[core] allow overriding prior config values (fixes #2799)

introduce ":=" config file syntax to replace previously set value

github: closes #78

x-ref:
  "allow overriding configuration values"
  https://redmine.lighttpd.net/issues/2799
  https://github.com/lighttpd/lighttpd1.4/pull/78

src/configfile.c

@@ -967,6 +967,14 @@ static int config_tokenizer(server *srv, tokenizer_t *t, int *token_id, buffer *
 			}
 			break;
 
+		case ':':
+			if (t->input[t->offset+1] == '=') {
+				t->offset += 2;
+				tid = TK_FORCE_ASSIGN;
+				buffer_copy_string_len(token, CONST_STR_LEN(":="));
+			}
+			break;
+
 		case '{':
 			t->offset++;
 

src/configparser.y

@@ -206,6 +206,23 @@ varline ::= key(A) ASSIGN expression(B). {
   A = NULL;
 }
 
+varline ::= key(A) FORCE_ASSIGN expression(B). {
+  if (ctx->ok) {
+    if (strncmp(A->ptr, "env.", sizeof("env.") - 1) == 0) {
+      fprintf(stderr, "Setting env variable is not supported in conditional %d %s: %s\n",
+              ctx->current->context_ndx,
+              ctx->current->key->ptr, A->ptr);
+      ctx->ok = 0;
+    } else {
+      buffer_copy_buffer(B->key, A);
+      array_replace(ctx->current->value, B);
+      B = NULL;
+    }
+  }
+  buffer_free(A);
+  A = NULL;
+}
+
 varline ::= key(A) APPEND expression(B). {
   if (ctx->ok) {
     array *vars = ctx->current->value;