core-util: Improve pa_replace() behaviour

- Assert that the search string isn't empty.
 - Add test.
 - Improve documentation.

src/pulsecore/core-util.c

@@ -3194,6 +3194,7 @@ char *pa_replace(const char*s, const char*a, const char *b) {
 
     pa_assert(s);
     pa_assert(a);
+    pa_assert(*a);
     pa_assert(b);
 
     an = strlen(a);

src/pulsecore/core-util.h

@@ -249,6 +249,10 @@ void pa_reduce(unsigned *num, unsigned *den);
 
 unsigned pa_ncpus(void);
 
+/* Replaces all occurrences of `a' in `s' with `b'. The caller has to free the
+ * returned string. All parameters must be non-NULL and additionally `a' must
+ * not be a zero-length string.
+ */
 char *pa_replace(const char*s, const char*a, const char *b);
 
 /* Escapes p by inserting backslashes in front of backslashes. chars is a

src/tests/core-util-test.c

@@ -228,6 +228,11 @@ START_TEST (modargs_test_escape) {
 }
 END_TEST
 
+START_TEST (modargs_test_replace_fail_4) {
+    pa_replace("abe", "", "bab");
+}
+END_TEST
+
 START_TEST (modargs_test_unescape) {
     char* value;
 
@@ -264,6 +269,7 @@ int main(int argc, char *argv[]) {
     tcase_add_test_raise_signal(tc, modargs_test_replace_fail_1, SIGABRT);
     tcase_add_test_raise_signal(tc, modargs_test_replace_fail_2, SIGABRT);
     tcase_add_test_raise_signal(tc, modargs_test_replace_fail_3, SIGABRT);
+    tcase_add_test_raise_signal(tc, modargs_test_replace_fail_4, SIGABRT);
     tcase_add_test(tc, modargs_test_escape);
     tcase_add_test(tc, modargs_test_unescape);