Controlling hairpin length

[David Sumbler]

Although I quite often find myself needing to use

\override Voice.Hairpin.minimum-length = #2

or similar, I find that the results are unpredictable (to me!)

Sometimes I have a very short hairpin, perhaps starting on a quaver and
ending on the next quaver.  In the output such a hairpin sometimes
appears as a short vertical line (i.e. with length=0), and a warning is
shown in the compiler output.

If I change Hairpin.minimum-length I have to do it by trial and error,
because the result does not seem necessarily to correspond to the value
I have specified.  (I am assuming that the units are staff spaces.)  I'm
guessing that this may perhaps be something to do with having an actual
dynamic such as mf attached to one or both notes.

The Internals Reference says that use of the minimum-length property
with a hairpin "requires an appropriate callback for the
springs-and-rods property".  Unfortunately I have no clear idea what
this means, and I haven't managed to find out yet either by searching or
by experimentation.  The best I have come up with so far is

\override Hairpin.springs-and-rods = #ly:spanner::set-spacing-rods

This doesn't improve the output at all, but at least it doesn't produce
any additional error or warning!

How can I guarantee that all hairpins will be printed with a minimum
length of, say, 1.5 staff spaces, and the notes spaced appropriately,
regardless of preceding or following dynamics?

David