Reject snapshot-less join against primary holding a more recent snapshot

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 [7.0.3]: https://github.com/microsoft/CCF/releases/tag/ccf-7.0.3
 
+### Fixed
+
+- On a joiner's first attempt, the primary now requires the joiner's startup seqno to be at least as recent as the primary's latest committed snapshot on disk, preventing snapshot-less joiners from replaying the entire ledger (#7844).
+
 ### Changed
 
 - Upgraded QuickJS from 2024-01-13 to 2025-09-13 (#7849).

src/node/node_state.h

@@ -476,6 +476,9 @@ namespace ccf
     ccf::tasks::Task snapshot_fetch_task;
     ccf::tasks::Task backup_snapshot_fetch_task;
 
+    // Number of times we have fetched the latest snapshot from the primary
+    size_t join_fetch_count = 0;
+
     std::shared_ptr<ccf::kv::AbstractTxEncryptor> make_encryptor()
     {
 #ifdef USE_NULL_ENCRYPTOR
@@ -570,6 +573,13 @@ namespace ccf
       last_recovered_idx = startup_seqno;
       last_recovered_signed_idx = last_recovered_idx;
 
+      if (start_type == StartType::Join)
+      {
+        // after fetching a snapshot, subsequent requests should use the
+        // required bound instead of the preferred bound
+        join_fetch_count += 1;
+      }
+
       if (start_type == StartType::Recover)
       {
         const auto segments = separate_segments(startup_snapshot_info->raw);
@@ -1317,6 +1327,14 @@ namespace ccf
       join_params.public_encryption_key = node_encrypt_kp->public_key_pem();
       join_params.quote_info = quote_info;
       join_params.startup_seqno = startup_seqno;
+      if (config.join.fetch_recent_snapshot)
+      {
+        join_params.join_fetch_count = join_fetch_count;
+      }
+      else
+      {
+        join_params.join_fetch_count = 1;
+      }
       join_params.certificate_signing_request = node_sign_kp->create_csr(
         config.node_certificate.subject_name, subject_alt_names);
       join_params.node_data = config.node_data;

src/node/rpc/node_call_types.h

@@ -97,6 +97,9 @@ namespace ccf
       std::optional<std::vector<uint8_t>> code_transparent_statement =
         std::nullopt;
       std::optional<ccf::LedgerSignMode> ledger_sign_mode = std::nullopt;
+      // Incremented by the joiner each time it retries a join request after
+      // receiving a StartupSeqnoIsOld response.
+      std::optional<uint32_t> join_fetch_count = std::nullopt;
     };
 
     struct Out

src/node/rpc/node_frontend.h

@@ -32,6 +32,7 @@
 #include "service/tables/local_sealing.h"
 #include "service/tables/previous_service_identity.h"
 #include "service/tables/snapshot_status.h"
+#include "snapshots/filenames.h"
 
 #include <llhttp/llhttp.h>
 #include <stdexcept>
@@ -573,23 +574,62 @@ namespace ccf
         }
 
         // Joiner's snapshot too old => StartupSeqnoIsOld
-        // (cause it to fetch a more recent snapshot)
+        // (causes joiner to fetch a more recent snapshot)
+        //
+        // The joiner always wants to use the most recent snapshot.
+        // However this will result in the joiner chasing the primary if
+        // snapshot production period ~= snapshot fetching delay
+        //
+        // So we have hysteresis in the fetching constraint:
+        // If the joiner has already fetched a snapshot: joiner seqno > startup
+        // snapshot seqno Otherwise: joiner seqno > latest snapshot on disk
+        // seqno
         auto this_startup_seqno =
           this->node_operation.get_startup_snapshot_seqno();
+        ccf::kv::Version required_seqno = this_startup_seqno;
+        // If the joiner does not enable fetching, or is a legacy node,
+        // join_fetch_count is unset and we should use the required bound to
+        // prevent it chasing the primary.
+        // Otherwise if this is the first request, use the preferred bound
+        bool using_preferred_bound =
+          (in.join_fetch_count.has_value() && in.join_fetch_count.value() == 0);
+        if (using_preferred_bound)
+        {
+          auto node_configuration_subsystem =
+            this->context.get_subsystem<NodeConfigurationSubsystem>();
+          if (node_configuration_subsystem != nullptr)
+          {
+            const auto& snapshots_config =
+              node_configuration_subsystem->get().node_config.snapshots;
+            const auto latest_committed_snapshot =
+              snapshots::find_latest_committed_snapshot_in_directory(
+                snapshots_config.directory);
+            if (latest_committed_snapshot.has_value())
+            {
+              const auto latest_snapshot_seqno =
+                snapshots::get_snapshot_idx_from_file_name(
+                  latest_committed_snapshot->filename().string());
+              required_seqno = std::max(
+                required_seqno,
+                static_cast<ccf::kv::Version>(latest_snapshot_seqno));
+            }
+          }
+        }
         if (
           in.startup_seqno.has_value() &&
-          this_startup_seqno > in.startup_seqno.value())
+          in.startup_seqno.value() < required_seqno)
         {
           // Make sure that the joiner's snapshot is more recent than this
           // node's snapshot. Otherwise, the joiner may not be given all the
           // ledger secrets required to replay historical transactions.
           const std::string payload = fmt::format(
             "Node requested to join from seqno {} which is older than this "
-            "node startup seqno {}. A snapshot at least as recent as {} must "
+            "node {} {}. A snapshot at least as recent as {} must "
             "be used instead.",
             in.startup_seqno.value(),
-            this_startup_seqno,
-            this_startup_seqno);
+            using_preferred_bound ? "latest_on_disk_seqno" : "startup_seqno",
+            required_seqno,
+            required_seqno);
           LOG_INFO_FMT("Join request rejected: {}", payload);
           return make_error(
             HTTP_STATUS_BAD_REQUEST, ccf::errors::StartupSeqnoIsOld, payload);

src/node/rpc/serialization.h

@@ -38,7 +38,8 @@ namespace ccf
     node_data,
     sealing_recovery_data,
     code_transparent_statement,
-    ledger_sign_mode);
+    ledger_sign_mode,
+    join_fetch_count);
 
   DECLARE_JSON_TYPE(NetworkIdentity);
   DECLARE_JSON_REQUIRED_FIELDS(NetworkIdentity, cert, priv_key);

tests/reconfiguration.py

@@ -812,6 +812,7 @@ def run_all(args):
         test_ledger_invariants(network, args)
 
     run_join_old_snapshot(args)
+    run_join_no_snapshot_against_original_primary(args)
 
 
 def run_join_old_snapshot(const_args):
@@ -929,3 +930,55 @@ def run_join_old_snapshot(const_args):
                 fetch_recent_snapshot=True,
                 timeout=3,
             )
+
+
+def run_join_no_snapshot_against_original_primary(const_args):
+    # Regression test.
+    # Previously a node which should fetch a snapshot, would not as the lower limit for this was the startup snapshot of the node.
+    # This test ensures that the startup seqno of a joining node is higher than the startup snapshot of the primary
+    txs = app.LoggingTxs("user0")
+    args = deepcopy(const_args)
+    args.nodes = infra.e2e_args.nodes(args, 1)
+    args.label += "_no_snapshot_against_original_primary"
+
+    with infra.network.network(
+        args.nodes,
+        args.binary_dir,
+        args.debug_nodes,
+        pdb=args.pdb,
+        txs=txs,
+    ) as network:
+        network.start_and_open(args)
+        primary, _ = network.find_primary()
+
+        # The original primary started without a snapshot; sanity-check this.
+        with primary.client() as c:
+            body = c.get("/node/state").body.json()
+            assert (
+                body["startup_seqno"] == 0
+            ), f"Original primary should have startup_seqno == 0, got {body['startup_seqno']}"
+
+        # Issue enough transactions for the primary to generate and commit a
+        # snapshot. Wait until that snapshot is on disk.
+        txs.issue(network, number_txs=args.snapshot_tx_interval)
+        committed_snapshots_dir = network.get_committed_snapshots(primary)
+        assert os.listdir(
+            committed_snapshots_dir
+        ), f"Expected committed snapshot in {committed_snapshots_dir}"
+
+        # Assert that fetch_recent_snapshot fetches a snapshot and starts from it
+        new_node = network.create_node()
+        network.join_node(
+            new_node,
+            args.package,
+            args,
+            from_snapshot=False,
+            fetch_recent_snapshot=True,
+            timeout=10,
+        )
+        network.trust_node(new_node, args)
+        with new_node.client() as c:
+            body = c.get("/node/state").body.json()
+            assert (
+                body["startup_seqno"] > 0
+            ), f"Joiner should have started from a fetched snapshot, got startup_seqno={body['startup_seqno']}"