Track source files of dependencies so we rebuild as necessary.

The fix is a bit of a hack but it seems to work.

Fixes #4202.
Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

cabal-install/Distribution/Client/ProjectBuilding.hs

@@ -1073,9 +1073,10 @@ buildInplaceUnpackedPackage verbosity
               | otherwise
               -> listSimple
 
+          let dep_monitors = map monitorFileHashed (elabInplaceDependencyBuildCacheFiles pkg)
           updatePackageBuildFileMonitor packageFileMonitor srcdir timestamp
                                         pkg buildStatus
-                                        monitors buildResult
+                                        (monitors ++ dep_monitors) buildResult
 
         -- PURPOSELY omitted: no copy!
 

cabal-install/Distribution/Client/ProjectPlanning.hs

@@ -1314,6 +1314,8 @@ elaborateInstallPlan verbosity platform compiler compilerprogdb pkgConfigDB
 
             external_lib_dep_sids = CD.select (== compSolverName) deps0
             external_lib_dep_pkgs = concatMap (elaborateLibSolverId' mapDep) external_lib_dep_sids
+            compInplaceDependencyBuildCacheFiles
+                = concatMap (elaborateLibBuildCacheFile mapDep) external_lib_dep_sids
             external_exe_dep_sids = CD.select (== compSolverName) exe_deps0
             external_cc_map = Map.fromList (map mkPkgNameMapping external_lib_dep_pkgs)
             external_lc_map = Map.fromList (map mkShapeMapping external_lib_dep_pkgs)
@@ -1359,6 +1361,24 @@ elaborateInstallPlan verbosity platform compiler compilerprogdb pkgConfigDB
                       -> SolverId -> [ConfiguredId]
     elaborateLibSolverId mapDep = map configuredId . elaborateLibSolverId' mapDep
 
+    elaborateLibBuildCacheFile :: (SolverId -> [ElaboratedPlanPackage])
+                               -> SolverId -> [FilePath]
+    elaborateLibBuildCacheFile mapDep = concatMap get_cache_file . mapDep
+      where
+        get_cache_file (InstallPlan.PreExisting _) = []
+        get_cache_file (InstallPlan.Installed elab) = go elab
+        get_cache_file (InstallPlan.Configured elab) = go elab
+
+        go elab
+            | elabBuildStyle elab == BuildInplaceOnly
+            , case elabPkgOrComp elab of
+                ElabPackage _ -> True
+                ElabComponent comp -> compSolverName comp == CD.ComponentLib
+            = [ distPackageCacheFile
+                  (elabDistDirParams elaboratedSharedConfig elab)
+                  "build" ]
+            | otherwise = []
+
     elaborateExeSolverId :: (SolverId -> [ElaboratedPlanPackage])
                       -> SolverId -> [ConfiguredId]
     elaborateExeSolverId mapDep = map configuredId . filter is_exe . mapDep
@@ -1420,6 +1440,7 @@ elaborateInstallPlan verbosity platform compiler compilerprogdb pkgConfigDB
             }
 
         deps = fmap (concatMap (elaborateLibSolverId mapDep)) deps0
+        pkgInplaceDependencyBuildCacheFiles = fmap (concatMap (elaborateLibBuildCacheFile mapDep)) deps0
 
         requires_reg = PD.hasPublicLib elabPkgDescription
         pkgInstalledId
@@ -2191,7 +2212,8 @@ pruneInstallPlanPass2 pkgs =
                   pkgStanzasEnabled = stanzas,
                   pkgLibDependencies   = CD.filterDeps keepNeeded (pkgLibDependencies pkg),
                   pkgExeDependencies   = CD.filterDeps keepNeeded (pkgExeDependencies pkg),
-                  pkgExeDependencyPaths = CD.filterDeps keepNeeded (pkgExeDependencyPaths pkg)
+                  pkgExeDependencyPaths = CD.filterDeps keepNeeded (pkgExeDependencyPaths pkg),
+                  pkgInplaceDependencyBuildCacheFiles = CD.filterDeps keepNeeded (pkgInplaceDependencyBuildCacheFiles pkg)
                 }
               r@(ElabComponent _) -> r
         }

cabal-install/Distribution/Client/ProjectPlanning/Types.hs

@@ -21,6 +21,7 @@ module Distribution.Client.ProjectPlanning.Types (
     elabOrderExeDependencies,
     elabSetupDependencies,
     elabPkgConfigDependencies,
+    elabInplaceDependencyBuildCacheFiles,
 
     elabPlanPackageName,
     elabConfiguredName,
@@ -408,6 +409,32 @@ elabPkgConfigDependencies ElaboratedConfiguredPackage { elabPkgOrComp = ElabPack
 elabPkgConfigDependencies ElaboratedConfiguredPackage { elabPkgOrComp = ElabComponent comp }
     = compPkgConfigDependencies comp
 
+-- | The cache files of all our inplace dependencies which,
+-- when updated, require us to rebuild.  See #4202 for
+-- more details.  Essentially, this is a list of filepaths
+-- that, if our dependencies get rebuilt, will themselves
+-- get updated.
+--
+-- Note: the hash of these cache files gets built into
+-- the build cache ourselves, which means that we end
+-- up tracking transitive dependencies!
+--
+-- Note: This tracks the "build" cache file, but not
+-- "registration" or "config" cache files.  Why not?
+-- Arguably we should...
+--
+-- Note: This is a bit of a hack, because it is not really
+-- the hashes of the SOURCES of our (transitive) dependencies
+-- that we should use to decide whether or not to rebuild,
+-- but the output BUILD PRODUCTS.  The strategy we use
+-- here will never work if we want to implement unchanging
+-- rebuilds.
+elabInplaceDependencyBuildCacheFiles :: ElaboratedConfiguredPackage -> [FilePath]
+elabInplaceDependencyBuildCacheFiles ElaboratedConfiguredPackage { elabPkgOrComp = ElabPackage pkg }
+    = CD.flatDeps (pkgInplaceDependencyBuildCacheFiles pkg)
+elabInplaceDependencyBuildCacheFiles ElaboratedConfiguredPackage { elabPkgOrComp = ElabComponent comp }
+    = compInplaceDependencyBuildCacheFiles comp
+
 -- | Some extra metadata associated with an
 -- 'ElaboratedConfiguredPackage' which indicates that the "package"
 -- in question is actually a single component to be built.  Arguably
@@ -446,7 +473,8 @@ data ElaboratedComponent
     compNonSetupDependencies :: [UnitId],
     -- | The setup dependencies.  TODO: Remove this when setups
     -- are components of their own.
-    compSetupDependencies :: [ConfiguredId]
+    compSetupDependencies :: [ConfiguredId],
+    compInplaceDependencyBuildCacheFiles :: [FilePath]
    }
   deriving (Eq, Show, Generic)
 
@@ -491,6 +519,8 @@ data ElaboratedPackage
        --
        pkgPkgConfigDependencies :: [(PkgconfigName, Maybe Version)],
 
+       pkgInplaceDependencyBuildCacheFiles :: ComponentDeps [FilePath],
+
        -- | Which optional stanzas (ie testsuites, benchmarks) will actually
        -- be enabled during the package configure step.
        pkgStanzasEnabled :: Set OptionalStanza

cabal-testsuite/PackageTests/Regression/T3294/setup.test.hs

@@ -4,12 +4,11 @@ import Control.Monad.IO.Class
 -- https://github.com/haskell/setup/issues/3294
 main = setupAndCabalTest $ do
     withDelay $ do
-        cwd <- fmap testCurrentDir getTestEnv
-        liftIO $ writeFile (cwd </> "Main.hs") "main = putStrLn \"aaa\""
+        writeSourceFile "Main.hs" "main = putStrLn \"aaa\""
         setup "configure" []
         setup "build" []
         runExe' "T3294" [] >>= assertOutputContains "aaa"
         delay
-        liftIO $ writeFile (cwd </> "Main.hs") "main = putStrLn \"bbb\""
+        writeSourceFile "Main.hs" "main = putStrLn \"bbb\""
         setup "build" []
         runExe' "T3294" [] >>= assertOutputContains "bbb"

cabal-testsuite/PackageTests/Regression/T4202/.gitignore

+p/P.hs

cabal-testsuite/PackageTests/Regression/T4202/cabal.project

+packages: p q

cabal-testsuite/PackageTests/Regression/T4202/cabal.test.hs

+import Test.Cabal.Prelude
+main = cabalTest $
+    withDelay $ do
+        writeSourceFile ("p/P.hs") "module P where\np = \"AAA\""
+        cabal "new-build" ["p","q"]
+        delay
+        writeSourceFile ("p/P.hs") "module P where\np = \"BBB\""
+        cabal "new-build" ["p"]
+        cabal "new-build" ["q"]
+        withPlan $
+            runPlanExe' "q" "qexe" []
+                >>= assertOutputContains "BBB"

cabal-testsuite/PackageTests/Regression/T4202/p/p.cabal

+name: p
+version: 1.0
+build-type: Simple
+cabal-version: >= 1.10
+
+library
+  exposed-modules: P
+  build-depends: base
+  default-language: Haskell2010

cabal-testsuite/PackageTests/Regression/T4202/q/Q.hs

+import P
+main = putStrLn p

cabal-testsuite/PackageTests/Regression/T4202/q/q.cabal

+name: q
+version: 1.0
+build-type: Simple
+cabal-version: >= 1.10
+
+executable qexe
+  main-is: Q.hs
+  build-depends: base, p
+  default-language: Haskell2010

cabal-testsuite/Test/Cabal/Prelude.hs

@@ -683,3 +683,8 @@ withSymlink oldpath newpath0 act = do
   bracket_ (liftIO $ createSymbolicLink oldpath newpath)
            (liftIO $ removeFile newpath) act
 #endif
+
+writeSourceFile :: FilePath -> String -> TestM ()
+writeSourceFile fp s = do
+    cwd <- fmap testCurrentDir getTestEnv
+    liftIO $ writeFile (cwd </> fp) s