In the current SA code, we need to remember to call Rollback on any error.
If we don't, we'll leave dangling transactions, which are hard to spot but eventually
clog up the database and cause availability problems.
This change attempts to deal with rollbacks more rigorously, by implementing a
withTransaction function that takes a closure as input. withTransaction opens
a transaction, applies a context.Context to it, and then runs the closure. If the
closure returns an error, withTransaction rolls back and return the error; otherwise
it commits and returns nil.
One of the quirks of this implementation is that it relies on the closure modifying
variables from its parent scope in order to return values. An alternate implementation
could define the return value of the closure as interface{}, nil, and have the calling
function do a type assertion. I'm seeking feedback on that; not sure yet which is cleaner.
This is a subset of the functions that need this treatment. I've got more coming, but
some of the changes break tests so I'm checking into why.
Updates #4337
16235b6839