Keep complete-source synchronization byte exact

.agents/skills/write-bpf-production-tutorial/references/drafting-process.md

@@ -10,6 +10,8 @@ Add a source-fidelity ledger. Inventory every supplied fact, number, command, co
 
 Add a code inventory that identifies the complete core files to reproduce in the tutorial. Include the kernel-mode eBPF source, user-space source, and any shared header needed to understand their contract. Keep tests, generated skeletons, vendored code, and build boilerplate as links unless the teaching mechanism depends on them.
 
+Every inventoried source must be non-empty UTF-8 text with LF line endings and a final LF byte. A fenced Markdown block cannot represent the absence of the final newline without changing the payload, so the synchronizer rejects an empty source or a source without the final LF instead of silently changing it. Do not normalize or edit an implementation file merely to make documentation synchronization pass without explicit authorization.
+
 ## Establish the teaching path
 
 Write one sentence for the tutorial's promise and one sentence for its reader. Map each planned section to a distinct reader question. Confirm that the path covers the problem, relevant background, whole-system flow, complete code, focused kernel explanation, brief user-space explanation, deeper concepts, compilation and execution, real output, requirements, failure and cleanup behavior, limitations, summary, and references.

.agents/skills/write-bpf-production-tutorial/scripts/sync-source-blocks.py

@@ -78,13 +78,19 @@ def render(
             raise ValueError(f"missing matching generated fence and {END!r} after {source_name}")
 
         language = LANGUAGES[source.suffix]
-        source_text = source.read_bytes().decode("utf-8")
+        source_bytes = source.read_bytes()
+        if not source_bytes:
+            raise ValueError(f"complete source cannot be empty: {source_name}")
+        if not source_bytes.endswith(b"\n"):
+            raise ValueError(
+                f"complete source must end with an LF byte for exact fenced inclusion: {source_name}"
+            )
+        source_text = source_bytes.decode("utf-8")
         if "\r" in source_text:
             raise ValueError(f"complete source must use LF line endings: {source_name}")
         max_backticks = max((len(run) for run in re.findall(r"`+", source_text)), default=0)
         generated_fence = "`" * max(3, max_backticks + 1)
-        if source_text.endswith("\n"):
+        source_text = source_text[:-1]
-            source_text = source_text[:-1]
         rendered.extend(
             [
                 lines[index],

.agents/skills/write-bpf-production-tutorial/scripts/test_sync_source_blocks.py

@@ -214,6 +214,50 @@ class SyncSourceBlocksTest(unittest.TestCase):
         self.assertEqual(first_english, english.read_bytes())
         self.assertEqual(first_chinese, chinese.read_bytes())
 
+    def test_rejects_empty_source_without_changing_either_readme(self) -> None:
+        source = self.root / "src/lesson/tool.c"
+        source.write_bytes(b"")
+        english = self.write_readme("src/lesson/README.md")
+        chinese = self.write_readme("src/lesson/README.zh.md")
+        before = (english.read_bytes(), chinese.read_bytes())
+
+        result = self.run_script(
+            "--readme", "src/lesson/README.md",
+            "--readme", "src/lesson/README.zh.md",
+            "--write",
+        )
+        self.assertEqual(1, result.returncode)
+        self.assertIn("complete source cannot be empty", result.stderr)
+        self.assertEqual(before, (english.read_bytes(), chinese.read_bytes()))
+
+    def test_rejects_source_without_final_lf(self) -> None:
+        source = self.root / "src/lesson/tool.c"
+        source.write_bytes(b"int value = 1;")
+        self.write_readme("src/lesson/README.md")
+        self.write_readme("src/lesson/README.zh.md")
+
+        result = self.run_script(
+            "--readme", "src/lesson/README.md",
+            "--readme", "src/lesson/README.zh.md",
+            "--write",
+        )
+        self.assertEqual(1, result.returncode)
+        self.assertIn("must end with an LF byte", result.stderr)
+
+    def test_preserves_multiple_trailing_lfs(self) -> None:
+        source = self.root / "src/lesson/tool.c"
+        source.write_bytes(b"int value = 1;\n\n\n")
+        english = self.write_readme("src/lesson/README.md")
+        self.write_readme("src/lesson/README.zh.md")
+
+        result = self.run_script(
+            "--readme", "src/lesson/README.md",
+            "--readme", "src/lesson/README.zh.md",
+            "--write",
+        )
+        self.assertEqual(0, result.returncode, result.stderr)
+        self.assertIn("int value = 1;\n\n\n```", english.read_text(encoding="utf-8"))
+
     def test_requires_bilingual_pair(self) -> None:
         self.write_readme("src/lesson/README.md")
         result = self.run_script(