stop trying to handle substvars idempotently

In the beginning, I tried to be careful to have commands that added a
substvar remove it when ran again with different options that caused it
to not be needed. However, now when I look over the code, I see 3 places
that got it right, 1 that was right but I just broke, and a dozen that
don't even try to handle this case.

Also, handling the case is hard; code that adds substvars may be complex
and calculate versioned dependencies. The removal code then has to somehow
also come up with those same exact dependency strings. It's a recipe for
nasty code and maintenance headache even if I went and fixed everything
right now.

Instead, I dropped the whole thing. Many debhelper commands make no
pretense of being idempotent anyway; it's easy and normal to call dh_prep
when starting a binary package build, with the exact purpose of not needing
to worry about idempotency.

I did leave in the delsubstvar function, as well as the option to
addsubstvar that, confusingly, causes an item to be removed. Just for
library compatability reasons.

1 files changed, 4 insertions, 3 deletions

diff --git a/doc/PROGRAMMING b/doc/PROGRAMMING
index 0fca4722..ca76b9c9 100644
--- a/doc/PROGRAMMING
+++ b/doc/PROGRAMMING
@@ -216,9 +216,10 @@ addsubstvar($package, $substvar, $deppackage, $verinfo, $remove)
 	- version info for the package (optional) (ie: ">= 1.1")
 	- if this last parameter is passed, the thing that would be added
 	  is removed instead. This can be useful to ensure that a debhelper
-	  command is idempotent. Note that without this parameter, if you
-	  call the function twice with the same values it will only add one
-	  item to the substvars file.
+	  command is idempotent. (However, we generally don't bother,
+	  and rely on the user calling dh_prep.) Note that without this
+	  parameter, if you call the function twice with the same values it
+	  will only add one item to the substvars file.
 delsubstvar($package, $substvar)
 	This function removes the entire line for the substvar from the
 	package's shlibs file.