fix: Increment sequence when accepting new manifests (#6059)

The `ManifestCache::applyManifest` function was returning early without incrementing `seq_`. `OverlayImpl `uses this sequence to identify/invalidate a cached `TMManifests` message, which is exchanged with peers on connection. Depending on network size, startup sequencing, and topology, this can cause syncing issues. This change therefore increments `seq_` when a new manifest is accepted.

src/test/app/Manifest_test.cpp

@@ -827,8 +827,13 @@ public:
 
             // applyManifest should accept new manifests with
             // higher sequence numbers
+            auto const seq0 = cache.sequence();
             BEAST_EXPECT(cache.applyManifest(clone(s_a0)) == ManifestDisposition::accepted);
+            BEAST_EXPECT(cache.sequence() > seq0);
+
+            auto const seq1 = cache.sequence();
             BEAST_EXPECT(cache.applyManifest(clone(s_a0)) == ManifestDisposition::stale);
+            BEAST_EXPECT(cache.sequence() == seq1);
 
             BEAST_EXPECT(cache.applyManifest(clone(s_a1)) == ManifestDisposition::accepted);
             BEAST_EXPECT(cache.applyManifest(clone(s_a1)) == ManifestDisposition::stale);

src/xrpld/app/misc/detail/Manifest.cpp

@@ -459,6 +459,10 @@ ManifestCache::applyManifest(Manifest m)
 
         auto masterKey = m.masterKey;
         map_.emplace(std::move(masterKey), std::move(m));
+
+        // Something has changed. Keep track of it.
+        seq_++;
+
         return ManifestDisposition::accepted;
     }