test(lance): cover the runtime config boundary

src/runtime/models/lance/MODEL.toml

@@ -6,5 +6,6 @@ runtime_library = "libtrtmc_model_lance.so"
 runtime_plugins = ["plugin.cpp|register_lance_plugin"]
 runtime_strategies = ["lance_vision_language"]
 runtime_tests = [
+  "test_lance_runtime_config_contract|test_lance_runtime_config_contract.cpp|_|image_preprocessor.cpp|EXTRA_INCLUDES,third_party/stb",
   "test_lance_vl_pipeline|test_lance_vl_pipeline.cpp|trtmc_model_lance,trtmc_backend_trt|_|REQUIRES_TRT,REQUIRES_GPU",
 ]

tests/cpp/models/lance/test_lance_runtime_config_contract.cpp

@@ -0,0 +1,141 @@
+/*
+ * SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+// CPU-only consumer contract for Lance's serialized VL preprocessing config.
+//
+// Lance is the only family whose preprocessing config is resolved from two
+// bundle sections at once: plugin.cpp passes config.json and
+// preprocessor_config.json to lance_parse_preprocess_config(), and the two
+// sources do not merge in a single direction.
+//
+//   patch_size / merge_size / temporal_patch_size -> preprocessor_config.json wins
+//   image_mean / image_std                        -> config.json wins
+//
+// The precedence is opposite for the two groups because the overrides are
+// applied in a fixed order, and nothing pinned either direction. This test
+// fixes both, plus the resample fallback that must stay suppressed whenever
+// config.json states an interpolation of its own.
+
+#include "runtime/models/lance/image_preprocessor.h"
+
+#include <cmath>
+#include <iostream>
+#include <string>
+
+namespace {
+
+bool near(float actual, float expected) {
+    return std::fabs(actual - expected) < 1e-6F;
+}
+
+} // namespace
+
+int main() {
+    bool ok = true;
+    const auto check = [&](bool condition, const char* name) {
+        if (!condition) {
+            std::cerr << "FAIL: " << name << '\n';
+            ok = false;
+        }
+    };
+
+    // Deliberately non-default values throughout, so a consumer that stopped
+    // reading a key and fell back to the struct default fails here.
+    const std::string config = R"({
+        "model_type": "lance",
+        "preprocessor_type": "pad_center_chw",
+        "interpolation": "nearest",
+        "image_token_id": 151655,
+        "fixed_image_size": 392,
+        "patch_size": 16,
+        "merge_size": 3,
+        "temporal_patch_size": 4,
+        "num_image_pad_tokens": 81,
+        "vision_output_dim": 2048,
+        "image_token_str": "<|image_pad|>",
+        "vl_prompt_template": "<|im_start|>user\\n{image_pads}{prompt}<|im_end|>\\n<|im_start|>assistant\\n",
+        "image_mean": [0.5, 0.5, 0.5],
+        "image_std": [0.25, 0.25, 0.25]
+    })";
+
+    // Same three geometry keys, different values, plus its own normalization
+    // triplet that config.json is expected to overrule.
+    const std::string preprocessor_config = R"({
+        "patch_size": 28,
+        "merge_size": 5,
+        "temporal_patch_size": 6,
+        "resample": 2,
+        "image_mean": [0.1, 0.2, 0.3],
+        "image_std": [0.9, 0.8, 0.7]
+    })";
+
+    // The family's own consumer: plugin.cpp builds its LancePreprocessConfig
+    // with exactly this call over the bundle's config.json and
+    // preprocessor_config.json sections.
+    const auto cfg = trtmc::lance_parse_preprocess_config(config, preprocessor_config);
+
+    // Keys that only config.json carries.
+    check(cfg.preprocessor_type == "pad_center_chw", "preprocessor strategy");
+    check(cfg.image_token_id == 151655, "image token id");
+    check(cfg.fixed_image_size == 392, "fixed image size");
+    check(cfg.num_image_pad_tokens == 81, "image pad token count");
+    check(cfg.vision_output_dim == 2048, "vision output dim");
+    check(cfg.image_token_str == "<|image_pad|>", "image placeholder token");
+
+    // Escaped newlines in the prompt template are decoded once, at parse time.
+    check(cfg.vl_prompt_template.find("\\n") == std::string::npos,
+          "prompt template carries no literal backslash-n");
+    check(cfg.vl_prompt_template ==
+              "<|im_start|>user\n{image_pads}{prompt}<|im_end|>\n<|im_start|>assistant\n",
+          "prompt template");
+
+    // Direction one: preprocessor_config.json overrules config.json.
+    check(cfg.patch_size == 28, "patch size comes from preprocessor_config.json");
+    check(cfg.merge_size == 5, "merge size comes from preprocessor_config.json");
+    check(cfg.temporal_patch_size == 6, "temporal patch size comes from preprocessor_config.json");
+
+    // Direction two: config.json overrules preprocessor_config.json.
+    check(near(cfg.image_mean[0], 0.5F) && near(cfg.image_mean[1], 0.5F) &&
+              near(cfg.image_mean[2], 0.5F),
+          "image mean comes from config.json");
+    check(near(cfg.image_std[0], 0.25F) && near(cfg.image_std[1], 0.25F) &&
+              near(cfg.image_std[2], 0.25F),
+          "image std comes from config.json");
+
+    // An interpolation stated in config.json suppresses the resample fallback,
+    // so "resample": 2 must not turn "nearest" into "bilinear".
+    check(cfg.interpolation == "nearest", "stated interpolation survives the resample fallback");
+
+    // With no interpolation stated, resample resolves it: 0 nearest, 2
+    // bilinear, 3 bicubic, anything else leaves the bicubic default.
+    const std::string config_without_interpolation = R"({"model_type": "lance"})";
+    const auto from_resample = [&](const char* preproc) {
+        return trtmc::lance_parse_preprocess_config(config_without_interpolation, preproc)
+            .interpolation;
+    };
+    check(from_resample(R"({"resample": 0})") == "nearest", "resample 0 resolves to nearest");
+    check(from_resample(R"({"resample": 2})") == "bilinear", "resample 2 resolves to bilinear");
+    check(from_resample(R"({"resample": 3})") == "bicubic", "resample 3 resolves to bicubic");
+    check(from_resample(R"({"resample": 7})") == "bicubic", "unknown resample keeps the default");
+
+    // A preprocessor_config.json that omits the geometry keys still overrides
+    // them, resetting config.json's values to the override defaults rather
+    // than preserving them. Pinned because it is the surprising half of
+    // direction one, and a merge that only wrote present keys would change it.
+    const auto reset = trtmc::lance_parse_preprocess_config(config, R"({"resample": 3})");
+    check(reset.patch_size == 14, "an absent patch_size override resets to 14");
+    check(reset.merge_size == 2, "an absent merge_size override resets to 2");
+    check(reset.temporal_patch_size == 2, "an absent temporal_patch_size override resets to 2");
+
+    // With no preprocessor_config.json section at all the overrides are
+    // skipped entirely and config.json's geometry stands.
+    const auto config_only = trtmc::lance_parse_preprocess_config(config, "");
+    check(config_only.patch_size == 16, "config.json patch size stands without an override file");
+    check(config_only.merge_size == 3, "config.json merge size stands without an override file");
+    check(config_only.temporal_patch_size == 4,
+          "config.json temporal patch size stands without an override file");
+
+    return ok ? 0 : 1;
+}