new_git_repository: support date-based shallow clones

Add an optional 'shallow_since' parameter to new_git_repository to allow
more shallow clones. As opposed to a fixed depths, specifying a date does
not get outdated if the branch progresses further.

Also, use fully shallow clones if no commit is specified. A (floating)
branch or tag can always be cloned in a truly shallow fashion.

Improves on bazelbuild#4359.

Change-Id: I8459c8f55042c98da2e43f57b7960a291004d26b
PiperOrigin-RevId: 185504534

tools/build_defs/repo/git.bzl

@@ -14,37 +14,47 @@
 """Rules for cloning external git repositories."""
 
 def _clone_or_update(ctx):
-  if (ctx.attr.verbose):
-    print('git.bzl: Cloning or updating repository %s using strip_prefix of [%s]' %
-    (ctx.name, ctx.attr.strip_prefix if ctx.attr.strip_prefix else 'None'))
   if ((not ctx.attr.tag and not ctx.attr.commit) or
       (ctx.attr.tag and ctx.attr.commit)):
     fail('Exactly one of commit and tag must be provided')
+  shallow = ''
   if ctx.attr.commit:
     ref = ctx.attr.commit
   else:
     ref = 'tags/' + ctx.attr.tag
+    shallow = '--depth=1'
   directory=str(ctx.path('.'))
   if ctx.attr.strip_prefix:
     directory = directory + "-tmp"
+  if ctx.attr.shallow_since:
+    if ctx.attr.tag:
+      fail('shallow_since not allowed if a tag is specified; --depth=1 will be used for tags')
+    shallow='--shallow-since=%s' % ctx.attr.shallow_since
 
+  if (ctx.attr.verbose):
+    print('git.bzl: Cloning or updating%s repository %s using strip_prefix of [%s]' %
+    (' (%s)' % shallow if shallow else '',
+     ctx.name,
+     ctx.attr.strip_prefix if ctx.attr.strip_prefix else 'None',
+    ))
   bash_exe = ctx.os.environ["BAZEL_SH"] if "BAZEL_SH" in ctx.os.environ else "bash"
   st = ctx.execute([bash_exe, '-c', """
 set -ex
 ( cd {working_dir} &&
     if ! ( cd '{dir_link}' && [[ "$(git rev-parse --git-dir)" == '.git' ]] ) >/dev/null 2>&1; then
       rm -rf '{directory}' '{dir_link}'
-      git clone '{remote}' '{directory}'
+      git clone {shallow} '{remote}' '{directory}'
     fi
     cd '{directory}'
-    git reset --hard {ref} || (git fetch origin {ref}:{ref} && git reset --hard {ref})
+    git reset --hard {ref} || (git fetch {shallow} origin {ref}:{ref} && git reset --hard {ref})
     git clean -xdf )
   """.format(
       working_dir=ctx.path('.').dirname,
       dir_link=ctx.path('.'),
       directory=directory,
       remote=ctx.attr.remote,
       ref=ref,
+      shallow=shallow,
   )])
 
   if st.return_code:
@@ -86,6 +96,7 @@ def _git_repository_implementation(ctx):
 _common_attrs = {
     'remote': attr.string(mandatory=True),
     'commit': attr.string(default=''),
+    'shallow_since': attr.string(default=''),
     'tag': attr.string(default=''),
     'init_submodules': attr.bool(default=False),
     'verbose': attr.bool(default=False),
@@ -124,6 +135,12 @@ Args:
   commit: specific commit to be checked out
     Either tag or commit must be specified.
 
+  shallow_since: an optional date, not after the specified commit; the
+    argument is not allowed if a tag is specified (which allows cloning
+    with depth 1). Setting such a date close to the specified commit
+    allows for a more shallow clone of the repository, saving bandwith and
+    wall-clock time.
+
   init_submodules: Whether to clone submodules in the repository.
 
   remote: The URI of the remote Git repository.
@@ -152,6 +169,11 @@ Args:
   commit: specific commit to be checked out
     Either tag or commit must be specified.
 
+  shallow_since: an optional date, not after the specified commit; the
+    argument is not allowed if a tag is specified (which allows cloning
+    with depth 1). Setting such a date close to the specified commit
+    allows for a more shallow clone of the repository, saving bandwith and
+    wall-clock time.
 
   strip_prefix: A directory prefix to strip from the extracted files.
 """