Bugfix: Looping LinSweepers

docs/changes/newsfragments/7607.improved

@@ -0,0 +1,2 @@
+Fixes a bug in the LinSweeper iterator that caused it to always raise StopIteration after
+completing a single sweep. This bug meant LinSweeper could not be used in a nested measurement function.

src/qcodes/dataset/measurement_extensions.py

@@ -236,4 +236,5 @@ def __next__(self) -> float:
             self._iter_index += 1
             return set_val
         else:
+            self._iter_index = 0
             raise StopIteration

tests/dataset/test_measurement_extensions.py

@@ -1,4 +1,5 @@
 import gc
+import itertools
 from functools import partial
 from itertools import product
 from pathlib import Path
@@ -238,6 +239,30 @@ def test_linsweeper(default_params, default_database_and_experiment):
     )
 
 
+def test_nested_linsweeper(default_params):
+    set1, set2, _, _, _, _ = default_params
+    linsweeper1 = LinSweeper(set1, 0, 1, 11, 0.001)
+    linsweeper2 = LinSweeper(set2, -1, 0, 6, 0.001)
+    data_pairs = []
+    for _ in linsweeper1:
+        for _ in linsweeper2:
+            data_pairs.append([set1(), set2()])
+
+    assert len(data_pairs) == 11 * 6
+    assert np.all(
+        np.isclose(
+            np.array(data_pairs),
+            np.array(
+                list(
+                    itertools.product(
+                        linsweeper1.get_setpoints(), linsweeper2.get_setpoints()
+                    )
+                )
+            ),
+        )
+    )
+
+
 def test_context_with_pws(pws_params, default_database_and_experiment):
     _ = default_database_and_experiment
     pws1, set1 = pws_params