fix(core): Fix logs and metrics flush timeout starvation with continuous logging (#18211)

The flush timeout was being reset on every incoming log, preventing
flushes when logs arrived continuously. Now, the timer starts on the
first log and won't get reset, ensuring logs flush within the configured
interval.

Fixes #18204, getsentry/sentry-react-native#5378

v9 backport: #18214

Author: andreiborza

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', () => {