Documentation: coding-style: Document error handling rules

Following a conversation on the mailing list about the use of
assertions, document the error handling strategy in libcamera. This is
an initial set of rules that are expected be extended and detailed in
the future.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Documentation/coding-style.rst

@@ -217,6 +217,36 @@ global variable, should run a minimal amount of code in the constructor and
 destructor, and code that contains dependencies should be moved to a later
 point in time. 
 
+Error Handling
+~~~~~~~~~~~~~~
+
+Proper error handling is crucial to the stability of libcamera. The project
+follows a set of high-level rules:
+
+* Make errors impossible through API design. The best way to handle errors is
+  to prevent them from happening in the first place. The preferred option is
+  thus to prevent error conditions at the API design stage when possible.
+* Detect errors at compile time. Compile-test checking of errors not only
+  reduces the runtime complexity, but also ensures that errors are caught early
+  on during development instead of during testing or, worse, in production. The
+  static_assert() declaration should be used where possible for this purpose.
+* Validate all external API contracts. Explicit pre-condition checks shall be
+  used to validate API contracts. Whenever possible, appropriate errors should
+  be returned directly. As libcamera doesn't use exceptions, errors detected in
+  constructors shall result in the constructed object being marked as invalid,
+  with a public member function available to check validity. The checks should
+  be thorough for the public API, and may be lighter for internal APIs when
+  pre-conditions can reasonably be considered to be met through other means.
+* Use assertions for fatal issues only. The ASSERT() macro causes a program
+  abort when compiled in debug mode, and is a no-op otherwise. It is useful to
+  abort execution synchronously with the error check instead of letting the
+  error cause problems (such as segmentation faults) later, and to provide a
+  detailed backtrace. Assertions shall only be used to catch conditions that are
+  never supposed to happen without a serious bug in libcamera that would prevent
+  safe recovery. They shall never be used to validate API contracts. The
+  assertion conditions shall not cause any side effect as they are compiled out
+  in non-debug mode.
+
 C Compatibility Headers
 ~~~~~~~~~~~~~~~~~~~~~~~