multicall: hookwrapper: use yielded results, support firstresult=True

Ref: pytest-dev/pytest#5301 (comment)

src/pluggy/callers.py

@@ -179,8 +179,12 @@ def _multicall(hook_impls, caller_kwargs, firstresult=False):
                 if hook_impl.hookwrapper:
                     try:
                         gen = hook_impl.function(*args)
-                        next(gen)  # first yield
+                        res = next(gen)  # first yield
                         teardowns.append(gen)
+                        if res is not None:
+                            results.append(res)
+                            if firstresult:
+                                break
                     except StopIteration:
                         _raise_wrapfail(gen, "did not yield")
                 else:

testing/test_multicall.py

@@ -142,7 +142,7 @@ def m2():
         out.append("m2 finish")
 
     res = MC([m2, m1], {})
-    assert res == []
+    assert res == [1, 2]
     assert out == ["m1 init", "m2 init", "m2 finish", "m1 finish"]
 
 
@@ -184,3 +184,26 @@ def m2():
     with pytest.raises(exc):
         MC([m2, m1], {})
     assert out == ["m1 init", "m1 finish"]
+
+
+def test_hookwrapper_result():
+    out = []
+
+    @hookimpl(hookwrapper=True)
+    def m1():
+        out.append("m1 init")
+        yield "hookwrapper_result"
+        out.append("m1 finish")
+
+    @hookimpl
+    def m2():
+        out.append("m2")
+        return 2
+
+    res = MC([m2, m1], {})
+    assert res == ["hookwrapper_result", 2]
+    assert out == ["m1 init", "m2", "m1 finish"]
+    out[:] = []
+    res = MC([m2, m1], {}, firstresult=True)
+    assert res == "hookwrapper_result"
+    assert out == ["m1 init", "m1 finish"]