test(bench): add F17 forward-iteration coverage symmetric to F16

tests/bench/bench_failure_modes.c

@@ -7,7 +7,7 @@
  * bench_failure_modes.c
  * vim: expandtab:ts=4:sts=4:sw=4
  *
- * F1–F16 failure-injection tests. Each test crafts a deliberately corrupt
+ * F1–F17 failure-injection tests. Each test crafts a deliberately corrupt
  * or pathological flat-file and asserts how the backend handles it:
  *
  *   F1   Truncated last line (crash mid-fwrite simulation)
@@ -22,6 +22,7 @@
  *   F14  mtime/inode flip — file replaced under us, ensure_fresh must rebuild
  *   F15  Empty file (0 bytes) — verify reports INFO and continues
  *   F16  Page-Up cursor must not stick on entries[0] (regression guard)
+ *   F17  Forward iteration via start_time covers every message (no gaps)
  *
  * Each test prints PASS/FAIL with detail and writes a CSV row:
  *   F#, "failure", file_size, line_count, wall_ms, peak_rss_kb, note
@@ -692,6 +693,112 @@ test_F16(fail_ctx_t* ctx)
     be->close();
 }
 
+// ---------------------------------------------------------------------------
+// F17 — Forward iteration (start_time + from_start=TRUE) reaches every
+// message without gaps.
+//
+// Symmetric counterpart to F16: instead of paging from newest backwards
+// via end_time, page from oldest forwards via start_time. Each iteration
+// passes start_time = timestamp of the newest message returned in the
+// previous batch; the backend should return the next batch of strictly
+// newer messages until the file is exhausted.
+//
+// This exercises a different branch of _flatfile_get_previous_chat:
+//   - read_from = ff_state_offset_for_time(start_time)  (not bom_len)
+//   - read_to = EOF                                     (no end_time bisect)
+//   - filter drops msgs with timestamp <= start_dt
+//   - pagination keeps FIRST MESSAGES_TO_RETRIEVE       (not last)
+// The path bypasses cursor entirely (start_time triggers cursor=-1 reset).
+//
+// Pass criterion: total messages received == total written.
+
+static void
+test_F17(fail_ctx_t* ctx)
+{
+    if (!test_enabled(ctx->tests, "F17")) return;
+    auto_gchar gchar* path = setup_test_dir(ctx->tmp_dir, "F17");
+
+    const int total_msgs = 2000;
+    FILE* fp = fopen(path, "w");
+    fprintf(fp, "%s", FLATFILE_HEADER);
+    for (int i = 0; i < total_msgs; i++)
+        write_normal_line(fp, i, "forward-iter regression payload");
+    fclose(fp);
+
+    g_free(g_flatfile_account_jid);
+    g_flatfile_account_jid = g_strdup(ACCOUNT_JID);
+    setenv("BENCH_ACCOUNT_JID", ACCOUNT_JID, 1);
+
+    db_backend_t* be = db_backend_flatfile();
+    if (!be || !be->init || !be->get_previous_chat || !be->close) {
+        report(ctx, "F17", FALSE, 0, path, "flatfile backend not available");
+        return;
+    }
+    ProfAccount fake = { 0 };
+    fake.jid = (gchar*)ACCOUNT_JID;
+    if (!be->init(&fake)) {
+        report(ctx, "F17", FALSE, 0, path, "flatfile init failed");
+        return;
+    }
+
+    int total_received = 0;
+    int empty_responses = 0;
+    int iterations = 0;
+    // Initial start_time before the first written message so the first
+    // call returns msgs starting at index 0.
+    auto_gchar gchar* start_time = g_strdup("2025-06-15T11:00:00Z");
+
+    double t0 = bench_now_ms();
+
+    while (iterations < 200) {
+        iterations++;
+        GSList* batch = NULL;
+        db_history_result_t r = be->get_previous_chat(
+            CONTACT_JID, start_time, NULL, TRUE, FALSE, &batch);
+
+        if (r == DB_RESPONSE_EMPTY) {
+            empty_responses++;
+            break;
+        }
+        if (r != DB_RESPONSE_SUCCESS || !batch) {
+            empty_responses++;
+            break;
+        }
+
+        // pre_result is sorted oldest-first; with from_start=TRUE we keep
+        // the first MESSAGES_TO_RETRIEVE. The newest in the returned batch
+        // is therefore the LAST element of the list.
+        ProfMessage* newest = NULL;
+        int batch_n = 0;
+        for (GSList* l = batch; l; l = l->next) {
+            batch_n++;
+            newest = l->data;
+        }
+        if (!newest || !newest->timestamp) {
+            g_slist_free_full(batch, (GDestroyNotify)message_free);
+            break;
+        }
+        total_received += batch_n;
+
+        g_free(start_time);
+        start_time = g_date_time_format_iso8601(newest->timestamp);
+
+        g_slist_free_full(batch, (GDestroyNotify)message_free);
+    }
+
+    double dt = bench_now_ms() - t0;
+
+    gboolean ok = (total_received == total_msgs)
+                  && (empty_responses == 1)
+                  && (iterations < 200);
+
+    auto_gchar gchar* note = g_strdup_printf(
+        "wrote=%d received=%d iters=%d empty=%d (forward-iter regression)",
+        total_msgs, total_received, iterations, empty_responses);
+    report(ctx, "F17", ok, dt, path, note);
+    be->close();
+}
+
 // ---------------------------------------------------------------------------
 // Driver
 
@@ -732,6 +839,7 @@ main(int argc, char** argv)
     test_F14(&ctx);
     test_F15(&ctx);
     test_F16(&ctx);
+    test_F17(&ctx);
 
     fprintf(stderr, "\nfailure-modes summary: %d passed, %d failed\n",
             ctx.passed, ctx.failed);