fix: raise on short buffer in get_string/get_wstring

CHANGES.md

@@ -13,6 +13,8 @@ Unreleased
 
 ### Bug fixes
 
+* `get_string` and `get_wstring` now raise `ValueError` when the buffer is shorter than the declared current length,
+  instead of silently returning a truncated value (#945).
 * `get_wstring` compared the declared maximum in bytes with the 16382-character limit, so any WSTRING with a
   capacity of 8192 characters or more was rejected, including ones written by `set_wstring` (#923).
 * The get-block-info request now places the trailing `A` after the block number, as native Snap7 does

snap7/util/getters.py

@@ -218,6 +218,10 @@ def get_string(bytearray_: Buffer, byte_index: int, encoding: str = "latin-1") -
             "String contains {str_length} chars, but max. {max_string_size} chars are expected or is "
             "larger than 254. Bytearray doesn't seem to be a valid string."
         )
+    if byte_index + 2 + str_length > len(bytearray_):
+        raise ValueError(
+            f"String contains {str_length} chars, but only {len(bytearray_) - byte_index - 2} bytes are in the buffer"
+        )
     raw = bytes(bytearray_[byte_index + 2 : byte_index + 2 + str_length])
     return raw.decode(encoding)
 
@@ -795,4 +799,8 @@ def get_wstring(bytearray_: Buffer, byte_index: int) -> str:
         )
 
     wstr_bytes = wstr_chars * 2
+    if wstring_start + wstr_bytes > len(bytearray_):
+        raise ValueError(
+            f"WString contains {wstr_chars} chars, but only {len(bytearray_) - wstring_start} bytes are in the buffer"
+        )
     return bytes(bytearray_[wstring_start : wstring_start + wstr_bytes]).decode("utf-16-be")

tests/test_util.py

@@ -8,7 +8,7 @@
 
 from snap7 import DB, Row
 from snap7.type import Area, WordLen
-from snap7.util import get_byte, get_time, get_fstring, get_int
+from snap7.util import get_byte, get_time, get_fstring, get_int, get_string, get_wstring
 from snap7.util import set_byte, set_time, set_fstring, set_int
 from snap7.util.db import print_row
 
@@ -1495,3 +1495,16 @@ def test_write_with_row_offset(self) -> None:
 
 if __name__ == "__main__":
     unittest.main()
+
+
+@pytest.mark.parametrize(
+    "func,data",
+    [
+        (get_string, bytearray([3, 3, 65])),
+        (get_string, bytearray([5, 4, 65, 66])),
+        (get_wstring, bytearray([0, 3, 0, 3, 0, 65, 0, 66])),
+    ],
+)
+def test_get_string_short_buffer_raises(func, data) -> None:  # type: ignore[no-untyped-def]
+    with pytest.raises(ValueError):
+        func(data, 0)