saveInMemoryData()
 		os.Exit(1)
 	}