Add test when there are no segments (#12646)

Summary:

The deprecation warning should only appear when there are values inside the constant_buffer and nothing inside the constant_segment.

Even when there are no constants in the program, we expect to have one 'segment' storing the placeholder for non-constants.

Fix failing jobs caused by error logging from #12295
Test failures: https://hud.pytorch.org/pr/pytorch/executorch/12631#46239138027

Arm test is using a recently-generated PTE, however hitting the deprecation warning because the pte has no constants.

Differential Revision: D78568839

Author: lucylq

runtime/executor/program.cpp

@@ -217,18 +217,21 @@ Result<executorch_flatbuffer::ExecutionPlan*> get_execution_plan(
         std::move(constant_segment_data.get()),
         std::move(pte_data_map));
   } else {
-    // The constant data is stored inside the flatbuffer, so this program does
-    // not contain a separate segment for it.
-
-    // NOTE: This branch is deprecated from ExecuTorch 0.7 onwards.
-    // Please regenerate your PTE file to ensure newer ExecuTorch runtimes can
-    // support it. ExecuTorch deprecation policy:
-    // https://docs.pytorch.org/executorch/stable/api-life-cycle.html#deprecation-policy.
-    // For support, contact the PyTorch Edge team or make an issue in:
-    // https://github.com/pytorch/executorch/issues.
-    ET_LOG(
-        Error,
-        "!!DEPRECATED!! This branch is deprecated from ExecuTorch 0.7; re-export this PTE file to ensure support on newer runtimes.");
+    if (constant_segment->offsets()->size() == 0) {
+      // NOTE: Storing constants in the constant_buffer is deprecated from
+      // ExecuTorch 0.7 onwards. Please regenerate your PTE file to ensure newer
+      // ExecuTorch runtimes can support it. ExecuTorch deprecation policy:
+      // https://docs.pytorch.org/executorch/stable/api-life-cycle.html#deprecation-policy.
+      // For support, contact the PyTorch Edge team or make an issue in:
+      // https://github.com/pytorch/executorch/issues.
+      ET_LOG(
+          Error,
+          "!!DEPRECATED!! This branch is deprecated from ExecuTorch 0.7; re-export this PTE file to ensure support on newer runtimes.");
+    }
+
+    // The constant data is either stored inside the flatbuffer, or there are
+    // no constants; the constant segment is empty and does not
+    // need to be loaded.
     return Program(
         loader,
         segment_base_offset,

runtime/executor/test/program_test.cpp

@@ -457,6 +457,49 @@ TEST_F(ProgramTest, LoadConstantSegment) {
   EXPECT_GE(flatbuffer_program->constant_segment()->offsets()->size(), 1);
 }
 
+TEST_F(ProgramTest, LoadConstantSegment_WhenEmpty) {
+  // Load the serialized ModuleAddMul data.
+  const char* add_path = std::getenv("ET_MODULE_ADD_PATH");
+  Result<FileDataLoader> add_loader = FileDataLoader::from(add_path);
+  ASSERT_EQ(add_loader.error(), Error::Ok);
+
+  // This file should always be compatible.
+  Result<FreeableBuffer> add_header = add_loader->load(
+      /*offset=*/0,
+      Program::kMinHeadBytes,
+      /*segment_info=*/
+      DataLoader::SegmentInfo(DataLoader::SegmentInfo::Type::Program));
+  ASSERT_EQ(add_header.error(), Error::Ok);
+  EXPECT_EQ(
+      Program::check_header(add_header->data(), add_header->size()),
+      Program::HeaderStatus::CompatibleVersion);
+
+  Result<Program> program = Program::load(&add_loader.get());
+  ASSERT_EQ(program.error(), Error::Ok);
+
+  // Expect no real segments when we try to load it.
+  const auto segment_info = DataLoader::SegmentInfo(
+      DataLoader::SegmentInfo::Type::Constant,
+      /*segment_index_=*/0);
+  Result<FreeableBuffer> segment =
+      ProgramTestFriend::LoadSegment(&program.get(), segment_info);
+  EXPECT_EQ(segment.error(), Error::NotFound);
+
+  const executorch_flatbuffer::Program* flatbuffer_program =
+      ProgramTestFriend::GetInternalProgram(&program.get());
+
+  // Expect empty data segment for the placeholder constant.
+  EXPECT_EQ(flatbuffer_program->segments()->size(), 1);
+
+  // The constant buffer should be empty.
+  EXPECT_EQ(flatbuffer_program->constant_buffer()->size(), 0);
+
+  // Check constant segment offsets.
+  EXPECT_EQ(flatbuffer_program->constant_segment()->segment_index(), 0);
+  // Expect one offset for the placeholder constant.
+  EXPECT_GE(flatbuffer_program->constant_segment()->offsets()->size(), 1);
+}
+
 TEST_F(ProgramTest, LoadConstantSegmentWhenConstantBufferExists) {
   // Load the serialized ModuleAddMul data, with constants in the flatbuffer and
   // no constants in the segment.