Add --all argument to spack dependents

--all causes spack dependents to list all possible dependents for a
package, rather than actual dependents for an installed spec.

lib/spack/spack/cmd/dependents.py

@@ -25,6 +25,7 @@
 import argparse
 
 import llnl.util.tty as tty
+from llnl.util.tty.colify import colify
 
 import spack
 import spack.store
@@ -36,20 +37,69 @@
 
 
 def setup_parser(subparser):
+    subparser.add_argument(
+        '-a', '--all', action='store_true', default=False,
+        help="List all potential dependents of the package instead of actual "
+        "dependents of an installed spec")
     subparser.add_argument(
         'spec', nargs=argparse.REMAINDER,
         help="specs to list dependencies of")
 
 
+def inverted_dag():
+    """Returns inverted package DAG as adjacency lists."""
+    dag = {}
+    for pkg in spack.repo.all_packages():
+        dag.setdefault(pkg.name, set())
+        for dep in pkg.dependencies:
+            deps = [dep]
+
+            # expand virtuals if necessary
+            if spack.repo.is_virtual(dep):
+                deps = [s.name for s in spack.repo.providers_for(dep)]
+
+            for d in deps:
+                dag.setdefault(d, set()).add(pkg.name)
+    return dag
+
+
+def all_dependents(name, inverted_dag, dependents=None):
+    if dependents is None:
+        dependents = set()
+
+    if name in dependents:
+        return set()
+    dependents.add(name)
+
+    direct = inverted_dag[name]
+    for dname in direct:
+        all_dependents(dname, inverted_dag, dependents)
+    dependents.update(direct)
+    return dependents
+
+
 def dependents(parser, args):
     specs = spack.cmd.parse_specs(args.spec)
     if len(specs) != 1:
         tty.die("spack dependents takes only one spec.")
-    spec = spack.cmd.disambiguate_spec(specs[0])
 
-    tty.msg("Dependents of %s" % spec.cformat('$_$@$%@$/'))
+    if args.all:
-    deps = spack.store.db.installed_dependents(spec)
+        spec = specs[0]
-    if deps:
+        idag = inverted_dag()
-        spack.cmd.display_specs(deps)
+
+        dependents = all_dependents(spec.name, idag)
+        dependents.remove(spec.name)
+        if dependents:
+            colify(sorted(dependents))
+        else:
+            print("No dependents")
+
     else:
-        print("No dependents")
+        spec = spack.cmd.disambiguate_spec(specs[0])
+
+        tty.msg("Dependents of %s" % spec.cformat('$_$@$%@$/'))
+        deps = spack.store.db.installed_dependents(spec)
+        if deps:
+            spack.cmd.display_specs(deps)
+        else:
+            print("No dependents")