add WITH-CHANGE-IF-CHANGES-FILE{,-CONTENT}

Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>

3 files changed, 34 insertions, 5 deletions

diff --git a/doc/ideas.rst b/doc/ideas.rst
index d32ffed..1209e7d 100644
--- a/doc/ideas.rst
+++ b/doc/ideas.rst
@@ -49,11 +49,6 @@ Core
   calling its :HOSTATTRS subroutine too -- could we figure out catching and
   ignoring the condition when its :HOSTATTRS subroutine did get run?
 
-- Macro for use in DEFPROP which works like Propellor's changesFile.  Will
-  probably output ``(:check ...)`` expression and then substitute user's code
-  into a ``(:apply ...)`` expression.  Use this or variants thereof in most of
-  the entries in ``PROPERTY.FILE``.
-
 - HOSTDEPLOY and HOSTDEPLOY-THESE functions which are like DEPLOY and
   DEPLOY-THESE but take the CONNECTION argument from the :DEPLOY argument to
   DEFHOST.
diff --git a/src/package.lisp b/src/package.lisp
index 5631860..efce2ee 100644
--- a/src/package.lisp
+++ b/src/package.lisp
@@ -130,6 +130,8 @@
            #:assert-euid-root
            #:assert-connection-supports
            #:call-with-os
+           #:with-change-if-changes-file
+           #:with-change-if-changes-file-content
 
            ;; propspec.lisp
            #:in-consfig
diff --git a/src/property.lisp b/src/property.lisp
index 7ff6c92..4403eb7 100644
--- a/src/property.lisp
+++ b/src/property.lisp
@@ -468,3 +468,35 @@ apply or unapply properties.")
   (unless (or (eq type :posix) (lisp-connection-p))
     (failed-change
      "Cannot apply :LISP properties using a POSIX-type connection")))
+
+(defun cksum (file)
+  (ignore-errors (parse-integer (car (split-string (mrun "cksum" file))))))
+
+;; this is a safe parse of ls(1) output given its POSIX specification
+(defun ls-cksum (file)
+  (let ((ls (ignore-errors
+             (split-string (mrun :env '(:LOCALE "C") "ls" "-dlL" file))))
+        (cksum (cksum file)))
+    (when (and ls cksum)
+      (list* (car ls) cksum (subseq ls 2 8)))))
+
+(defmacro with-change-if-changes-file ((file) &body forms)
+  "Execute FORMS and yield :NO-CHANGE if FILE does not change.
+Since stat(1) is not POSIX, this is implemented by calling `ls -dlL' and
+cksum(1), and seeing if any of the information reported there, except for the
+number of links, has changed.  Thus, you should not use this macro to detect
+changes in properties which will change the file but not the output of `ls
+-dlL' and cksum(1)."
+  (with-gensyms (before)
+    `(let* ((,before (ls-cksum ,file))
+            (result (progn ,@forms)))
+       (if (and ,before (equal ,before (ls-cksum ,file)))
+           :no-change result))))
+
+(defmacro with-change-if-changes-file-content ((file) &body forms)
+  "Execute FORMS and yield :NO-CHANGE if FILE has the same content afterwards."
+  (with-gensyms (before)
+    `(let* ((,before (cksum ,file))
+            (result (progn ,@forms)))
+       (if (and ,before (eql ,before (cksum ,file)))
+           :no-change result))))