bpo-30249: Improve struct.unpack_from() error messages (GH-6059)

2 files changed, 32 insertions, 8 deletions

diff --git a/Modules/_struct.c b/Modules/_struct.c
index 66f74d6..0539706 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1553,7 +1553,8 @@ Return a tuple containing unpacked values.
 
 Values are unpacked according to the format string Struct.format.
 
-The buffer's size in bytes, minus offset, must be at least Struct.size.
+The buffer's size in bytes, starting at position offset, must be
+at least Struct.size.
 
 See help(struct) for more on format strings.
 [clinic start generated code]*/
@@ -1561,16 +1562,38 @@ See help(struct) for more on format strings.
 static PyObject *
 Struct_unpack_from_impl(PyStructObject *self, Py_buffer *buffer,
                         Py_ssize_t offset)
-/*[clinic end generated code: output=57fac875e0977316 input=97ade52422f8962f]*/
+/*[clinic end generated code: output=57fac875e0977316 input=cafd4851d473c894]*/
 {
     assert(self->s_codes != NULL);
 
-    if (offset < 0)
+    if (offset < 0) {
+        if (offset + self->s_size > 0) {
+            PyErr_Format(StructError,
+                         "not enough data to unpack %zd bytes at offset %zd",
+                         self->s_size,
+                         offset);
+            return NULL;
+        }
+
+        if (offset + buffer->len < 0) {
+            PyErr_Format(StructError,
+                         "offset %zd out of range for %zd-byte buffer",
+                         offset,
+                         buffer->len);
+            return NULL;
+        }
         offset += buffer->len;
-    if (offset < 0 || buffer->len - offset < self->s_size) {
+    }
+
+    if ((buffer->len - offset) < self->s_size) {
         PyErr_Format(StructError,
-            "unpack_from requires a buffer of at least %zd bytes",
-            self->s_size);
+                     "unpack_from requires a buffer of at least %zu bytes for "
+                     "unpacking %zd bytes at offset %zd "
+                     "(actual buffer size is %zd)",
+                     (size_t)self->s_size + (size_t)offset,
+                     self->s_size,
+                     offset,
+                     buffer->len);
         return NULL;
     }
     return s_unpack_internal(self, (char*)buffer->buf + offset);
diff --git a/Modules/clinic/_struct.c.h b/Modules/clinic/_struct.c.h
index 6e43215..2ecadfb 100644
--- a/Modules/clinic/_struct.c.h
+++ b/Modules/clinic/_struct.c.h
@@ -79,7 +79,8 @@ PyDoc_STRVAR(Struct_unpack_from__doc__,
 "\n"
 "Values are unpacked according to the format string Struct.format.\n"
 "\n"
-"The buffer\'s size in bytes, minus offset, must be at least Struct.size.\n"
+"The buffer\'s size in bytes, starting at position offset, must be\n"
+"at least Struct.size.\n"
 "\n"
 "See help(struct) for more on format strings.");
 
@@ -302,4 +303,4 @@ exit:
 
     return return_value;
 }
-/*[clinic end generated code: output=9119f213a951e4cc input=a9049054013a1b77]*/
+/*[clinic end generated code: output=d79b009652ae0b89 input=a9049054013a1b77]*/