Add directive is-leaf to force selecting node

Expected usage with virtual tests while keeping the parent
test as well

Fix: #221

docs/features.rst

@@ -194,6 +194,14 @@ The key content attributes are not supposed to be hard-coded in
 the Flexible Metadata Format but freely configurable. Multiple key
 content attributes (e.g. script & backend) could be used as well.
 
+Sometimes it is necessary to select node from the metadata tree
+even though it is not leaf. For example, when virtual tests are
+created from a parent test but one wants to keep the parent available
+as a test as well. To do so, one can set fhe following directive::
+
+    /:
+        is-leaf: true
+
 
 .. _virtual:
 

fmf/base.py

@@ -250,10 +250,12 @@ def check(value, type_, name=None):
         for key, value in directives.items():
             if key == "inherit":
                 check(value, bool, name="inherit")
-                continue
-            # No other directive supported
-            raise fmf.utils.FormatError(
-                f"Unknown fmf directive '{key}' in '{self.name}'.")
+            elif key == "is-leaf":
+                check(value, bool, name="is-leaf")
+            else:
+                # No other directive supported
+                raise fmf.utils.FormatError(
+                    f"Unknown fmf directive '{key}' in '{self.name}'.")
 
         # Everything ok, store the directives
         self._directives.update(directives)
@@ -572,7 +574,7 @@ def grow(self, path):
 
     def climb(self, whole=False):
         """ Climb through the tree (iterate leaf/all nodes) """
-        if whole or not self.children:
+        if whole or not self.children or self._directives.get("is-leaf"):
             yield self
         for name, child in self.children.items():
             for node in child.climb(whole):

tests/core/is-leaf/data/.fmf/version

@@ -0,0 +1 @@
+1

tests/core/is-leaf/data/foo/main.fmf

@@ -0,0 +1,3 @@
+/:
+    is-leaf: true
+test: echo

tests/core/is-leaf/main.fmf

@@ -0,0 +1,7 @@
+summary: Verify functionality of the `is-leaf` directive
+description:
+    Sometimes the node should stay reported as a object
+    even though it has child nodes. Using this directive
+    allows it. Real life example is to keep "/test" available
+    after virtual tests "/test/virtual_foo" and "test/virtual_bar"
+    are created.

tests/core/is-leaf/test.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+. /usr/share/beakerlib/beakerlib.sh || exit 1
+
+rlJournalStart
+    rlPhaseStartTest
+        rlRun "pushd data"
+
+        rlRun -s "fmf ls"
+        rlAssertGrep "^/foo$" $rlRun_LOG
+        rlAssertGrep "^/foo/child$" $rlRun_LOG
+
+        rlRun "popd"
+    rlPhaseEnd
+rlJournalEnd