From 54016ac01f6834b2fbc5fc3723acaebb4e396db6 Mon Sep 17 00:00:00 2001
From: kipp-ing <account@kipp.ing>
Date: Wed, 29 Jul 2026 21:00:01 +0200
Subject: [PATCH] fix: VLSD signal data is unreadable after a composed channel

`MDF4._read_channels` appends an extra `grp.signal_data.append(None)` for
every channel that carries a `component_addr` (a structure or array
composition), while `dependencies` gets exactly one entry per channel.
`Group.signal_data` is indexed by channel index, so each composed channel
shifts the VLSD block info of every following channel by one and the payload
can no longer be located:

    MdfException: Wrong signal data block refence (0x...) for VLSD channel "..."

This bites every CAN-FD bus-logging file recorded with variable-length
payloads: `CAN_DataFrame.DataBytes` is a VLSD channel inside the composed
`CAN_DataFrame` structure, so `MDF.extract_bus_logging` cannot read it at all.

Drop the stray append and cover it with a regression test that writes a VLSD
channel after a structure channel in the same group.
---
 src/asammdf/blocks/mdf_v4.py |  1 -
 test/test_mdf4.py            | 29 +++++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/asammdf/blocks/mdf_v4.py b/src/asammdf/blocks/mdf_v4.py
index df71d52b1..7124e2e6d 100644
--- a/src/asammdf/blocks/mdf_v4.py
+++ b/src/asammdf/blocks/mdf_v4.py
@@ -1065,7 +1065,6 @@ def _read_channels(
 
                 index = ch_cntr - 1
                 dependencies.append(None)
-                grp.signal_data.append(None)
 
                 # check if it is a CABLOCK or CNBLOCK
                 stream.seek(component_addr)
diff --git a/test/test_mdf4.py b/test/test_mdf4.py
index c538dfeb4..025e40821 100644
--- a/test/test_mdf4.py
+++ b/test/test_mdf4.py
@@ -189,6 +189,35 @@ def test_channel_with_boolean_array(self) -> None:
 
         self.assertTrue((record == signal.samples).all())
 
+    def test_vlsd_channel_after_structure_composition(self) -> None:
+        """A VLSD channel that sits after a structure (composed) channel in the
+        same group must still find its signal data.
+
+        ``Group.signal_data`` is indexed by channel index, so an extra entry
+        pushed for a composed channel shifts every following channel's VLSD
+        block info and makes the payload unreadable.
+        """
+        count = 20
+        timestamps = np.arange(count, dtype="<f8")
+
+        record = np.rec.fromarrays(
+            [np.arange(count, dtype="<u2"), np.arange(count, dtype="<u1") % 5],
+            dtype=np.dtype([("a", "<u2"), ("b", "<u1")]),
+        )
+        structure = Signal(record, timestamps=timestamps, name="Structure")
+        texts = np.array([("x" * (i % 5 + 1)).encode("latin-1") for i in range(count)], dtype="S8")
+        text = Signal(texts, timestamps=timestamps, name="Text", encoding="latin-1")
+
+        path = Path(TestMDF4.tempdir.name) / "vlsd_after_structure.mf4"
+        with MDF(version="4.10") as mdf:
+            mdf.append([structure, text])
+            mdf.save(path, overwrite=True)
+
+        with MDF(path) as mdf:
+            group = mdf.groups[0]
+            self.assertEqual(len(group.signal_data), len(group.channels))
+            self.assertTrue(np.array_equal(mdf.get("Text", group=0).samples, texts))
+
 
 if __name__ == "__main__":
     unittest.main()