[PATCH] submodule--helper: use parallel processor correctly.

[Stefan Beller <sbeller@google.com>]

When developing another patch series I had a temporary state in which
git-clone would segfault, when the call was prepared in
prepare_to_clone_next_submodule. This lead to the call failing, i.e. in
`update_clone_task_finished` the task was scheduled to be tried again.
The second call to prepare_to_clone_next_submodule would return 0, as the
segfaulted clone did create the .git file already, such that was not
considered to need to be cloned again. I was seeing the "BUG: ce was
a submodule before?\n" message, which was the correct behavior at the
time as my local code was buggy. When trying to debug this failure, I
tried to use printing messages into the strbuf that is passed around,
but these messages were never printed as the die(..) doesn't
flush the `err` strbuf.

When implementing the die() in 665b35ecc (2016-06-09, "submodule--helper:
initial clone learns retry logic"), I considered this condition to be
a severe condition, which should lead to an immediate abort as we do not
trust ourselves any more. However the queued messages in `err` are valuable
so let's not toss them out by immediately dying, but a graceful return.

Another thing to note: The error message itself was misleading. A return
value of 0 doesn't indicate the passed in `ce` is not a submodule any more,
but just that we do not consider cloning it any more.

Signed-off-by: Stefan Beller <sbeller@google.com>
---

 Here is the last patch of the series as an independant bug fix applicable
 on master.
 
 Thanks,
 Stefan

 builtin/submodule--helper.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 6f6d67a..bd7cce6 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -748,8 +748,12 @@ static int update_clone_get_next_task(struct child_process *child,
 	if (index < suc->failed_clones_nr) {
 		int *p;
 		ce = suc->failed_clones[index];
-		if (!prepare_to_clone_next_submodule(ce, child, suc, err))
-			die("BUG: ce was a submodule before?");
+		if (!prepare_to_clone_next_submodule(ce, child, suc, err)) {
+			suc->current ++;
+			strbuf_addf(err, "BUG: submodule considered for cloning,"
+				    "doesn't need cloning any more?\n");
+			return 0;
+		}
 		p = xmalloc(sizeof(*p));
 		*p = suc->current;
 		*idx_task_cb = p;
-- 
2.9.2.583.gd6329be.dirty