From 0079346d748f262a41f75096df1083ea8a4fa6a4 Mon Sep 17 00:00:00 2001
From: Matthew Treinish <mtreinish@kortar.org>
Date: Fri, 20 Oct 2023 16:06:19 -0400
Subject: [PATCH] Emit a descriptive error when the QPY version is too new

This commit updates the qpy load() function to ensure we are raising a
descriptive error message when a user attempts to load a qpy version
that's not supported by this version of qiskit. Previously it would
fail but with a hard to understand internal error message which was just
confusing and not helpful. For example, when trying to load a QPY
version 10 payload with qiskit 0.25.2 (which only supported up to
version 9) it would raise an error message like:

```
Invalid payload format data kind 'b'p''."
```

which doesn't tell you anything meaningful unless you are intimately
familiar with the QPY file format and how the load() function works.
With this commit it now checks if the version number is supported
immediately and if it's too new it will raise an error message that says
exactly that. So in the above scenario instead of that error message it
will say:

```
The QPY format version being read, 10, isn't supported by this Qiskit
version. Please upgrade your version of Qiskit to load this QPY payload.
```
---
 qiskit/qpy/interface.py                       |  6 ++++++
 ...-message-qpy-version-cf0763da22ce2224.yaml |  9 +++++++++
 test/python/qpy/test_circuit_load_from_qpy.py | 19 ++++++++++++++++++-
 3 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 releasenotes/notes/fix-error-message-qpy-version-cf0763da22ce2224.yaml

diff --git a/qiskit/qpy/interface.py b/qiskit/qpy/interface.py
index 9886160dacc0..f4a77ec14598 100644
--- a/qiskit/qpy/interface.py
+++ b/qiskit/qpy/interface.py
@@ -231,6 +231,12 @@ def load(
     version = struct.unpack("!6sB", file_obj.read(7))[1]
     file_obj.seek(0)
 
+    if version > common.QPY_VERSION:
+        raise QiskitError(
+            f"The QPY format version being read, {version}, isn't supported by "
+            "this Qiskit version. Please upgrade your version of Qiskit to load this QPY payload"
+        )
+
     if version < 10:
         data = formats.FILE_HEADER._make(
             struct.unpack(
diff --git a/releasenotes/notes/fix-error-message-qpy-version-cf0763da22ce2224.yaml b/releasenotes/notes/fix-error-message-qpy-version-cf0763da22ce2224.yaml
new file mode 100644
index 000000000000..67690c011126
--- /dev/null
+++ b/releasenotes/notes/fix-error-message-qpy-version-cf0763da22ce2224.yaml
@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixed an issue with :func:`.qpy.load` when attempting to load a QPY format
+    version that is not supported by this version of Qiskit it will now display
+    a descriptive error message. Previously, it would raise an internal error
+    because of the incompatibility between the formats which was difficult to
+    debug. If the QPY format verison is not supported that indicates the Qiskit
+    version will need to be upgraded to read the QPY payload.
diff --git a/test/python/qpy/test_circuit_load_from_qpy.py b/test/python/qpy/test_circuit_load_from_qpy.py
index 3684e8aacfe6..ba6aeb81f290 100644
--- a/test/python/qpy/test_circuit_load_from_qpy.py
+++ b/test/python/qpy/test_circuit_load_from_qpy.py
@@ -13,12 +13,15 @@
 """Test cases for the schedule block qpy loading and saving."""
 
 import io
+import struct
 
 from ddt import ddt, data
 
 from qiskit.circuit import QuantumCircuit, QuantumRegister, Qubit
 from qiskit.providers.fake_provider import FakeHanoi, FakeSherbrooke
-from qiskit.qpy import dump, load
+from qiskit.exceptions import QiskitError
+from qiskit.qpy import dump, load, formats
+from qiskit.qpy.common import QPY_VERSION
 from qiskit.test import QiskitTestCase
 from qiskit.transpiler import PassManager, TranspileLayout
 from qiskit.transpiler import passes
@@ -71,6 +74,20 @@ def test_rzx_calibration_echo(self, angle):
         self.assert_roundtrip_equal(rzx_qc)
 
 
+class TestVersions(QpyCircuitTestCase):
+    """Test version handling in qpy."""
+
+    def test_invalid_qpy_version(self):
+        """Test a descriptive exception is raised if QPY version is too new."""
+        with io.BytesIO() as buf:
+            buf.write(
+                struct.pack(formats.FILE_HEADER_PACK, b"QISKIT", QPY_VERSION + 4, 42, 42, 1, 2)
+            )
+            buf.seek(0)
+            with self.assertRaisesRegex(QiskitError, str(QPY_VERSION + 4)):
+                load(buf)
+
+
 @ddt
 class TestLayout(QpyCircuitTestCase):
     """Test circuit serialization for layout preservation."""