feat(logger): Emit a warning on buffer flush (#3639)

Author: ConnorKirk

Diff for: packages/logger/src/Logger.ts

@@ -1255,7 +1255,10 @@ class Logger extends Utility implements LoggerInterface {
       return;
     }
 
-    const buffer = this.#buffer.get(traceId) || [];
+    const buffer = this.#buffer.get(traceId);
+    if (buffer === undefined) {
+      return;
+    }
 
     for (const item of buffer) {
       const consoleMethod =
@@ -1266,6 +1269,16 @@ class Logger extends Utility implements LoggerInterface {
             ).toLowerCase() as keyof Omit<LogFunction, 'critical'>);
       this.console[consoleMethod](item.value);
     }
+    if (buffer.hasEvictedLog) {
+      this.printLog(
+        LogLevelThreshold.WARN,
+        this.createAndPopulateLogItem(
+          LogLevelThreshold.WARN,
+          'Some logs are not displayed because they were evicted from the buffer. Increase buffer size to store more logs in the buffer',
+          []
+        )
+      );
+    }
 
     this.#buffer.delete(traceId);
   }

Diff for: packages/logger/src/logBuffer.ts

@@ -17,6 +17,7 @@ export class SizedItem<V> {
 
 export class SizedSet<V> extends Set<SizedItem<V>> {
   public currentBytesSize = 0;
+  public hasEvictedLog = false;
 
   add(item: SizedItem<V>): this {
     this.currentBytesSize += item.byteSize;
@@ -89,6 +90,7 @@ export class CircularMap<V> extends Map<string, SizedSet<V>> {
   ) => {
     while (buffer.currentBytesSize + item.byteSize >= this.#maxBytesSize) {
       buffer.shift();
+      buffer.hasEvictedLog = true;
     }
   };
 }

Diff for: packages/logger/tests/unit/logBuffer.test.ts

@@ -107,4 +107,34 @@ describe('flushBuffer', () => {
     expect(console.debug).toHaveBeenCalledTimes(0);
     expect(console.warn).toHaveBeenCalledTimes(1);
   });
+
+  it('outputs a warning when buffered logs have been evicted', () => {
+    // Prepare
+    const logger = new TestLogger({ logLevel: 'ERROR' });
+    logger.enableBuffering();
+    logger.setbufferLevelThreshold(LogLevelThreshold.INFO);
+
+    // Act
+    const longMessage = 'blah'.repeat(10);
+
+    let i = 0;
+    while (i < 4) {
+      logger.info(
+        `${i} This is a really long log message intended to exceed the buffer ${longMessage}`
+      );
+      i++;
+    }
+
+    // Act
+    logger.flushBufferWrapper();
+
+    // Assess
+    expect(console.warn).toHaveLogged(
+      expect.objectContaining({
+        level: 'WARN',
+        message:
+          'Some logs are not displayed because they were evicted from the buffer. Increase buffer size to store more logs in the buffer',
+      })
+    );
+  });
 });