Return non-zero error code on skipped env

In #2206, we said that tox should fail if all envs are skipped. This is
broken when only a single env runs. We print an error message but return
0. Correct this behavior.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #2827

docs/changelog/2827.bugfix.rst

@@ -0,0 +1,2 @@
+tox returns a non-zero error code if all envs are skipped. It will now correctly do this if only a single env was
+requested and this was skipped.

src/tox/session/cmd/run/common.py

@@ -187,7 +187,9 @@ def _print(color_: int, message: str) -> None:
         _print(Fore.GREEN, f"  congratulations :) ({duration:.2f} seconds)")
         return Outcome.OK
     _print(Fore.RED, f"  evaluation failed :( ({duration:.2f} seconds)")
-    return runs[0].code if len(runs) == 1 else -1
+    if len(runs) == 1:
+        return runs[0].code if not runs[0].skipped else -1
+    return -1
 
 
 def _get_outcome_message(run: ToxEnvRunResult) -> tuple[str, int]:

tests/session/cmd/test_sequential.py

@@ -224,7 +224,7 @@ def test_missing_interpreter_skip_on(tox_project: ToxProjectCreator) -> None:
     proj = tox_project({"tox.ini": ini})
 
     result = proj.run("r")
-    result.assert_success()
+    result.assert_failed()
     assert "py: SKIP" in result.out
 
 
@@ -367,7 +367,7 @@ def test_platform_does_not_match_run_env(tox_project: ToxProjectCreator) -> None
     proj = tox_project({"tox.ini": ini})
 
     result = proj.run("r")
-    result.assert_success()
+    result.assert_failed()
     exp = f"py: skipped because platform {sys.platform} does not match wrong_platform"
     assert exp in result.out
 

tests/tox_env/python/test_python_runner.py

@@ -145,7 +145,7 @@ def test_config_skip_missing_interpreters(
     [
         ("true", f"py{''.join(str(i) for i in sys.version_info[0:2])}", 0),
         ("false", f"py{''.join(str(i) for i in sys.version_info[0:2])}", 0),
-        ("true", "py31", 0),
+        ("true", "py31", -1),
         ("false", "py31", 1),
         ("true", None, 0),
         ("false", None, -1),