Improve error handling around yieldctx fixtures which do not yield a value (#7083)

Author: symonk

changelog/7061.bugfix.rst

@@ -0,0 +1 @@
+When a yielding fixture fails to yield a value, report a test setup error instead of crashing.

src/_pytest/fixtures.py

@@ -786,13 +786,18 @@ def fail_fixturefunc(fixturefunc, msg):
 def call_fixture_func(fixturefunc, request, kwargs):
     yieldctx = is_generator(fixturefunc)
     if yieldctx:
-        it = fixturefunc(**kwargs)
-        res = next(it)
-        finalizer = functools.partial(_teardown_yield_fixture, fixturefunc, it)
+        generator = fixturefunc(**kwargs)
+        try:
+            fixture_result = next(generator)
+        except StopIteration:
+            raise ValueError(
+                "{} did not yield a value".format(request.fixturename)
+            ) from None
+        finalizer = functools.partial(_teardown_yield_fixture, fixturefunc, generator)
         request.addfinalizer(finalizer)
     else:
-        res = fixturefunc(**kwargs)
-    return res
+        fixture_result = fixturefunc(**kwargs)
+    return fixture_result
 
 
 def _teardown_yield_fixture(fixturefunc, it):

testing/python/fixtures.py

@@ -3,6 +3,7 @@
 
 import pytest
 from _pytest import fixtures
+from _pytest.config import ExitCode
 from _pytest.fixtures import FixtureRequest
 from _pytest.pathlib import Path
 from _pytest.pytester import get_public_names
@@ -4290,3 +4291,23 @@ def test_suffix(fix_combined):
     )
     result = testdir.runpytest("-vv", str(p1))
     assert result.ret == 0
+
+
+def test_yield_fixture_with_no_value(testdir):
+    testdir.makepyfile(
+        """
+        import pytest
+        @pytest.fixture(name='custom')
+        def empty_yield():
+            if False:
+                yield
+
+        def test_fixt(custom):
+            pass
+        """
+    )
+    expected = "E               ValueError: custom did not yield a value"
+    result = testdir.runpytest()
+    result.assert_outcomes(error=1)
+    result.stdout.fnmatch_lines([expected])
+    assert result.ret == ExitCode.TESTS_FAILED