Add --only-deps option for caching

alibuild_helpers/args.py

@@ -107,6 +107,8 @@ def doParseArgs():
                                   "in multiple packages. The comment will only be stored if "
                                   "PACKAGE is compiled or downloaded during this run; if it "
                                   "already exists, this does not happen."))
+  build_parser.add_argument("--only-deps", dest="onlyDeps", default=False, action="store_true",
+                            help="Only build dependencies, not the main package (e.g. for caching)")
 
   build_docker = build_parser.add_argument_group(title="Build inside a container", description="""\
   Builds can be done inside a Docker container, to make it easier to get a

alibuild_helpers/build.py

@@ -697,6 +697,12 @@ def doBuild(args, parser):
     deps=",".join(buildOrder[:-1]),
   ), args.architecture)
 
+  # If we are building only the dependencies, the last package in
+  # the build order can be considered done.
+  if args.onlyDeps:
+    mainPackage = buildOrder.pop()
+    warning("Not rebuilding %s because --only-deps option provided.", mainPackage["package"])
+
   while buildOrder:
     p = buildOrder[0]
     spec = specs[p]