Fix thread safety and robustness in Poll adapter and Sync middleware

- Add mutex to Poll#sync to prevent redundant concurrent imports
- Use rescue StandardError instead of bare rescue in Poll and CloudPoll
- Cache poll adapter lookup by identity in Sync middleware to avoid
  object_id reuse issues after GC
- Document write-through eventual consistency trade-off

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jnunemaker

lib/flipper/adapters/cloud_poll.rb

@@ -27,7 +27,7 @@ def initialize(poller, adapter)
         if adapter.features.empty?
           begin
             @poller.sync
-          rescue
+          rescue StandardError
             # TODO: Warn here that it's possible that no data has been synced
             # and flags are being evaluated without flag data being present
             # until a sync completes. We rescue to avoid flipper being down

lib/flipper/adapters/poll.rb

@@ -18,6 +18,9 @@ class Poll
       # Public: The Poller instance used to sync in the background.
       attr_reader :poller
 
+      # Public: The mutex used to synchronize sync operations.
+      attr_reader :sync_mutex
+
       # Public: The local memory adapter that serves reads.
       attr_reader :local
 
@@ -46,13 +49,14 @@ def initialize(source, options = {})
         @local = Adapters::Memory.new
         @remote = source
         @last_synced_at = Concurrent::AtomicFixnum.new(0)
+        @sync_mutex = Mutex.new
 
         # Block the main thread for the initial sync so we don't serve
         # empty/default values before the first poll completes.
         begin
           @poller.sync
           sync
-        rescue
+        rescue StandardError
           # Rescue to avoid source adapter being down causing processes to crash.
         end
       end
@@ -67,11 +71,13 @@ def adapter_stack
       # If given a block, syncs once at the start and suppresses further syncs
       # for the duration of the block (useful for per-request sync).
       def sync
-        poller_last_synced_at = @poller.last_synced_at.value
-        last = @last_synced_at.value
-        if poller_last_synced_at > last
-          @local.import(@poller.adapter)
-          @last_synced_at.update { poller_last_synced_at }
+        @sync_mutex.synchronize do
+          poller_last_synced_at = @poller.last_synced_at.value
+          last = @last_synced_at.value
+          if poller_last_synced_at > last
+            @local.import(@poller.adapter)
+            @last_synced_at.update { poller_last_synced_at }
+          end
         end
 
         if block_given?
@@ -106,7 +112,10 @@ def get_all(**kwargs)
         @local.get_all(**kwargs)
       end
 
-      # Writes - go to source first, then update local memory
+      # Writes - go to source first, then update local memory.
+      # Note: There is a small window where another thread's sync could overwrite
+      # the local adapter with a stale poller snapshot that doesn't include the
+      # write yet. The write will be picked up on the next poll cycle.
 
       def add(feature)
         @remote.add(feature).tap { @local.add(feature) }

lib/flipper/middleware/sync.rb

@@ -8,7 +8,7 @@ def initialize(app, options = {})
 
       def call(env)
         flipper = env.fetch(@env_key) { Flipper }
-        poll_adapter = find_poll_adapter(flipper.adapter)
+        poll_adapter = poll_adapter_for(flipper)
 
         if poll_adapter
           poll_adapter.sync { @app.call(env) }
@@ -19,6 +19,17 @@ def call(env)
 
       private
 
+      # Cache the poll adapter lookup since the adapter stack doesn't change
+      # after initialization. Uses the flipper instance itself as key to avoid
+      # object_id reuse issues after GC.
+      def poll_adapter_for(flipper)
+        @poll_adapters ||= {}.compare_by_identity
+        unless @poll_adapters.key?(flipper)
+          @poll_adapters[flipper] = find_poll_adapter(flipper.adapter)
+        end
+        @poll_adapters[flipper]
+      end
+
       # Walk the adapter stack to find a Poll adapter, which may be wrapped
       # by Strict, ActorLimit, or other Wrapper adapters.
       def find_poll_adapter(adapter)