Bad array indexing is considered deadly

[Vladimir Panteleev via Digitalmars-d]

On Wednesday, 31 May 2017 at 13:04:52 UTC, Steven Schveighoffer 
wrote:
> I have discovered an annoyance in using vibe.d instead of 
> another web framework. Simple errors in indexing crash the 
> entire application.

Since I wrote/run a bunch of websites/network services written in 
D, here's my experience/advice:

First, this is not something specific to array indexing, but an 
entire class of logic errors which are sometimes recoverable. 
Other examples are associative array indexing, division by zero, 
and out-of-memory errors resulting from underflows. All of these 
are due to bugs in the program, but could hypothetically be 
handled without compromising the integrity of the process.

My advice:

1. Let the program crash. Make sure it's restarted afterwards, 
either via a looping script, or a watchdog.

2. Make sure you are notified of the error. I don't mean just 
recorded in a log file somewhere, but set it up so you receive an 
email any time it happens, with the stack trace. I run all my D 
network services from a cronjob, which automatically sends output 
by email. If you have the stack trace, most of these bugs take 
only a few minutes to fix - at the very least, turning the error 
into an exception is a trivial modification if you don't have 
time for a full root cause analysis at that moment.

3. Design your program so that it can be terminated at any point 
without resulting in data corruption. I don't know if Vibe.d can 
satisfy this constraint, but e.g. the ae.net.http.server workflow 
is to build/send the entire response atomically, meaning that the 
Content-Length will always be populated. Wrap your database 
updates in transactions. Use the "write to temporary file then 
rename over the original file" pattern when updating files. Etc.