overlay repositories: resolve labels before downloading (#1349)

Fixes #1338

go/private/tools/overlay_repository.bzl

@@ -27,14 +27,14 @@
 # recursive workspaces.
 
 def _http_archive_impl(ctx):
+  overlay = _resolve_overlay(ctx, ctx.attr.overlay)
   ctx.download_and_extract(
       url = ctx.attr.urls,
       sha256 = ctx.attr.sha256,
       type = ctx.attr.type,
       stripPrefix = ctx.attr.strip_prefix,
   )
-
-  _apply_overlay(ctx, ctx.attr.overlay)
+  _apply_overlay(ctx, overlay)
 
 http_archive = repository_rule(
     implementation = _http_archive_impl,
@@ -57,12 +57,14 @@ def _git_repository_impl(ctx):
   if ctx.attr.commit and ctx.attr.tag:
     fail("'commit' and 'tag' may not both be specified")
 
+  overlay = _resolve_overlay(ctx, ctx.attr.overlay)
+
   # TODO(jayconrod): sanitize inputs passed to git.
   revision = ctx.attr.commit if ctx.attr.commit else ctx.attr.tag
   _check_execute(ctx, ["git", "clone", "-n", ctx.attr.remote, "."], "failed to clone %s" % ctx.attr.remote)
   _check_execute(ctx, ["git", "checkout", revision], "failed to checkout revision %s in remote %s" % (revision, ctx.attr.remote))
 
-  _apply_overlay(ctx, ctx.attr.overlay)
+  _apply_overlay(ctx, overlay)
 
 git_repository = repository_rule(
     implementation = _git_repository_impl,
@@ -74,11 +76,23 @@ git_repository = repository_rule(
     },
 )
 
+def _resolve_overlay(ctx, overlay):
+  """Resolve overlay labels to paths.
+
+  This should be done before downloading the repository, since it may
+  trigger restarts.
+  """
+  return [(ctx.path(src_label), dst_rel) for src_label, dst_rel in overlay.items()]
+
 def _apply_overlay(ctx, overlay):
+  """Copies overlay files into the repository.
+
+  This should be done after downloading the repository, since it may replace
+  downloaded files.
+  """
   # TODO(jayconrod): sanitize destination paths.
-  for src_label, dst_rel in overlay.items():
-    src_path = ctx.path(src_label)
-    _check_execute(ctx, ["cp", src_path, dst_rel], "failed to copy file from %s" % src_label)
+  for src_path, dst_rel in overlay:
+    _check_execute(ctx, ["cp", src_path, dst_rel], "failed to copy file from %s" % src_path)
 
 def _check_execute(ctx, arguments, message):
   res = ctx.execute(arguments)