test(nixos): add S3 multipart upload integration tests

Author: lovesegfault

tests/nixos/s3-binary-cache-store.nix

@@ -597,6 +597,76 @@ in
 
           print("  ✓ File content verified correct (hash matches)")
 
+      @setup_s3()
+      def test_multipart_upload_basic(bucket):
+          """Test basic multipart upload with a large file"""
+          print("\n--- Test: Multipart Upload Basic ---")
+
+          large_file_size = 10 * 1024 * 1024
+          large_pkg = server.succeed(
+              "nix-store --add $(dd if=/dev/urandom of=/tmp/large-file bs=1M count=10 2>/dev/null && echo /tmp/large-file)"
+          ).strip()
+
+          chunk_size = 5 * 1024 * 1024
+          expected_parts = 3  # 10 MB raw becomes ~10.5 MB compressed (NAR + xz overhead)
+
+          store_url = make_s3_url(
+              bucket,
+              **{
+                  "multipart-upload": "true",
+                  "multipart-threshold": str(5 * 1024 * 1024),
+                  "multipart-chunk-size": str(chunk_size),
+              }
+          )
+
+          print(f"  Uploading {large_file_size} byte file (expect {expected_parts} parts)")
+          output = server.succeed(f"{ENV_WITH_CREDS} nix copy --to '{store_url}' {large_pkg} --debug 2>&1")
+
+          if "using S3 multipart upload" not in output:
+              raise Exception("Expected multipart upload to be used")
+
+          expected_msg = f"{expected_parts} parts uploaded"
+          if expected_msg not in output:
+              print("Debug output:")
+              print(output)
+              raise Exception(f"Expected '{expected_msg}' in output")
+
+          print(f"  ✓ Multipart upload used with {expected_parts} parts")
+
+          client.succeed(f"{ENV_WITH_CREDS} nix copy --from '{store_url}' {large_pkg} --no-check-sigs")
+          verify_packages_in_store(client, large_pkg, should_exist=True)
+
+          print("  ✓ Large file downloaded and verified")
+
+      @setup_s3()
+      def test_multipart_threshold(bucket):
+          """Test that files below threshold use regular upload"""
+          print("\n--- Test: Multipart Threshold Behavior ---")
+
+          store_url = make_s3_url(
+              bucket,
+              **{
+                  "multipart-upload": "true",
+                  "multipart-threshold": str(1024 * 1024 * 1024),
+              }
+          )
+
+          print("  Uploading small file with high threshold")
+          output = server.succeed(f"{ENV_WITH_CREDS} nix copy --to '{store_url}' {PKGS['A']} --debug 2>&1")
+
+          if "using S3 multipart upload" in output:
+              raise Exception("Should not use multipart for file below threshold")
+
+          if "using S3 regular upload" not in output:
+              raise Exception("Expected regular upload to be used")
+
+          print("  ✓ Regular upload used for file below threshold")
+
+          client.succeed(f"{ENV_WITH_CREDS} nix copy --no-check-sigs --from '{store_url}' {PKGS['A']}")
+          verify_packages_in_store(client, PKGS['A'], should_exist=True)
+
+          print("  ✓ Small file uploaded and verified")
+
       # ============================================================================
       # Main Test Execution
       # ============================================================================
@@ -626,6 +696,8 @@ in
       test_compression_mixed()
       test_compression_disabled()
       test_nix_prefetch_url()
+      test_multipart_upload_basic()
+      test_multipart_threshold()
 
       print("\n" + "="*80)
       print("✓ All S3 Binary Cache Store Tests Passed!")