fix(core): Fix logs and metrics flush timeout starvation with continuous logging

packages/core/src/client.ts

@@ -94,7 +94,7 @@ function _isDoNotSendEventError(error: unknown): error is DoNotSendEventError {
  * This helper function encapsulates the common pattern of:
  * 1. Tracking accumulated weight of items
  * 2. Flushing when weight exceeds threshold (800KB)
- * 3. Flushing after idle timeout if no new items arrive
+ * 3. Flushing after timeout period from the first item
  *
  * Uses closure variables to track weight and timeout state.
  */
@@ -112,11 +112,13 @@ function setupWeightBasedFlushing<
   // Track weight and timeout in closure variables
   let weight = 0;
   let flushTimeout: ReturnType<typeof setTimeout> | undefined;
+  let isTimerActive = false;
 
   // @ts-expect-error - TypeScript can't narrow generic hook types to match specific overloads, but we know this is type-safe
   client.on(flushHook, () => {
     weight = 0;
     clearTimeout(flushTimeout);
+    isTimerActive = false;
   });
 
   // @ts-expect-error - TypeScript can't narrow generic hook types to match specific overloads, but we know this is type-safe
@@ -127,10 +129,15 @@ function setupWeightBasedFlushing<
     // The weight is a rough estimate, so we flush way before the payload gets too big.
     if (weight >= 800_000) {
       flushFn(client);
-    } else {
-      clearTimeout(flushTimeout);
+    } else if (!isTimerActive) {
+      // Only start timer if one isn't already running.
+      // This prevents flushing being delayed by items that arrive close to the timeout limit
+      // and thus resetting the flushing timeout and delaying items being flushed.
+      isTimerActive = true;
       flushTimeout = setTimeout(() => {
         flushFn(client);
+        // Note: isTimerActive is reset by the flushHook handler above, not here,
+        // to avoid race conditions when new items arrive during the flush.
       }, DEFAULT_FLUSH_INTERVAL);
     }
   });

packages/core/test/lib/client.test.ts

@@ -2772,7 +2772,7 @@ describe('Client', () => {
       expect(sendEnvelopeSpy).toHaveBeenCalledTimes(1);
     });
 
-    it('resets idle timeout when new logs are captured', () => {
+    it('does not reset idle timeout when new logs are captured', () => {
       const options = getDefaultTestClientOptions({
         dsn: PUBLIC_DSN,
         enableLogs: true,
@@ -2783,26 +2783,52 @@ describe('Client', () => {
 
       const sendEnvelopeSpy = vi.spyOn(client, 'sendEnvelope');
 
-      // Add initial log
+      // Add initial log (starts the timer)
       _INTERNAL_captureLog({ message: 'test log 1', level: 'info' }, scope);
 
       // Fast forward part of the idle timeout
       vi.advanceTimersByTime(2500);
 
-      // Add another log which should reset the timeout
+      // Add another log which should NOT reset the timeout
       _INTERNAL_captureLog({ message: 'test log 2', level: 'info' }, scope);
 
-      // Fast forward the remaining time
+      // Fast forward the remaining time to reach the full timeout from the first log
       vi.advanceTimersByTime(2500);
 
-      // Should not have flushed yet since timeout was reset
-      expect(sendEnvelopeSpy).not.toHaveBeenCalled();
+      // Should have flushed both logs since timeout was not reset
+      expect(sendEnvelopeSpy).toHaveBeenCalledTimes(1);
+    });
+
+    it('starts new timer after timeout completes and flushes', () => {
+      const options = getDefaultTestClientOptions({
+        dsn: PUBLIC_DSN,
+        enableLogs: true,
+      });
+      const client = new TestClient(options);
+      const scope = new Scope();
+      scope.setClient(client);
 
-      // Fast forward the full timeout
+      const sendEnvelopeSpy = vi.spyOn(client, 'sendEnvelope');
+
+      // First batch: Add a log and let it flush
+      _INTERNAL_captureLog({ message: 'test log 1', level: 'info' }, scope);
+
+      // Fast forward to trigger the first flush
       vi.advanceTimersByTime(5000);
 
-      // Now should have flushed both logs
       expect(sendEnvelopeSpy).toHaveBeenCalledTimes(1);
+
+      // Second batch: Add another log after the first flush completed
+      _INTERNAL_captureLog({ message: 'test log 2', level: 'info' }, scope);
+
+      // Should not have flushed yet
+      expect(sendEnvelopeSpy).toHaveBeenCalledTimes(1);
+
+      // Fast forward to trigger the second flush
+      vi.advanceTimersByTime(5000);
+
+      // Should have flushed the second log
+      expect(sendEnvelopeSpy).toHaveBeenCalledTimes(2);
     });
 
     it('flushes logs on flush event', () => {