Configure the compiler ProgramDb once per project

[sheaf]

This PR ensures we configure the compiler program database (constituted of ghc and attendant programs such as ghc-pkg, haddock, toolchain programs such as ar, ld etc) ahead of time within cabal-install, so that we don't need to reconfigure programs such as ghc, ghc-pkg, haddock etc once per package within Cabal.

This should be a net performance win without any other change in behaviour.

---

Template Α: This PR modifies behaviour or interface

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• No manual QA notes necessary (no change to the command line interface).
• Testing approach: the subtle tests like BuildToolPaths ensure we don't regress when users pass --with-alex (for example).

[sheaf]

This is a thorny area, see e.g. #10692 (commits 24f8395 and 2c19bf3) and #11373 (040a97d). So I would appreciate a careful review.

[sheaf]

I have put up a spreadsheet here, compiling pandoc from scratch and using the --build-timings feature from #11769.

Base average time spent in configure: 2.9 seconds.
With this patch: it goes down to 1.2 seconds.
With #11767: it goes down to 1.7 seconds.
With both: it goes down to 0.15 seconds.

[Mikolaj]

This really looks tricky --- so few changes, but such a great effect.

Would it possible to add some tests that try to break things by performing a few scenarios that previously took many configuration invocations and now that just one? Maybe to these twice for a good measure?

A question: does the "once per project" mean once per project per cabal run, or until cabal clean happens or something else invalidates things?

[philderbeast]

Are you able to add haddocks to configureCompiler, including what is in ProgramDb and when it is set up and cleared?

[philderbeast]

The absence of a casing change in compilerprogdb is a bit odd. Maybe compilerProgDb? Is there a problem with the progdb name?

[philderbeast]

This one could be called unconfiguredProgDb and the other one progdb or progDb, once configured.

[philderbeast]

This doesn't render well:

Is that markdown bold in place of haddocks bold?

@@ -1640,7 +1641,7 @@ elaborateInstallPlan
   -> Platform
   -> Compiler
   -> ProgramDb
-  -- ^ **Configured** compiler program database (ghc, ghc-pkg, haddock, ld, etc)
+  -- ^ __Configured__ compiler program database (ghc, ghc-pkg, haddock, ld, etc)
   -> Maybe PkgConfigDb

[philderbeast]

Is the comment "Passes user-supplied arguments to e.g. GHC" no longer relevant?

[philderbeast]

That's part of configuring a program, isn't it?

cabal/Cabal/src/Distribution/Simple/Program/Db.hs

Lines 397 to 404 in 5ac41c2

	-- | Try to configure a specific program. If the program is already included in
	-- the collection of unconfigured programs then we use any user-supplied
	-- location and arguments.
	configureUnconfiguredProgram
	:: Verbosity
	-> Program
	-> ProgramDb
	-> IO (Maybe ConfiguredProgram)