[PATCH v2 1/5] merge-ort: propagate callback errors from traverse_trees_wrapper()

["Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Elijah Newren <newren@gmail.com>

traverse_trees_wrapper() saves entries from a first pass through
traverse_trees() and then replays them through the real callback
(collect_merge_info_callback).  However, the replay loop silently
discards the callback return value.  This is not a deferred error;
it is an ignored error.

Today the only originator of a negative return in this entire call
graph is traverse_trees()'s "exceeded maximum allowed tree depth"
check; everything else (collect_merge_info_callback,
traverse_trees_wrapper, the inner traverse_trees recursion) only
relays that.  So in current Git, the visible effect of dropping the
replay callback's return value is narrow but bad: a tree nested past
core.maxTreeDepth has its -1 swallowed, the subtree below the limit
is silently pruned, and the merge completes as if that were the
correct result.

A later patch in this series will teach collect_merge_info_callback()
to return -1 on an additional path -- detecting duplicate
entries in malformed trees -- which is similarly handled today by
just ignoring the problem (resulting in mostly a "last one wins" rule,
though the non-last entry can mutate various state flags).

Capture the return value, stop the loop on negative returns, and
propagate the error to the caller.  The callback returns a positive mask
value on success, so normalize non-negative returns to
0 for the caller.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 merge-ort.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/merge-ort.c b/merge-ort.c
index 00923ce3cd..4b8e32209d 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -1008,18 +1008,20 @@ static int traverse_trees_wrapper(struct index_state *istate,
 	info->traverse_path = renames->callback_data_traverse_path;
 	info->fn = old_fn;
 	for (i = old_offset; i < renames->callback_data_nr; ++i) {
-		info->fn(n,
-			 renames->callback_data[i].mask,
-			 renames->callback_data[i].dirmask,
-			 renames->callback_data[i].names,
-			 info);
+		ret = info->fn(n,
+			       renames->callback_data[i].mask,
+			       renames->callback_data[i].dirmask,
+			       renames->callback_data[i].names,
+			       info);
+		if (ret < 0)
+			break;
 	}
 
 	renames->callback_data_nr = old_offset;
 	free(renames->callback_data_traverse_path);
 	renames->callback_data_traverse_path = old_callback_data_traverse_path;
 	info->traverse_path = NULL;
-	return 0;
+	return ret < 0 ? ret : 0;
 }
 
 static void setup_path_info(struct merge_options *opt,
-- 
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