feat(raft-log-replication): Log Matching assertion helper (task 8.3)

raft_sim_check_log_matching: for every node pair and every shared index i, if
both entries at i have equal term, assert the whole prefix 1..i is term-and-byte
identical (Req 8.4, raft.tla Log Matching). Read-only, framework-agnostic (returns
1/0), bounded. Self-tests: pass on a converged cluster, fail on a hand-divergent
pair sharing (i=2,term=7) but differing at the index-1 prefix. ASAN green.

src/lib/raft/raft.h

@@ -498,6 +498,21 @@ void raft_sim_fire_timeout(raft_sim_cluster_t *c, int node_idx);
  */
 void raft_sim_partition(raft_sim_cluster_t *c, int node_idx, int isolated);
 
+/*
+ * raft_sim_check_log_matching: assert the Raft Log Matching property over the
+ * cluster's in-memory logs (Req 8.4).
+ *
+ * For every pair of nodes (a, b) and every index i present in BOTH logs
+ * (1 <= i <= min(log_count_a, log_count_b)), if the entries at i carry the
+ * SAME term, then the logs must be identical for all indices 1..i — same term
+ * AND same payload (length + bytes) at every k in 1..i.
+ *
+ * Returns 1 if Log Matching holds across the whole cluster, 0 if any pair/index
+ * violates it. Pure read-only over c (no I/O, no Unity assertions) so the lib
+ * stays test-framework-agnostic; the caller asserts on the returned value.
+ */
+int raft_sim_check_log_matching(const raft_sim_cluster_t *c);
+
 #endif /* XCE_TESTING */
 
 #endif /* RAFT_H */

src/lib/raft/raft_sim.c

@@ -373,4 +373,44 @@ void raft_sim_partition(raft_sim_cluster_t *c, int node_idx, int isolated)
         c->nodes[node_idx].partition = isolated;
 }
 
+/* Two log entries are identical iff their term, length, and payload bytes
+ * (over the recorded length) all match. */
+static int sim_entry_identical(const raft_sim_entry_t *a,
+                               const raft_sim_entry_t *b)
+{
+    if (a->term != b->term || a->len != b->len)
+        return 0;
+    return memcmp(a->bytes, b->bytes, a->len) == 0;
+}
+
+int raft_sim_check_log_matching(const raft_sim_cluster_t *c)
+{
+    /* For every ordered-pair-free pair (a < b) and every index i present in
+     * both logs: if the entries at i carry the same term, the prefixes 1..i
+     * must be entry-identical. Pairs are symmetric, so a < b suffices. */
+    for (int a = 0; a < c->n; a++) {
+        const raft_sim_node_t *na = &c->nodes[a];
+        for (int b = a + 1; b < c->n; b++) {
+            const raft_sim_node_t *nb = &c->nodes[b];
+
+            int shared = na->log_count < nb->log_count
+                       ? na->log_count : nb->log_count;
+
+            for (int i = 1; i <= shared; i++) {
+                const raft_sim_entry_t *ea = &na->log[i - 1];
+                const raft_sim_entry_t *eb = &nb->log[i - 1];
+
+                /* Same (index, term) ⇒ prefixes through i must be identical. */
+                if (ea->term != eb->term)
+                    continue;
+                for (int k = 1; k <= i; k++) {
+                    if (!sim_entry_identical(&na->log[k - 1], &nb->log[k - 1]))
+                        return 0; /* Log Matching violated */
+                }
+            }
+        }
+    }
+    return 1; /* holds */
+}
+
 #endif /* XCE_TESTING */

src/lib/testsuite/test_raft.c

@@ -3100,6 +3100,100 @@ static void test_sim_deliver_append_entries_replicates(void)
     TEST_ASSERT_EQUAL_INT(0, c.nodes[2].log_count);
 }
 
+/*
+ * test_sim_check_log_matching_pass_on_converged (task 8.3)
+ *
+ * Log Matching helper must return PASS (1) when two nodes hold identical logs.
+ * Build a 3-node cluster, elect node 0, append two entries, replicate both to
+ * node 1; assert raft_sim_check_log_matching == 1 and node 1's prefix matches
+ * node 0's byte-for-byte.
+ */
+static void test_sim_check_log_matching_pass_on_converged(void)
+{
+    raft_sim_cluster_t c;
+    memset(&c, 0, sizeof(c));
+
+    TEST_ASSERT_EQUAL_INT(0, raft_sim_init(&c, 3, 2000));
+
+    raft_start_election(&c.nodes[0].state);
+    raft_sim_deliver_pending(&c);
+    TEST_ASSERT_EQUAL_INT(RAFT_LEADER, c.nodes[0].state.role);
+
+    /* Append two client entries on the leader and replicate each to node 1.
+     * One AppendEntries carries at most one entry, so replicate index by index. */
+    unsigned char p1[3] = { 0x01, 0x02, 0x03 };
+    unsigned char p2[3] = { 0x04, 0x05, 0x06 };
+    uint64_t out_index = 0;
+
+    TEST_ASSERT_EQUAL_INT(RAFT_OK,
+        raft_log_append(&c.nodes[0].state, p1, sizeof(p1), &out_index));
+    TEST_ASSERT_EQUAL_UINT64(1, out_index);
+    TEST_ASSERT_EQUAL_INT(RAFT_OK,
+        raft_send_append_entries(&c.nodes[0].state, 1, /*leader_commit=*/0));
+    raft_sim_deliver_append_entries(&c);
+
+    TEST_ASSERT_EQUAL_INT(RAFT_OK,
+        raft_log_append(&c.nodes[0].state, p2, sizeof(p2), &out_index));
+    TEST_ASSERT_EQUAL_UINT64(2, out_index);
+    TEST_ASSERT_EQUAL_INT(RAFT_OK,
+        raft_send_append_entries(&c.nodes[0].state, 1, /*leader_commit=*/0));
+    raft_sim_deliver_append_entries(&c);
+
+    /* Node 1 holds the leader's prefix byte-for-byte. */
+    TEST_ASSERT_EQUAL_INT(2, c.nodes[0].log_count);
+    TEST_ASSERT_EQUAL_INT(2, c.nodes[1].log_count);
+
+    /* Log Matching holds across the cluster. */
+    TEST_ASSERT_EQUAL_INT(1, raft_sim_check_log_matching(&c));
+}
+
+/*
+ * test_sim_check_log_matching_fail_on_divergence (task 8.3)
+ *
+ * Log Matching helper must return FAIL (0) when two nodes share the same
+ * (index, term) at some i but their prefixes differ. Hand-craft node 0 and
+ * node 1 so that index 2 carries the SAME term but index 1 differs in bytes —
+ * a Log Matching violation (equal (i,term) but non-identical prefix through i).
+ */
+static void test_sim_check_log_matching_fail_on_divergence(void)
+{
+    raft_sim_cluster_t c;
+    memset(&c, 0, sizeof(c));
+
+    TEST_ASSERT_EQUAL_INT(0, raft_sim_init(&c, 3, 2000));
+
+    /* Hand-craft node 0: index1=(term 5, "AAA"), index2=(term 7, "XXX"). */
+    c.nodes[0].log[0].term = 5;
+    c.nodes[0].log[0].bytes[0] = 'A';
+    c.nodes[0].log[0].bytes[1] = 'A';
+    c.nodes[0].log[0].bytes[2] = 'A';
+    c.nodes[0].log[0].len = 3;
+    c.nodes[0].log[1].term = 7;
+    c.nodes[0].log[1].bytes[0] = 'X';
+    c.nodes[0].log[1].bytes[1] = 'X';
+    c.nodes[0].log[1].bytes[2] = 'X';
+    c.nodes[0].log[1].len = 3;
+    c.nodes[0].log_count = 2;
+
+    /* Hand-craft node 1: SAME (index2, term 7) but a DIFFERENT index1 prefix
+     * (term 5 but bytes "BBB"). This must be flagged: equal (i=2, term=7) yet
+     * the prefix through 2 is not byte-identical. */
+    c.nodes[1].log[0].term = 5;
+    c.nodes[1].log[0].bytes[0] = 'B';
+    c.nodes[1].log[0].bytes[1] = 'B';
+    c.nodes[1].log[0].bytes[2] = 'B';
+    c.nodes[1].log[0].len = 3;
+    c.nodes[1].log[1].term = 7;
+    c.nodes[1].log[1].bytes[0] = 'X';
+    c.nodes[1].log[1].bytes[1] = 'X';
+    c.nodes[1].log[1].bytes[2] = 'X';
+    c.nodes[1].log[1].len = 3;
+    c.nodes[1].log_count = 2;
+
+    /* Same (index2, term7) with divergent index1 ⇒ Log Matching violated. */
+    TEST_ASSERT_EQUAL_INT(0, raft_sim_check_log_matching(&c));
+}
+
 /* ---------- Group A: Basic Election Correctness (task 5.1) ---------- */
 
 /*
@@ -5348,6 +5442,8 @@ void test_raft(void)
     RUN_TEST(test_sim_partition_no_quorum);
     RUN_TEST(test_sim_fire_timeout_steps_down);
     RUN_TEST(test_sim_deliver_append_entries_replicates);
+    RUN_TEST(test_sim_check_log_matching_pass_on_converged);
+    RUN_TEST(test_sim_check_log_matching_fail_on_divergence);
 
     /* Group A: basic election correctness (task 5.1) */
     RUN_TEST(test_proto_A1_single_node_self_elects);