feat(sterni/mblog): implement environment based config mechanism

Change-Id: I091c0d5decc0a1eb3d24e81b713434ab391c677d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8347
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

users/sterni/mblog/config.lisp

@@ -0,0 +1,31 @@
+;; SPDX-License-Identifier: GPL-3.0-only
+;; SPDX-FileCopyrightText: Copyright (C) 2023 by sterni
+
+(in-package :config)
+
+(eval-when (:compile-toplevel :load-toplevel)
+  (defun plist-to-alist (lst)
+    (loop for (name . (default . (parser . nil))) on lst by #'cdddr
+          collect (cons name (list default parser))))
+
+  (defun symbol-to-env-var-name (symbol)
+    (concatenate 'string
+                 "MBLOG_"
+                 (string-upcase
+                  (remove #\* (substitute #\_ #\- (string symbol)))))))
+
+(defmacro define-configuration-variables (&rest args)
+  (let ((vars (plist-to-alist args))
+        (val-var-sym (gensym)))
+    `(progn
+       ,@(loop for (name . (default nil)) in vars
+              collect `(defvar ,name ,default))
+
+       (defun init-from-env ()
+         ,@(loop for (name . (nil parser)) in vars
+                 collect
+                 `(when-let ((,val-var-sym (getenv ,(symbol-to-env-var-name name))))
+                    (setf ,name (funcall ,parser ,val-var-sym))))))))
+
+(define-configuration-variables
+  *general-buffer-size* 4096 #'parse-integer)