[PortPather] complain if the user gives ambiguous port names

masque/builder/pather.py

@@ -857,20 +857,46 @@ class PortPather:
         self.ports = [pp for pp in self.ports if pp not in ports_set]
         return self
 
+    def _normalize_copy_map(self, name: str | Mapping[str, str], action: str) -> dict[str, str]:
+        if isinstance(name, str):
+            if len(self.ports) > 1:
+                raise BuildError(f'Use a mapping to {action} >1 port')
+            name_map = {self.ports[0]: name}
+        else:
+            name_map = dict(name)
+
+        missing_selected = set(name_map) - set(self.ports)
+        if missing_selected:
+            raise PortError(f'Can only {action} selected ports: {missing_selected}')
+
+        missing_pattern = set(name_map) - set(self.pather.pattern.ports)
+        if missing_pattern:
+            raise PortError(f'Ports to {action} were not found: {missing_pattern}')
+
+        targets = list(name_map.values())
+        duplicate_targets = {vv for vv in targets if targets.count(vv) > 1}
+        if duplicate_targets:
+            raise PortError(f'{action.capitalize()} targets would collide: {duplicate_targets}')
+
+        overwritten = {
+            dst for src, dst in name_map.items()
+            if dst in self.pather.pattern.ports and dst != src
+            }
+        if overwritten:
+            raise PortError(f'{action.capitalize()} would overwrite existing ports: {overwritten}')
+
+        return name_map
+
     def mark(self, name: str | Mapping[str, str]) -> Self:
         """ Bookmark current port(s). """
-        name_map: Mapping[str, str] = {self.ports[0]: name} if isinstance(name, str) else name
-        if isinstance(name, str) and len(self.ports) > 1:
-            raise BuildError('Use a mapping to mark >1 port')
+        name_map = self._normalize_copy_map(name, 'mark')
         for src, dst in name_map.items():
             self.pather.pattern.ports[dst] = self.pather.pattern[src].copy()
         return self
 
     def fork(self, name: str | Mapping[str, str]) -> Self:
         """ Split and follow new name. """
-        name_map: Mapping[str, str] = {self.ports[0]: name} if isinstance(name, str) else name
-        if isinstance(name, str) and len(self.ports) > 1:
-            raise BuildError('Use a mapping to fork >1 port')
+        name_map = self._normalize_copy_map(name, 'fork')
         for src, dst in name_map.items():
             self.pather.pattern.ports[dst] = self.pather.pattern[src].copy()
             self.ports = [(dst if pp == src else pp) for pp in self.ports]