[Reporting] Fix slow CSV with large max size bytes (elastic#120365)

* use Buffer.alloc + .set API instead of .concat

* refactor variable names and actually assign to this.buffer

* ok, looks like an array of buffers could work

* added large comment and refactored some variable name

* fix comment

* refactored logic to deal with an edge case where partial buffers should be added, also throw if bad config is detected

* added new test for detecting when the write stream throws for bad config

* updated logic to not ever call .slice(0), updated the guard for the config error, updated a comment

* refactor totalBytesConsumed -> bytesToFlush

* use the while loop mike wrote

* remove unused variable

* update comment

Co-authored-by: Kibana Machine <[email protected]>

x-pack/plugins/reporting/server/lib/content_stream.ts

@@ -66,7 +66,9 @@ export class ContentStream extends Duplex {
     return Math.floor(max / 2);
   }
 
-  private buffer = Buffer.from('');
+  private buffers: Buffer[] = [];
+  private bytesBuffered = 0;
+
   private bytesRead = 0;
   private chunksRead = 0;
   private chunksWritten = 0;
@@ -249,8 +251,43 @@ export class ContentStream extends Duplex {
     });
   }
 
-  private async flush(size = this.buffer.byteLength) {
-    const chunk = this.buffer.slice(0, size);
+  private async flush(size = this.bytesBuffered) {
+    const buffersToFlush: Buffer[] = [];
+    let bytesToFlush = 0;
+
+    /*
+     Loop over each buffer, keeping track of how many bytes we have added
+     to the array of buffers to be flushed. The array of buffers to be flushed
+     contains buffers by reference, not copies. This avoids putting pressure on
+     the CPU for copying buffers or for gc activity. Please profile performance
+     with a large byte configuration and a large number of records (900k+)
+     before changing this code. Config used at time of writing:
+
+      xpack.reporting:
+        csv.maxSizeBytes: 500000000
+        csv.scroll.size: 1000
+
+     At the moment this can put memory pressure on Kibana. Up to 1,1 GB in a dev
+     build. It is not recommended to have overly large max size bytes but we
+     need this code to be as performant as possible.
+    */
+    while (this.buffers.length) {
+      const remainder = size - bytesToFlush;
+      if (remainder <= 0) {
+        break;
+      }
+      const buffer = this.buffers.shift()!;
+      const chunkedBuffer = buffer.slice(0, remainder);
+      buffersToFlush.push(chunkedBuffer);
+      bytesToFlush += chunkedBuffer.byteLength;
+
+      if (buffer.byteLength > remainder) {
+        this.buffers.unshift(buffer.slice(remainder));
+      }
+    }
+
+    // We call Buffer.concat with the fewest number of buffers possible
+    const chunk = Buffer.concat(buffersToFlush);
     const content = this.encode(chunk);
 
     if (!this.chunksWritten) {
@@ -265,22 +302,21 @@ export class ContentStream extends Duplex {
     }
 
     this.bytesWritten += chunk.byteLength;
-    this.buffer = this.buffer.slice(size);
+    this.bytesBuffered -= bytesToFlush;
   }
 
   private async flushAllFullChunks() {
     const maxChunkSize = await this.getMaxChunkSize();
 
-    while (this.buffer.byteLength >= maxChunkSize) {
+    while (this.bytesBuffered >= maxChunkSize && this.buffers.length) {
       await this.flush(maxChunkSize);
     }
   }
 
   _write(chunk: Buffer | string, encoding: BufferEncoding, callback: Callback) {
-    this.buffer = Buffer.concat([
-      this.buffer,
-      Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk, encoding),
-    ]);
+    const buffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk, encoding);
+    this.bytesBuffered += buffer.byteLength;
+    this.buffers.push(buffer);
 
     this.flushAllFullChunks()
       .then(() => callback())