From: Davin McCall Date: Sun, 8 Jul 2018 17:00:43 +0000 (+0100) Subject: Some updates to the design document. X-Git-Tag: v0.4.0~64 X-Git-Url: https://git.librecmc.org/?a=commitdiff_plain;h=7366c8fbf79495cb69deb8b48283d8ebef5e8582;p=oweals%2Fdinit.git Some updates to the design document. --- diff --git a/doc/DESIGN b/doc/DESIGN index c776fa5..1c1f2be 100644 --- a/doc/DESIGN +++ b/doc/DESIGN @@ -1,6 +1,9 @@ Dinit Design Overview ===================== +NOTE: This document is intended to provide an overview. Specifics are generally included as + comments/documentation within source files. + Dinit's overall function is fairly straightforward but there are some considerations that cause its design to be more complicated than it might otherwise be. The heart of the issue is that Dinit should not stall on I/O and its essential operation should never fail, which means that many @@ -14,6 +17,9 @@ with the Dinit source, but has a separate web page and documentation: http://davmac.org/projects/dasynq/ +Note that blocking actions are avoided: non-blocking I/O is used where possible; Dinit must +remain responsive at all times. + In Dinit, service actions are often performed by setting "propagation" flags, inserting the service in a queue, and then processing the queues. Thus a lot of service actions are performed by first calling a function on a service and then draining the propagation queues. More @@ -66,8 +72,41 @@ control.cc - control protocol handling dinit-log.cc - logging subsystem +tests/ - various tests The utility sources are: dinitctl.cc - the control/status utility shutdown.cc - shutdown/halt/reboot utility + + +Testing +------- + +The tests work by mocking out parts of Dinit, and some system calls, in order to isolate +functional units. In the src/tests/test-includes directory are three mock headers. When compiling +tests, the src/tests/includes directory is populated with (linked) copies of the standard headers +from src/include, but with mocked headers taken from src/tests/test-includes instead. + +Note that systems calls are not mocked directly, instead: + + - system calls are wrapped in the bp_sys namespace, as defined in the baseproc-sys.h header; + - for testing, the header is replaced with a mock header. + +(This avoids problems that might arise from replacing important system calls, and in +particular avoids interfering with the test harness itself). + +It is important when writing new code in Dinit to avoid calling system calls directly, and to +instead call the wrapper in bp_sys. + + +Exception safety, error handling +-------------------------------- + +In general operation Dinit methods should avoid throwing exceptions and be declared as 'noexcept', +or otherwise be clearly documented as throwing exceptions. Errors should always be handled as +gracefully as possible and should not prevent Dinit's continued operation. Particular care is +needed for dynamic allocations: C++ style allocations (including adding elements to C++ +containers) will raise 'std::bad_alloc' if they cannot allocate memory, and this must be handled +appropriately. Once it has started regular operation, Dinit must not terminate due to an error +condition, even if the error is an allocation failure.