fix panic with Python 3.13 shim frames (#787)

File/function names in some Py3.13 frames are empty strings.

It appears that the caller (get_stack_trace) handles this condition and
expects the callee (copy_string) to return an error. The callee however
panics by copying zero bytes into a vector and then calling as_ptr on
it, which returns a wild pointer (0x01), which is then passed to
std::slice::from_raw_parts. This is not OK even if the length passed to
from_raw_parts is 0.

Author: karolinepauls

src/python_data_access.rs

@@ -15,6 +15,9 @@ pub fn copy_string<T: StringObject, P: ProcessMemory>(
     process: &P,
 ) -> Result<String, Error> {
     let obj = process.copy_pointer(ptr)?;
+    if obj.size() == 0 {
+        return Ok(String::new());
+    }
     if obj.size() >= 4096 {
         return Err(format_err!(
             "Refusing to copy {} chars of a string",

src/stack_trace.rs

@@ -155,7 +155,6 @@ where
         // would also have to figure out what the address of PyCode_Type is (which will be
         // easier if something like https://github.com/python/cpython/issues/100987#issuecomment-1487227139
         // is merged )
-        // Unset file/function name in py3.13 means this is a shim.
         if filename.is_err() || name.is_err() {
             frame_ptr = frame.back();
             set_last_frame_as_shim_entry(&mut frames);
@@ -165,7 +164,8 @@ where
         let name = name?;
 
         // skip <shim> entries in python 3.12+
-        if filename == "<shim>" {
+        // Unset file/function name in py3.13 means this is a shim.
+        if filename.is_empty() || filename == "<shim>" {
             frame_ptr = frame.back();
             set_last_frame_as_shim_entry(&mut frames);
             continue;

tests/integration_test.rs

@@ -280,7 +280,7 @@ fn test_local_vars() {
     let frame = &trace.frames[0];
     let locals = frame.locals.as_ref().unwrap();
 
-    assert_eq!(locals.len(), 28);
+    assert_eq!(locals.len(), 29);
 
     let arg1 = &locals[0];
     assert_eq!(arg1.name, "arg1");
@@ -422,6 +422,11 @@ fn test_local_vars() {
     let end = unicode_val.char_indices().map(|(i, _)| i).nth(4).unwrap();
     assert_eq!(unicode_val[0..end], *"\"测试1");
 
+    // Empty string
+    let local26 = &locals[28];
+    assert_eq!(local26.name, "local26");
+    assert_eq!(local26.repr, Some("\"\"".to_string()));
+
     // we only support dictionary lookup on python 3.6+ right now
     if runner.spy.version.major == 3 && runner.spy.version.minor >= 6 {
         assert_eq!(

tests/scripts/local_vars.py

@@ -42,6 +42,9 @@ def local_variable_lookup(arg1="foo", arg2=None, arg3=True):
     # https://github.com/benfred/py-spy/issues/766
     local25 = "测试1" * 500
 
+    # Empty strings should not be ignored
+    local26 = ""
+
     time.sleep(100000)