Add --no-history option to fetch and gclient for shallow clones.

gclient_scm.py

Index: gclient_scm.py
diff --git a/gclient_scm.py b/gclient_scm.py
index a87032e749902122c676b6621dde4fec33f8f232..6ac47172856e378890e08eb2eb0dad9d3f191a83 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -825,6 +825,25 @@ class GitWrapper(SCMWrapper):
     # create it, so we need to do it manually.
     parent_dir = os.path.dirname(self.checkout_path)
     gclient_utils.safe_makedirs(parent_dir)
+
+    template_dir = None
+    if options.no_history:
+      if gclient_utils.IsGitSha(revision):
+        # In the case of a subproject, the pinned sha is not necessarily the
+        # head of the remote branch (so we can't just use --depth=N). Instead,
+        # we tell git to fetch all the remote objects from SHA..HEAD by means of
+        # a template git dir which has a 'shallow' file pointing to the sha.
+        template_dir = tempfile.mkdtemp(
+            prefix='_gclient_gittmp_%s' % os.path.basename(self.checkout_path),
+            dir=parent_dir)
+        self._Run(['init', '--bare', template_dir], options, cwd=self._root_dir)
+        with open(os.path.join(template_dir, 'shallow'), 'w') as template_file:
+          template_file.write(revision)
+        clone_cmd.append('--template=' + template_dir)
+      else:
+        # Otherwise, we're just interested in the HEAD. Just use --depth.
+        clone_cmd.append('--depth=1')
+
     tmp_dir = tempfile.mkdtemp(
         prefix='_gclient_%s_' % os.path.basename(self.checkout_path),
         dir=parent_dir)
@@ -841,6 +860,9 @@ class GitWrapper(SCMWrapper):
       if os.listdir(tmp_dir):
         self.Print('_____ removing non-empty tmp dir %s' % tmp_dir)
       gclient_utils.rmtree(tmp_dir)
+      if template_dir:
+        self.Print('_____ removing template dir %s' % template_dir)
+        gclient_utils.rmtree(template_dir)

[wtc, 2014/08/04 22:32:50]

What I suggested was:

    if template_dir:
      if os.listdir(template_dir):
        self.Print('_____ removing non-empty template dir %s' % template_dir)
      gclient_utils.rmtree(template_dir)

So, if template_dir is not-None, we remove it, empty or not. But we print a
message only when template_dir is not empty.

Your new code is fine by me. Since you have thought about this code more than I
have, I'll let you decide which is better. I actually prefer your form and I
think lines 860-862 should also be changed accordinly:
    self.Print('_____ removing tmp dir %s' % tmp_dir)
    gclient_utils.rmtree(tmp_dir)

[Primiano Tucci (use gerrit), 2014/08/04 23:00:25]

But... template_dir will always be non-empty (Conversely to tmp_dir, which is
checked out from the remote, and its empty state depends on the outcome of the
clone).
That's why I removed also the "non-empty" from the message for template_dir
Actually looking closer at the code, I think we shouldn't probably add a Print
for template_dir but keep it as it is for tmp_dir. Rationale: in nominal
conditions (if everything goes right) tmp_dir should not exists at the end, as
line 853 renames it atomically to the final checkout dir. That tmp_dir stays
there in its original tmp location only if something goes wrong and we end up in
the finally block due to an exception. 
Essentially, it looks to me that the print on line 861 is there to say "I tried
to checked out/clone the remote repo, something went wrong, and now I'm deleting
the temporary directory where I attempted the checkout". In nominal condition
that print should never be hit.
Having said that, the situation is different for template_dir. That one is
supposed to be just a temporary scratch directory to be used for git clone.
After the clone (either successful or not), that directory will always be not
empty, and we always want to clean that up. Hence, probably a Print there would
be misleading (we'd print the message also in the case of success) and would add
too much noise.

I'd be tempted to remove lines 863-865 ad this point. WDYT?

[Primiano Tucci (use gerrit), 2014/08/04 23:01:52]

Ehm, sorry, I actually meant: "I'd be tempted to remove line 864" (just the
print, not the rmtree) 

[wtc, 2014/08/05 00:11:55]

I agree. Thanks for the analysis.

     self._UpdateBranchHeads(options, fetch=True)
     self._Checkout(options, revision.replace('refs/heads/', ''), quiet=True)
     if self._GetCurrentBranch() is None: