Addressed some feedback

Signed-off-by: Ben Andersen <ben@isohedron.com.au>

src/rez/cli/env.py

@@ -117,9 +117,9 @@ def setup_parser(parser, completions=False):
         "--no-pkg-cache", action="store_true",
         help="Disable package caching")
     parser.add_argument(
-        "--pkg-cache-sync", action="store_true",
-        help="Disable asynchronous package caching.  "
-             "Process will block until packages are cached.")
+        "--pkg-cache-mode", choices=["sync", "async"],
+        help="Optionally disable for force enable asynchronous package caching.  "
+             "If 'sync', the process will block until packages are cached.")
     parser.add_argument(
         "--pre-command", type=str, help=SUPPRESS)
     PKG_action = parser.add_argument(
@@ -202,6 +202,13 @@ def command(opts, parser, extra_arg_groups=None):
             rule = Rule.parse_rule(rule_str)
             package_filter.add_inclusion(rule)
 
+        if opts.pkg_cache_sync == "async":
+            package_cache_async = True
+        elif opts.pkg_cache_sync == "sync":
+            package_cache_async = False
+        else:
+            package_cache_async = None
+
         # perform the resolve
         context = ResolvedContext(
             package_requests=request,
@@ -217,7 +224,7 @@ def command(opts, parser, extra_arg_groups=None):
             suppress_passive=opts.no_passive,
             print_stats=opts.stats,
             package_caching=(not opts.no_pkg_cache),
-            package_cache_async=(not opts.pkg_cache_sync),
+            package_cache_async=package_cache_async,
         )
 
     success = (context.status == ResolverStatus.solved)

src/rez/package_cache.py

@@ -366,6 +366,16 @@ def remove_variant(self, variant):
 
         return self.VARIANT_REMOVED
 
+    def add_variants_async(self, variants):
+        """Update the package cache by adding some or all of the given variants.
+
+        This method is called when a context is created or sourced. Variants
+        are then added to the cache in a separate process.
+
+        This method is left for backwards compatibility.
+        """
+        return self.add_variants(variants, package_cache_async=True)
+
     def add_variants(self, variants, package_cache_async=True):
         """Update the package cache by adding some or all of the given variants.
 
@@ -467,7 +477,7 @@ def add_variants(self, variants, package_cache_async=True):
                     **kwargs
                 )
                 if not package_cache_async:
-                    process.wait()
+                    process.communicate()
 
         except Exception as e:
             print_warning(

src/rez/resolved_context.py

@@ -248,7 +248,6 @@ def __init__(self, package_requests, verbosity=0, timestamp=None,
                 package_caching = config.package_cache_during_build
             else:
                 package_caching = True
-
         self.package_caching = package_caching
 
         if package_cache_async is None:

src/rez/tests/test_package_cache.py

@@ -140,6 +140,9 @@ def test_caching_on_resolve(self):
                 "pyfoo-3.1.0"
             ])
 
+        # Prove that the resolved context used async mode.
+        self.assertTrue(c.package_cache_async)
+
         variant = c.get_resolved_package("timestamped")
 
         # Retry 50 times with 0.1 sec interval, 5 secs is more than enough for
@@ -154,7 +157,8 @@ def test_caching_on_resolve(self):
             resolve_not_always_cached = True
             time.sleep(0.1)
 
-        self.assertNotEqual(cached_root, None)
+        self.assertNotEqual(cached_root, None,
+                            msg="Packages were expected to be cached, but were not.")
 
         # Test that the package is not immediately cached, since it is asynchronous
         # WARNING: This is dangerous since it does open the test to a race condition and