getdeps: fix builds with cached project downloads

Summary:
D21364132 accidentally broke this; we can't run the fetcher
for projects for which we pulled the build out of cache, because there
is no source to update in that case.

This commit adjusts the logic so that we write out a marker file
to indicate that we installed a build from cache and to look for
that file being present to gate the new update logic.

Reviewed By: lnicco

Differential Revision: D21419122

fbshipit-source-id: 304670848add22531d88549d66f22c40ff255140

build/fbcode_builder/getdeps.py

@@ -233,6 +233,10 @@ class CachedProject(object):
         """ We only cache third party projects """
         return self.cache and self.m.shipit_project is None
 
+    def was_cached(self):
+        cached_marker = os.path.join(self.inst_dir, ".getdeps-cached-build")
+        return os.path.exists(cached_marker)
+
     def download(self):
         if self.is_cacheable() and not os.path.exists(self.inst_dir):
             print("check cache for %s" % self.cache_file_name)
@@ -247,6 +251,11 @@ class CachedProject(object):
                         "Extracting %s -> %s..." % (self.cache_file_name, self.inst_dir)
                     )
                     tf.extractall(self.inst_dir)
+
+                    cached_marker = os.path.join(self.inst_dir, ".getdeps-cached-build")
+                    with open(cached_marker, "w") as f:
+                        f.write("\n")
+
                     return True
             except Exception as exc:
                 print("%s" % str(exc))
@@ -471,7 +480,7 @@ class BuildCmd(ProjectCmdBase):
                     cached_project, fetcher, m, built_marker, project_hash
                 )
 
-                if os.path.exists(built_marker):
+                if os.path.exists(built_marker) and not cached_project.was_cached():
                     # We've previously built this. We may need to reconfigure if
                     # our deps have changed, so let's check them.
                     dep_reconfigure, dep_build = self.compute_dep_change_status(