[multiple] perf: power-2 alloc large headers

allocate large headers by power-2 as headers to backend is generated
(mod_proxy, mod_fastcgi, mod_scgi)

src/mod_fastcgi.c

@@ -118,7 +118,11 @@ static int fcgi_env_add(void *venv, const char *key, size_t key_len, const char
 	force_assert(key_len < 0x7fffffffu);
 	force_assert(val_len < 0x7fffffffu);
 
-	buffer_string_prepare_append(env, len);
+	if (buffer_string_space(env) < len) {
+		size_t extend = env->size * 2 - buffer_string_length(env);
+		extend = extend > len ? extend : len + 4095;
+		buffer_string_prepare_append(env, extend);
+	}
 
 	if (key_len > 127) {
 		len_enc[len_enc_len++] = ((key_len >> 24) & 0xff) | 0x80;

src/mod_proxy.c

@@ -792,6 +792,12 @@ static handler_t proxy_create_env(server *srv, gw_handler_ctx *gwhctx) {
 		vlen = buffer_string_length(ds->value);
 		if (0 == vlen) continue;
 
+		if (buffer_string_space(b) < klen + vlen + 4) {
+			size_t extend = b->size * 2 - buffer_string_length(b);
+			extend = extend > klen + vlen + 4 ? extend : klen + vlen + 4 + 4095;
+			buffer_string_prepare_append(b, extend);
+		}
+
 		buffer_append_string_len(b, ds->key->ptr, klen);
 		buffer_append_string_len(b, CONST_STR_LEN(": "));
 		buffer_append_string_len(b, ds->value->ptr, vlen);

src/mod_scgi.c

@@ -100,7 +100,11 @@ static int scgi_env_add_scgi(void *venv, const char *key, size_t key_len, const
 
 	len = key_len + val_len + 2;
 
-	buffer_string_prepare_append(env, len);
+	if (buffer_string_space(env) < len) {
+		size_t extend = env->size * 2 - buffer_string_length(env);
+		extend = extend > len ? extend : len + 4095;
+		buffer_string_prepare_append(env, extend);
+	}
 
 	buffer_append_string_len(env, key, key_len);
 	buffer_append_string_len(env, "", 1);