Fix incorrect aggregation of volume mount and dynamic volumes created…

… via Hera (#875)

**Pull Request Checklist**
- [x] Fixes #<!--issue number goes here-->
- [x] Tests added
- [x] Documentation/examples added
- [x] [Good commit messages](https://cbea.ms/git-commit/) and/or PR
title

**Description of PR**
Hera supports building dynamic volumes via the `Volume` object. That
creates PVCs + PVs dynamically. Users have the liberty to mount these
volumes wherever they want. However, if users have a volume mount
_already_, then the dynamic volumes are not combined with mounts
correctly. That happens because of the incorrect conditions inside of
`build_volume_mounts`. This PR fixes that

Signed-off-by: Flaviu Vadan <flaviuvadan@gmail.com>

docs/examples/workflows/volume_mount.md

@@ -0,0 +1,69 @@
+# Volume Mount
+
+
+
+This example showcases how to create a volume at a workflow level and use it in a container via a mount.
+
+
+=== "Hera"
+
+    ```python linenums="1"
+    from hera.workflows import Container, SecretVolume, Steps, Volume, Workflow
+    from hera.workflows.models import VolumeMount
+
+    with Workflow(
+        generate_name="test-",
+        volumes=[SecretVolume(name="service-account-credential", secret_name="service-account-credential")],
+        entrypoint="test",
+    ) as w:
+        v_tmp_pod = Volume(
+            name="tmp-pod",
+            size="100Mi",
+            mount_path="/tmp/pod",
+        )
+        init_container_example = Container(
+            name="git-sync",
+            volume_mounts=[VolumeMount(name="service-account-credential", mount_path="/secrets")],
+            volumes=[v_tmp_pod],
+        )
+        with Steps(name="test") as s:
+            init_container_example()
+    ```
+
+=== "YAML"
+
+    ```yaml linenums="1"
+    apiVersion: argoproj.io/v1alpha1
+    kind: Workflow
+    metadata:
+      generateName: test-
+    spec:
+      entrypoint: test
+      templates:
+      - container:
+          image: python:3.8
+          volumeMounts:
+          - mountPath: /secrets
+            name: service-account-credential
+          - mountPath: /tmp/pod
+            name: tmp-pod
+        name: git-sync
+      - name: test
+        steps:
+        - - name: git-sync
+            template: git-sync
+      volumeClaimTemplates:
+      - metadata:
+          name: tmp-pod
+        spec:
+          accessModes:
+          - ReadWriteOnce
+          resources:
+            requests:
+              storage: 100Mi
+      volumes:
+      - name: service-account-credential
+        secret:
+          secretName: service-account-credential
+    ```
+

examples/workflows/volume-mount.yaml

@@ -0,0 +1,32 @@
+apiVersion: argoproj.io/v1alpha1
+kind: Workflow
+metadata:
+  generateName: test-
+spec:
+  entrypoint: test
+  templates:
+  - container:
+      image: python:3.8
+      volumeMounts:
+      - mountPath: /secrets
+        name: service-account-credential
+      - mountPath: /tmp/pod
+        name: tmp-pod
+    name: git-sync
+  - name: test
+    steps:
+    - - name: git-sync
+        template: git-sync
+  volumeClaimTemplates:
+  - metadata:
+      name: tmp-pod
+    spec:
+      accessModes:
+      - ReadWriteOnce
+      resources:
+        requests:
+          storage: 100Mi
+  volumes:
+  - name: service-account-credential
+    secret:
+      secretName: service-account-credential

examples/workflows/volume_mount.py

@@ -0,0 +1,21 @@
+"""This example showcases how to create a volume at a workflow level and use it in a container via a mount."""
+from hera.workflows import Container, SecretVolume, Steps, Volume, Workflow
+from hera.workflows.models import VolumeMount
+
+with Workflow(
+    generate_name="test-",
+    volumes=[SecretVolume(name="service-account-credential", secret_name="service-account-credential")],
+    entrypoint="test",
+) as w:
+    v_tmp_pod = Volume(
+        name="tmp-pod",
+        size="100Mi",
+        mount_path="/tmp/pod",
+    )
+    init_container_example = Container(
+        name="git-sync",
+        volume_mounts=[VolumeMount(name="service-account-credential", mount_path="/secrets")],
+        volumes=[v_tmp_pod],
+    )
+    with Steps(name="test") as s:
+        init_container_example()

src/hera/workflows/_mixins.py

@@ -539,25 +539,25 @@ def _build_volume_mounts(self) -> Optional[List[VolumeMount]]:
         if self.volume_mounts is None and self.volumes is None:
             return None
 
-        if isinstance(self.volumes, list):
-            volumes = self.volumes
-        elif not isinstance(self.volumes, list) and self.volumes is not None:
-            volumes = [self.volumes]
-        else:
+        if self.volumes is None:
             volumes = []
+        else:
+            volumes = self.volumes if isinstance(self.volumes, list) else [self.volumes]
 
         result = (
             None
-            if self.volumes is None
+            if not volumes
             else [v._build_volume_mount() if issubclass(v.__class__, _BaseVolume) else v for v in volumes]
         )
+
         if result is None and self.volume_mounts is None:
             return None
         elif result is None and self.volume_mounts is not None:
             return self.volume_mounts
+        elif result is not None and self.volume_mounts is None:
+            return cast(List[VolumeMount], result)
 
-        mounts = cast(List[VolumeMount], self.volume_mounts) or [] + cast(List[VolumeMount], result) or []
-        return mounts or None
+        return cast(List[VolumeMount], self.volume_mounts) + cast(List[VolumeMount], result)
 
 
 class ArgumentsMixin(BaseMixin):