Cabal: Profiling with Custom setup < 1.24 doesn't work

[ezyang]

It seems that Cabal 1.22 doesn't understand how to install profiling objects, or maybe it doesn't understand the profiling flag? In any case, if we build a custom setup with Cabal 1.22 we will run into trouble if we try to do profiling; see @bitc's report at #3485 for where this cropped up in the wild.

One possible way to fix this is to add an extra solver parameter saying, "Want profiling" and then add extra Cabal library constraints (preferences?) accordingly. But I am a bit worried, because this means that the solver plan with and without profiling will vary, which is pretty confusing because you wouldn't expect turning on profiling to change what libraries get picked (you just wanted to profile the damn configuration you had!) So it's not altogether clear what we should do here. Maybe a compromise is to print a warning in this case?

[dcoutts]

I believe this is due to the new --enable-profiling vs the old --enable-library-profiling vs --enable-executable-profiling flags and so it should be just a matter of translating into the older flags when talking to an older Cabal setup cli version.

What would be nice is if internally we translate from all these flags into one canonical value representing what it means, and the translate back when we construct the Setup.hs command line, and have that translation be version dependent.

[dcoutts]

Oh and just to record what we said elsewhere, that ideally we should not have to force cabal version constraints depending on feature flags because it's rather nice to have the property that the versions chosen does not depend on whether we're testing/profiling etc.

In cases where we know it's impossible to support a request to use some new feature with an older Cabal lib (used for a custom setup hs) then we can perhaps suggest that users use a flag (e.g. --constraint="Cabal >= x.y" or in cabal.project.local) so that it is explicit. Note that there's also the old cabal-lib-version flag, which the old code path uses to select the Cabal lib version for Setup.hs, which we could re-purpose as an alias for --constraint="Cabal == x.y".

[23Skidoo]

I believe this is due to the new --enable-profiling vs the old --enable-library-profiling vs --enable-executable-profiling flags and so it should be just a matter of translating into the older flags when talking to an older Cabal setup cli version.

Hmm, I thought that we already did that?

/cc @ttuegel

[ezyang]

Related #3502; right now constraint on Cabal is an all-or-nothing deal.

OK, so if we think we can support this for 1.22 that would solve the problem. I've changed the ticket name.

[ezyang]

Apparently we do that:

    -- Cabal < 1.21.1 doesn't know about 'disable-relocatable'
    -- Cabal < 1.21.1 doesn't know about 'enable-profiling'
    flags_1_21_1 =
      flags_1_22_0 { configRelocatable = NoFlag
                   , configProf = NoFlag
                   , configProfExe = configProf flags
                   , configProfLib =
                     mappend (configProf flags) (configProfLib flags)
                   , configCoverage = NoFlag
                   , configLibCoverage = configCoverage flags
                   }

So the 1.22 bug is different in some way.

[ezyang]

Oh. Ohhhhhhh. mappend (Flag True) (Flag False) == Flag False. Oops. EDIT: Actually, this shouldn't cause the problem...

[ezyang]

OK, so the semantics of --enable-profiling, --enable-library-profiling and --enable-executable-profiling have changed over the releases of Cabal. Here's what's going on:

HEAD (modestly abridged)

  let tryExeProfiling = fromFlagOrDefault False
                        (mappend (configProf cfg) (configProfExe cfg))
      tryLibProfiling = fromFlagOrDefault tryExeProfiling
                        (mappend (configProf cfg) (configProfLib cfg))

The mappend business is tricky, because if both flags are set, the later one is preferred. So here are some particularly tricky cases:

• --enable-executable-profiling: lib YES, exe YES
• --enable-profiling --disable-library-profiling: lib NO, exe YES
• --disable-profiling --enable-executable-profiling: lib NO, exe YES

1.24.0.0

    let profEnabledBoth    = fromFlagOrDefault False (configProf cfg)
        profEnabledLib = fromFlagOrDefault profEnabledBoth (configProfLib cfg)
        profEnabledExe = fromFlagOrDefault profEnabledBoth (configProfExe cfg)

Hard cases:

• --enable-executable-profiling: lib NO, exe YES (DIFFERENT!)
• --enable-profiling --disable-library-profiling: lib NO, exe YES
• --disable-profiling --enable-executable-profiling: lib NO, exe YES

1.22.0.0

        let withProfExe_ = fromFlagOrDefault False $ configProfExe cfg
            withProfLib_ = fromFlagOrDefault withProfExe_ $ configProfLib cfg

Hard case: --enable-executable-profiling: lib YES, exe YES (BACK TO HEAD BEHAVIOR!)

--enable-profiling was introduced in 1.24 which is when this stuff got complicated.

There is a hack in 1.22.5.0:

      ,option "" ["profiling"]
         "Executable profiling (requires library profiling)"
         -- HACK: See #2409. Thankfully, this is 1.22-specific.
         (\flags ->
           case (configProfLib flags, configProfExe flags) of
             (Flag a, Flag b)
               | (a == b)
               && ("cabalConfProf", "/TRUE") `elem` configProgramPaths flags
                    -> configProfExe flags
             _      -> NoFlag)
         (\v flags -> flags
               { configProfLib = v, configProfExe = v
               , configProgramPaths = ("cabalConfProf", "/TRUE")
               : configProgramPaths flags })
         (boolOpt [] [])

I... have no idea what this is.

1.20.0.0, 1.18.0 and probably everything older.

                    withProfLib         = fromFlag $ configProfLib cfg,
                    withProfExe         = fromFlag $ configProfExe cfg,

Hard case: --enable-library-profiling: lib NO, exe YES (AND IT'S BROKEN AGAIN)

So, I think what our setup wrapper script should do is just smooth over all the compatibility differences. EDIT: But none of these problems should be causing the bug reported here.

CC @ttuegel

[ezyang]

OK, I've diagnosed the actual problem.

The problem is with 1.22.5.0 (and any other version with the hack mentioned in the comment above), the semantics of --disable-profiling/--enable-profiling depend on ordering (because the hack operates by looking at the current flag assignment and doing something). In particular, if I specify --enable-library-profiling --disable-profiling, I end up with library profiling DISABLED. This is NOT the case for 1.24, which has an order-invariant approach. If I swap the ordering things are OK.

[ezyang]

So, I think my suggested fix is that we NEVER pass --enable-profiling or --disable-profiling to Cabal. At the moment, and according to my historical analysis, Cabal ONLY uses configProf to affect the effective library/executable profiling, which means that anything we do with --enable-profiling, we can do using the library/executable profiling individually. Since these are always flags for the versions of Cabal library we support, we will get order invariance. Historical versions have varied on whether or not setting executable profiling implies library profiling, but if we set both explicitly this change in behavior doesn't matter. I'll code it up and test.

[ttuegel]

So, I think my suggested fix is that we NEVER pass --enable-profiling or --disable-profiling to Cabal.

That should be fine. Those flags exist to stop folks from shooting themselves in the foot by doing --enable-executable-profiling without --enable-library-profiling.

[23Skidoo]

Also fixed on the 1.24 branch in #3953.