string-util: readd string_erase()

This was dropped in 8e27167cc9, but is
actually useful for some usecases still.

src/basic/string-util.c

@@ -1064,3 +1064,13 @@ bool string_is_safe(const char *p) {
 
         return true;
 }
+
+char* string_erase(char *x) {
+        if (!x)
+                return NULL;
+
+        /* A delicious drop of snake-oil! To be called on memory where we stored passphrases or so, after we
+         * used them. */
+        explicit_bzero_safe(x, strlen(x));
+        return x;
+}

src/basic/string-util.h

@@ -278,3 +278,5 @@ static inline char* str_realloc(char **p) {
 
         return (*p = t);
 }
+
+char* string_erase(char *x);

src/test/test-string-util.c

@@ -9,6 +9,29 @@
 #include "utf8.h"
 #include "util.h"
 
+static void test_string_erase(void) {
+        char *x;
+
+        x = strdupa("");
+        assert_se(streq(string_erase(x), ""));
+
+        x = strdupa("1");
+        assert_se(streq(string_erase(x), ""));
+
+        x = strdupa("123456789");
+        assert_se(streq(string_erase(x), ""));
+
+        assert_se(x[1] == '\0');
+        assert_se(x[2] == '\0');
+        assert_se(x[3] == '\0');
+        assert_se(x[4] == '\0');
+        assert_se(x[5] == '\0');
+        assert_se(x[6] == '\0');
+        assert_se(x[7] == '\0');
+        assert_se(x[8] == '\0');
+        assert_se(x[9] == '\0');
+}
+
 static void test_free_and_strndup_one(char **t, const char *src, size_t l, const char *expected, bool change) {
         int r;
 
@@ -543,6 +566,7 @@ static void test_memory_startswith_no_case(void) {
 int main(int argc, char *argv[]) {
         test_setup_logging(LOG_DEBUG);
 
+        test_string_erase();
         test_free_and_strndup();
         test_ascii_strcasecmp_n();
         test_ascii_strcasecmp_nn();