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>

Documentation/config/advice.txt

@@ -107,4 +107,7 @@ advice.*::
 		editor input from the user.
 	nestedTag::
 		Advice shown if a user attempts to recursively tag a tag object.
+	submoduleAlternateErrorStrategyDie:
+		Advice shown when a submodule.alternateErrorStrategy option
+		configured to "die" causes a fatal error.
 --

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 }

advice.h

@@ -30,6 +30,7 @@ extern int advice_waiting_for_editor;
 extern int advice_graft_file_deprecated;
 extern int advice_checkout_ambiguous_remote_branch_name;
 extern int advice_nested_tag;
+extern int advice_submodule_alternate_error_strategy_die;
 
 int git_default_advice_config(const char *var, const char *value);
 __attribute__((format (printf, 1, 2)))

builtin/submodule--helper.c

@@ -19,6 +19,7 @@
 #include "diffcore.h"
 #include "diff.h"
 #include "object-store.h"
+#include "advice.h"
 
 #define OPT_QUIET (1 << 0)
 #define OPT_CACHED (1 << 1)
@@ -1268,6 +1269,13 @@ struct submodule_alternate_setup {
 #define SUBMODULE_ALTERNATE_SETUP_INIT { NULL, \
 	SUBMODULE_ALTERNATE_ERROR_IGNORE, NULL }
 
+static const char alternate_error_advice[] = N_(
+"An alternate computed from a superproject's alternate is invalid.\n"
+"To allow Git to clone without an alternate in such a case, set\n"
+"submodule.alternateErrorStrategy to 'info' or, equivalently, clone with\n"
+"'--reference-if-able' instead of '--reference'."
+);
+
 static int add_possible_reference_from_superproject(
 		struct object_directory *odb, void *sas_cb)
 {
@@ -1299,6 +1307,8 @@ static int add_possible_reference_from_superproject(
 		} else {
 			switch (sas->error_mode) {
 			case SUBMODULE_ALTERNATE_ERROR_DIE:
+				if (advice_submodule_alternate_error_strategy_die)
+					advise(_(alternate_error_advice));
 				die(_("submodule '%s' cannot add alternate: %s"),
 				    sas->submodule_name, err.buf);
 			case SUBMODULE_ALTERNATE_ERROR_INFO: