add template_node node attribute

closes #399

bundlewrap/node.py

@@ -364,6 +364,7 @@ def __init__(self, name, attributes=None):
         self._node_metadata = attributes.get('metadata', {})
         self._ssh_conn_established = False
         self._ssh_first_conn_lock = Lock()
+        self._template_node_name = attributes.get('template_node')
         self.hostname = attributes.get('hostname', name)
         self.name = name
 
@@ -731,6 +732,20 @@ def log_function(msg):
             wrapper_outer=self.cmd_wrapper_outer,
         )
 
+    @property
+    def template_node(self):
+        if not self._template_node_name:
+            return None
+        else:
+            target_node = self.repo.get_node(self._template_node_name)
+            if target_node._template_node_name:
+                raise RepositoryError(_(
+                    "{template_node} cannot use template_node because {node} uses {template_node} "
+                    "as template_node"
+                ).format(node=self.name, template_node=target_node.name))
+            else:
+                return target_node
+
     def upload(self, local_path, remote_path, mode=None, owner="", group="", may_fail=False):
         assert self.os in self.OS_FAMILY_UNIX
         return operations.upload(
@@ -777,6 +792,10 @@ def method(self):
                 attr_source = "group:{}".format(group.name)
                 attr_value = getattr(group, attr)
 
+        if self.template_node:
+            attr_source = "template_node"
+            attr_value = getattr(self.template_node, attr)
+
         if getattr(self, "_{}".format(attr)) is not None:
             attr_source = "node"
             attr_value = getattr(self, "_{}".format(attr))

bundlewrap/repo.py

@@ -513,18 +513,21 @@ def _build_node_metadata(self, blame=False):
                     # start messing with these objects in metadata processors. Every
                     # time we would edit one of these objects, the changes would be
                     # shared amongst multiple nodes.
-                    new_metadata = deepcopy_metadata(merge_dict(
-                        self._node_metadata_partial[node.name],
-                        node._node_metadata,
-                    ))
-                    if blame:
-                        blame_changed_paths(
+                    for source_node in (node.template_node, node):
+                        if not source_node:  # template_node might be None
+                            continue
+                        new_metadata = deepcopy_metadata(merge_dict(
                             self._node_metadata_partial[node.name],
-                            new_metadata,
-                            node_blame,
-                            "node:{}".format(node.name),
-                        )
-                    self._node_metadata_partial[node.name] = new_metadata
+                            source_node._node_metadata,
+                        ))
+                        if blame:
+                            blame_changed_paths(
+                                self._node_metadata_partial[node.name],
+                                new_metadata,
+                                node_blame,
+                                "node:{}".format(source_node.name),
+                            )
+                        self._node_metadata_partial[node.name] = new_metadata
 
             # Now for the interesting part: We run all metadata processors
             # until none of them return DONE anymore (indicating that they're

docs/content/repo/nodes.py.md

@@ -128,6 +128,14 @@ Tuples of integers can be used for easy comparison of versions: `(12, 4) < (16,
 
 <br>
 
+### template_node
+
+Copy all attributes and merge all metadata from this node. This is useful for temporary clones of single specific nodes, where you don't want to create a group to deduplicate all the node-level configuration.
+
+Cannot be set at group level.
+
+<br>
+
 ## OS compatibility overrides
 
 ### cmd_wrapper_outer

tests/integration/bw_metadata.py

@@ -78,6 +78,65 @@ def test_merge(tmpdir):
     assert rcode == 0
 
 
+def test_template_node(tmpdir):
+    make_repo(
+        tmpdir,
+        nodes={
+            "node1": {
+                'template_node': "node2",
+            },
+            "node2": {
+                'metadata': {
+                    "foo": 2,
+                },
+            },
+        },
+        groups={
+            "group1": {
+                'members': ["node1"],
+                'metadata': {
+                    "foo": 3,
+                },
+            },
+        },
+    )
+    stdout, stderr, rcode = run("bw metadata node1", path=str(tmpdir))
+    assert loads(stdout.decode()) == {"foo": 2}
+    assert stderr == b""
+    assert rcode == 0
+
+
+def test_template_node_override(tmpdir):
+    make_repo(
+        tmpdir,
+        nodes={
+            "node1": {
+                'metadata': {
+                    "foo": 1,
+                },
+                'template_node': "node2",
+            },
+            "node2": {
+                'metadata': {
+                    "foo": 2,
+                },
+            },
+        },
+        groups={
+            "group1": {
+                'members': ["node1"],
+                'metadata': {
+                    "foo": 3,
+                },
+            },
+        },
+    )
+    stdout, stderr, rcode = run("bw metadata node1", path=str(tmpdir))
+    assert loads(stdout.decode()) == {"foo": 1}
+    assert stderr == b""
+    assert rcode == 0
+
+
 def test_metadatapy(tmpdir):
     make_repo(
         tmpdir,

tests/integration/bw_nodes.py

@@ -46,3 +46,29 @@ def test_bundles(tmpdir):
     assert stdout.decode().strip().split("\n") == ["bundle1", "bundle2"]
     assert stderr == b""
     assert rcode == 0
+
+
+def test_template_node(tmpdir):
+    make_repo(
+        tmpdir,
+        nodes={
+            "node1": {'template_node': "node2"},
+            "node2": {'dummy': True},
+        },
+    )
+    stdout, stderr, rcode = run("BW_TABLE_STYLE=grep bw nodes node1 dummy | grep node1 | cut -f 2", path=str(tmpdir))
+    assert stdout.decode().strip() == "True"
+    assert stderr == b""
+    assert rcode == 0
+
+
+def test_template_node_cascade(tmpdir):
+    make_repo(
+        tmpdir,
+        nodes={
+            "node1": {'template_node': "node2"},
+            "node2": {'template_node': "node1"},
+        },
+    )
+    stdout, stderr, rcode = run("BW_TABLE_STYLE=grep bw nodes node1 dummy", path=str(tmpdir))
+    assert rcode == 1