UTIL: sss_create_internal_fqname() optimization (caching)

This helper is heavily used, including in hot paths.

Since number of domains used is very limited, hash table used for caching
should be very small and lookup much more efficient as compared with
`sss_tc_utf8_str_tolower()`

Author: alexey-tikhonov

src/util/usertools.c

@@ -638,29 +638,75 @@ errno_t sss_parse_internal_fqname(TALLOC_CTX *mem_ctx,
     return EOK;
 }
 
+/* This is a wrapper around `sss_tc_utf8_str_tolower()` that
+ * maintains run time cache.
+ */
+static const char *sss_get_lc_dom_name(const char *dom_name)
+{
+    static TALLOC_CTX *cache_ctx;
+    static hash_table_t *lc_dom_name_cache;
+    hash_key_t key;
+    hash_value_t value;
+    char *lc_dom_name;
+    int hret;
+
+    key.type = HASH_KEY_STRING;
+    key.str = discard_const(dom_name);
+
+    if (lc_dom_name_cache != NULL) {
+        hret = hash_lookup(lc_dom_name_cache, &key, &value);
+        if (hret == HASH_SUCCESS) {
+            return (const char *)value.ptr;
+        }
+    } else {
+        cache_ctx = talloc_new(NULL);
+        if (!cache_ctx) return NULL;
+        hret = hash_create(0, &lc_dom_name_cache, NULL, NULL);
+        if (hret != HASH_SUCCESS) {
+            DEBUG(SSSDBG_CRIT_FAILURE,
+                  "hash_create() failed [%s]\n", hash_error_string(hret));
+            return NULL;
+        }
+    }
+
+    lc_dom_name = sss_tc_utf8_str_tolower(cache_ctx, dom_name);
+    if (lc_dom_name == NULL) {
+        return NULL;
+    }
+
+    value.type = HASH_VALUE_PTR;
+    value.ptr = lc_dom_name;
+
+    hret = hash_enter(lc_dom_name_cache, &key, &value);
+    if (hret != HASH_SUCCESS) {
+        DEBUG(SSSDBG_CRIT_FAILURE,
+              "hash_enter() failed [%s]\n", hash_error_string(hret));
+        talloc_free(lc_dom_name);
+        return NULL;
+    }
+
+    return lc_dom_name;
+}
+
 /* Creates internal fqname in format shortname@domname.
  * The domain portion is lowercased. */
 char *sss_create_internal_fqname(TALLOC_CTX *mem_ctx,
                                  const char *shortname,
                                  const char *dom_name)
 {
-    char *lc_dom_name;
-    char *fqname = NULL;
+    const char *lc_dom_name;
 
     if (shortname == NULL || dom_name == NULL) {
         /* Avoid allocating null@null */
         return NULL;
     }
 
-    lc_dom_name = sss_tc_utf8_str_tolower(mem_ctx, dom_name);
+    lc_dom_name = sss_get_lc_dom_name(dom_name);
     if (lc_dom_name == NULL) {
-        goto done;
+        return NULL;
     }
 
-    fqname = talloc_asprintf(mem_ctx, "%s@%s", shortname, lc_dom_name);
-    talloc_free(lc_dom_name);
-done:
-    return fqname;
+    return talloc_asprintf(mem_ctx, "%s@%s", shortname, lc_dom_name);
 }
 
 /* Creates a list of internal fqnames in format shortname@domname.