Merge branch 'mt/cleanly-die-upon-missing-required-filter'

We had a code to diagnose and die cleanly when a required
clean/smudge filter is missing, but an assert before that
unnecessarily fired, hiding the end-user facing die() message.

* mt/cleanly-die-upon-missing-required-filter:
  convert: fail gracefully upon missing clean cmd on required filter

Author: gitster

convert.c

@@ -1456,7 +1456,6 @@ void convert_to_git_filter_fd(const struct index_state *istate,
 	convert_attrs(istate, &ca, path);
 
 	assert(ca.drv);
-	assert(ca.drv->clean || ca.drv->process);
 
 	if (!apply_filter(path, NULL, 0, fd, dst, ca.drv, CAP_CLEAN, NULL, NULL))
 		die(_("%s: clean filter '%s' failed"), path, ca.drv->name);

t/t0021-conversion.sh

@@ -257,6 +257,30 @@ test_expect_success 'required filter clean failure' '
 	test_must_fail git add test.fc
 '
 
+test_expect_success 'required filter with absent clean field' '
+	test_config filter.absentclean.smudge cat &&
+	test_config filter.absentclean.required true &&
+
+	echo "*.ac filter=absentclean" >.gitattributes &&
+
+	echo test >test.ac &&
+	test_must_fail git add test.ac 2>stderr &&
+	test_i18ngrep "fatal: test.ac: clean filter .absentclean. failed" stderr
+'
+
+test_expect_success 'required filter with absent smudge field' '
+	test_config filter.absentsmudge.clean cat &&
+	test_config filter.absentsmudge.required true &&
+
+	echo "*.as filter=absentsmudge" >.gitattributes &&
+
+	echo test >test.as &&
+	git add test.as &&
+	rm -f test.as &&
+	test_must_fail git checkout -- test.as 2>stderr &&
+	test_i18ngrep "fatal: test.as: smudge filter absentsmudge failed" stderr
+'
+
 test_expect_success 'filtering large input to small output should use little memory' '
 	test_config filter.devnull.clean "cat >/dev/null" &&
 	test_config filter.devnull.required true &&