submodule--helper: advise on fatal alternate error

When recursively cloning a superproject with some shallow modules
defined in its .gitmodules, then recloning with "--reference=<path>", an
error occurs. For example:

  git clone --recurse-submodules --branch=master -j8 \
    https://android.googlesource.com/platform/superproject \
    master
  git clone --recurse-submodules --branch=master -j8 \
    https://android.googlesource.com/platform/superproject \
    --reference master master2

fails with:

  fatal: submodule '<snip>' cannot add alternate: reference repository
  '<snip>' is shallow

When a alternate computed from the superproject's alternate cannot be
added, whether in this case or another, advise about configuring the
"submodule.alternateErrorStrategy" configuration option and using
"--reference-if-able" instead of "--reference" when cloning.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/advice.c b/advice.c
index 3ee0ee2..249c60d 100644
--- a/advice.c
+++ b/advice.c
@@ -30,6 +30,7 @@ int advice_waiting_for_editor = 1;
 int advice_graft_file_deprecated = 1;
 int advice_checkout_ambiguous_remote_branch_name = 1;
 int advice_nested_tag = 1;
+int advice_submodule_alternate_error_strategy_die = 1;
 
 static int advice_use_color = -1;
 static char advice_colors[][COLOR_MAXLEN] = {
@@ -89,6 +90,7 @@ static struct {
 	{ "graftFileDeprecated", &advice_graft_file_deprecated },
 	{ "checkoutAmbiguousRemoteBranchName", &advice_checkout_ambiguous_remote_branch_name },
 	{ "nestedTag", &advice_nested_tag },
+	{ "submoduleAlternateErrorStrategyDie", &advice_submodule_alternate_error_strategy_die },
 
 	/* make this an alias for backward compatibility */
 	{ "pushNonFastForward", &advice_push_update_rejected }