From 1e5962b7860e2e5fc91e89721a176e4f6e61d135 Mon Sep 17 00:00:00 2001
From: Ivan Levkivskyi <levkivskyi@gmail.com>
Date: Wed, 8 Oct 2025 02:24:18 +0100
Subject: [PATCH] Fix infinite loop on empty Buffer

---
 mypyc/build.py                   | 1 -
 mypyc/lib-rt/librt_internal.c    | 8 +++++---
 mypyc/test-data/run-classes.test | 9 +++++++++
 mypyc/test/test_run.py           | 6 ++++++
 4 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/mypyc/build.py b/mypyc/build.py
index 40638b31d000..13648911c0b5 100644
--- a/mypyc/build.py
+++ b/mypyc/build.py
@@ -664,7 +664,6 @@ def mypycify(
             )
 
     if install_librt:
-        os.makedirs("librt", exist_ok=True)
         for name in RUNTIME_C_FILES:
             rt_file = os.path.join(build_dir, name)
             with open(os.path.join(include_dir(), name), encoding="utf-8") as f:
diff --git a/mypyc/lib-rt/librt_internal.c b/mypyc/lib-rt/librt_internal.c
index cb9aa1025821..b97d6665b515 100644
--- a/mypyc/lib-rt/librt_internal.c
+++ b/mypyc/lib-rt/librt_internal.c
@@ -70,12 +70,14 @@ Buffer_init_internal(BufferObject *self, PyObject *source) {
             PyErr_SetString(PyExc_TypeError, "source must be a bytes object");
             return -1;
         }
-        self->size = PyBytes_GET_SIZE(source);
-        self->end = self->size;
+        self->end = PyBytes_GET_SIZE(source);
+        // Allocate at least one byte to simplify resizing logic.
+        // The original bytes buffer has last null byte, so this is safe.
+        self->size = self->end + 1;
         // This returns a pointer to internal bytes data, so make our own copy.
         char *buf = PyBytes_AsString(source);
         self->buf = PyMem_Malloc(self->size);
-        memcpy(self->buf, buf, self->size);
+        memcpy(self->buf, buf, self->end);
     } else {
         self->buf = PyMem_Malloc(START_SIZE);
         self->size = START_SIZE;
diff --git a/mypyc/test-data/run-classes.test b/mypyc/test-data/run-classes.test
index 8755169fdb0b..84704ce66c81 100644
--- a/mypyc/test-data/run-classes.test
+++ b/mypyc/test-data/run-classes.test
@@ -2866,6 +2866,15 @@ test_buffer_roundtrip_interpreted()
 test_buffer_int_size_interpreted()
 test_buffer_str_size_interpreted()
 
+[case testBufferEmpty_librt_internal]
+from librt.internal import Buffer, write_int, read_int
+
+def test_empty() -> None:
+    b = Buffer(b"")
+    write_int(b, 42)
+    b1 = Buffer(b.getvalue())
+    assert read_int(b1) == 42
+
 [case testEnumMethodCalls]
 from enum import Enum
 from typing import overload, Optional, Union
diff --git a/mypyc/test/test_run.py b/mypyc/test/test_run.py
index 22ab18e97293..953f61329395 100644
--- a/mypyc/test/test_run.py
+++ b/mypyc/test/test_run.py
@@ -285,6 +285,12 @@ def run_case_step(self, testcase: DataDrivenTestCase, incremental_step: int) ->
                 )
             )
 
+        if librt:
+            # This hack forces Python to prefer the local "installation".
+            os.makedirs("librt", exist_ok=True)
+            with open(os.path.join("librt", "__init__.py"), "a"):
+                pass
+
         if not run_setup(setup_file, ["build_ext", "--inplace"]):
             if testcase.config.getoption("--mypyc-showc"):
                 show_c(cfiles)