contributor-guide/style-guide: Add a note about task idempotence

(From yocto-docs rev: 681e6a0a4305057f7b96f5bef804391d8aff812b)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
CC: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/contributor-guide/recipe-style-guide.rst

@@ -257,6 +257,20 @@ Tips and Guidelines for Writing Recipes
    and ``-nativesdk`` ones, whenever possible. This avoids having to maintain multiple
    recipe files at the same time.
 
+-  Recipes should have tasks which are idempotent, i.e. that executing a given task
+   multiple times shouldn't change the end result. The build environment is built upon
+   this assumption and breaking it can cause obscure build failures.
+
+-  For idempotence when modifying files in tasks, it is usually best to:
+
+   - copy a file ``X`` to ``X.orig`` (only if it doesn't exist already)
+   - then, copy ``X.orig`` back to ``X``,
+   - and, finally, modify ``X``.
+
+   This ensures if rerun the task always has the same end result and the
+   original file can be preserved to reuse. It also guards against an
+   interrupted build corrupting the file.
+
 Patch Upstream Status
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