Keep data in mem for local query (#381)

When using the disk writer, keep a copy of record in 
a mutable memory writer. This allows querying hot data 
of past minute by cloning the records and provide to 
query table. 

The memory is discarded when finalise is called at 
the end of minute.

server/src/event/writer.rs

@@ -44,7 +44,7 @@ pub static STREAM_WRITERS: Lazy<WriterTable> = Lazy::new(WriterTable::default);
 
 pub enum StreamWriter {
     Mem(InMemWriter),
-    Disk(FileWriter),
+    Disk(FileWriter, InMemWriter),
 }
 
 impl StreamWriter {
@@ -58,8 +58,9 @@ impl StreamWriter {
             StreamWriter::Mem(mem) => {
                 mem.push(rb);
             }
-            StreamWriter::Disk(disk) => {
+            StreamWriter::Disk(disk, mem) => {
                 disk.push(stream_name, schema_key, &rb)?;
+                mem.push(rb);
             }
         }
         Ok(())
@@ -112,7 +113,7 @@ impl WriterTable {
                     let mut writer = if CONFIG.parseable.in_mem_ingestion {
                         StreamWriter::Mem(InMemWriter::default())
                     } else {
-                        StreamWriter::Disk(FileWriter::default())
+                        StreamWriter::Disk(FileWriter::default(), InMemWriter::default())
                     };
 
                     writer.push(stream_name, schema_key, record)?;
@@ -146,7 +147,7 @@ impl WriterTable {
                             buf: rb,
                         });
                 }
-                StreamWriter::Disk(disk) => disk.close_all(),
+                StreamWriter::Disk(disk, _) => disk.close_all(),
             }
         }
     }
@@ -155,13 +156,15 @@ impl WriterTable {
         let hashmap_guard = self.read().unwrap();
         let (writer, context) = hashmap_guard.get(stream_name)?;
         let writer = writer.lock().unwrap();
-        match &*writer {
-            StreamWriter::Mem(mem) => Some(ReadBuf {
-                time: context.time,
-                buf: mem.recordbatch_cloned(),
-            }),
-            StreamWriter::Disk(_) => None,
-        }
+        let mem = match &*writer {
+            StreamWriter::Mem(mem) => mem,
+            StreamWriter::Disk(_, mem) => mem,
+        };
+
+        Some(ReadBuf {
+            time: context.time,
+            buf: mem.recordbatch_cloned(),
+        })
     }
 }