xDS: fix DistributorWatcher thread safety in CertificateProvider - #13087
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`DistributorWatcher.updateSpiffeTrustMap()` is not `synchronized`, unlike `updateCertificate()` and `updateTrustedRoots()` in the same class. This causes `ConcurrentModificationException` when a watcher is added or removed while `updateSpiffeTrustMap` iterates `downstreamWatchers`. Additionally, all update methods iterate `downstreamWatchers` directly. If a callback reentrantly calls `addWatcher`/`removeWatcher`, the `HashSet` is modified during iteration, producing: ``` java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextNode(HashMap.java:1606) at java.util.HashMap$KeyIterator.next(HashMap.java:1629) at DistributorWatcher.updateSpiffeTrustMap(CertificateProvider.java:120) ``` Observed in production when a service creates many mTLS-enabled gRPC channels concurrently at startup (~60 channels across 7 threads). All channels share the same `file_watcher` cert provider instance. The burst of concurrent `addWatcher()` calls races with the provider's initial `updateSpiffeTrustMap()` call, triggering the CME. The exception propagates to `DynamicSslContextProvider.onError()`, preventing the TLS context from being created. Affected subchannels remain in CONNECTING until the deadline expires: ``` DEADLINE_EXCEEDED: CallOptions deadline exceeded after 4.998s. Name resolution delay 0.000000000 seconds. [closed=[], open=[[connecting_and_lb_delay=4999ms, was_still_waiting]]] ``` These tests demonstrate the race. They are expected to fail until the fix is applied.
Add `synchronized` to `updateSpiffeTrustMap()`, matching `updateCertificate()` and `updateTrustedRoots()`. Iterate a snapshot (`ImmutableSet.copyOf`) of `downstreamWatchers` in all notification methods, instead of iterating the live `HashSet`. This prevents `ConcurrentModificationException` when a callback calls `addWatcher()`/`removeWatcher()`. Mark other functions (`getLastIdentityCert()`, `close()` & `clearValues()`) which read/write state without holding the monitor as synchronized.
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A gRPC client using xDS-driven mTLS does two things concurrently:
The
DistributorWatchersits in the middle and fans out: one cert provider pushes updates to it, and it distributes to N channel watchers.I think there are two conditions in this that can lead to a
ConcurrentModificationException:updateSpiffeTrustMapisn't synchronized (unlikeupdateCertificateandupdateTrustedRoots), so it can race againstaddWatcher/removeWatcherfrom another thread.We observed
ConcurrentModificationExceptioninupdateSpiffeTrustMap()in production, consistent with the unsynchronized iteration described above. We also observed startup RPC deadlines during the rollout, but have not established the causal connection between these symptoms.These all occurred in a service that creates many (~60) channels on startup, while getting the CDS config over xDS. The service uses grpc-java / grpc-xds
1.83.0. When we switched it to use mTLS with SPIFFE certificates, we got the following errors: