merge: replace atoi() with strtol_i() for marker size validation

Replace atoi() with strtol_i() for parsing conflict-marker-size to
improve error handling. Invalid values, such as those containing letters
now trigger a clear error message.
Update the test to verify invalid input handling.

Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Author: Unique-Usman

merge-ll.c

@@ -15,6 +15,7 @@
 #include "merge-ll.h"
 #include "quote.h"
 #include "strbuf.h"
+#include "gettext.h"
 
 struct ll_merge_driver;
 
@@ -427,7 +428,10 @@ enum ll_merge_result ll_merge(mmbuffer_t *result_buf,
 	git_check_attr(istate, path, check);
 	ll_driver_name = check->items[0].value;
 	if (check->items[1].value) {
-		marker_size = atoi(check->items[1].value);
+		if (strtol_i(check->items[1].value, 10, &marker_size)) {
+			marker_size = DEFAULT_CONFLICT_MARKER_SIZE;
+			warning(_("invalid marker-size '%s', expecting an integer"), check->items[1].value);
+		}
 		if (marker_size <= 0)
 			marker_size = DEFAULT_CONFLICT_MARKER_SIZE;
 	}
@@ -454,7 +458,10 @@ int ll_merge_marker_size(struct index_state *istate, const char *path)
 		check = attr_check_initl("conflict-marker-size", NULL);
 	git_check_attr(istate, path, check);
 	if (check->items[0].value) {
-		marker_size = atoi(check->items[0].value);
+		if (strtol_i(check->items[0].value, 10, &marker_size)) {
+			marker_size = DEFAULT_CONFLICT_MARKER_SIZE;
+			warning(_("invalid marker-size '%s', expecting an integer"), check->items[0].value);
+		}
 		if (marker_size <= 0)
 			marker_size = DEFAULT_CONFLICT_MARKER_SIZE;
 	}

t/t6406-merge-attr.sh

@@ -118,6 +118,14 @@ test_expect_success 'retry the merge with longer context' '
 	grep "<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<" actual
 '
 
+test_expect_success 'invalid conflict-marker-size 3a' '
+	cp .gitattributes .gitattributes.bak &&
+	echo "text conflict-marker-size=3a" >>.gitattributes &&
+	test_when_finished "mv .gitattributes.bak .gitattributes" &&
+	git checkout -m text 2>err &&
+	test_grep "warning: invalid marker-size ${SQ}3a${SQ}, expecting an integer" err
+'
+
 test_expect_success 'custom merge backend' '
 
 	echo "* merge=union" >.gitattributes &&