Add cabal get --only-package-description

cabal-install/src/Distribution/Client/Get.hs

@@ -31,7 +31,7 @@ import Distribution.Package
 import Distribution.Simple.Setup
          ( Flag(..), fromFlag, fromFlagOrDefault, flagToMaybe )
 import Distribution.Simple.Utils
-         ( notice, die', info, writeFileAtomic )
+         ( notice, die', info, warn, writeFileAtomic )
 import qualified Distribution.PackageDescription as PD
 import Distribution.Simple.Program
          ( programName )
@@ -49,8 +49,11 @@ import qualified Distribution.Client.Tar as Tar (extractTarGzFile)
 import Distribution.Client.IndexUtils
         ( getSourcePackagesAtIndexState, TotalIndexState, ActiveRepos )
 import Distribution.Solver.Types.SourcePackage
+import Distribution.PackageDescription.PrettyPrint
+        ( writeGenericPackageDescription )
 
 import qualified Data.Map as Map
+import Control.Monad ( mapM_ )
 import System.Directory
          ( createDirectoryIfMissing, doesDirectoryExist, doesFileExist )
 import System.FilePath
@@ -94,16 +97,26 @@ get verbosity repoCtxt _ getFlags userTargets = do
   unless (null prefix) $
     createDirectoryIfMissing True prefix
 
-  if useSourceRepo
-    then clone  pkgs
-    else unpack pkgs
+  if onlyPkgDescr
+    then do
+      when useSourceRepo $
+        warn verbosity $
+          "Ignoring --source-repository for --only-package-description"
+
+      mapM_ (unpackOnlyPkgDescr verbosity prefix) pkgs
+    else
+      if useSourceRepo
+        then clone pkgs
+        else unpack pkgs
 
   where
     resolverParams :: SourcePackageDb -> [PackageSpecifier UnresolvedSourcePackage] -> DepResolverParams
     resolverParams sourcePkgDb pkgSpecifiers =
         --TODO: add command-line constraint and preference args for unpack
         standardInstallPolicy mempty sourcePkgDb pkgSpecifiers
 
+    onlyPkgDescr = fromFlagOrDefault False (getOnlyPkgDescr getFlags)
+
     prefix :: String
     prefix = fromFlagOrDefault "" (getDestDir getFlags)
 
@@ -189,6 +202,23 @@ unpackPackage verbosity prefix pkgid descOverride pkgPath = do
         writeFileAtomic descFilePath pkgtxt
 
 
+-- | Write a @pkgId.cabal@ file with the package description to the destination
+-- directory, unless one already exists.
+unpackOnlyPkgDescr :: Verbosity -> FilePath -> UnresolvedSourcePackage -> IO ()
+unpackOnlyPkgDescr verbosity dstDir pkg = do
+    let pkgFile = dstDir </> prettyShow (packageId pkg) <.> "cabal"
+    existsFile <- doesFileExist pkgFile
+    when existsFile $ die' verbosity $
+      "The file \"" ++ pkgFile ++ "\" already exists, not overwriting."
+    existsDir <- doesDirectoryExist (addTrailingPathSeparator pkgFile)
+    when existsDir $ die' verbosity $
+      "A directory \"" ++ pkgFile ++ "\" is in the way, not unpacking."
+    notice verbosity $ "Writing package description to " ++ pkgFile
+    case srcpkgDescrOverride pkg of
+      Just pkgTxt -> writeFileAtomic pkgFile pkgTxt
+      Nothing ->
+        writeGenericPackageDescription pkgFile (srcpkgDescription pkg)
+
 -- ------------------------------------------------------------
 -- * Cloning packages from their declared source repositories
 -- ------------------------------------------------------------

cabal-install/src/Distribution/Client/Setup.hs

@@ -1297,6 +1297,7 @@ instance Semigroup ReportFlags where
 
 data GetFlags = GetFlags {
     getDestDir          :: Flag FilePath,
+    getOnlyPkgDescr     :: Flag Bool,
     getPristine         :: Flag Bool,
     getIndexState       :: Flag TotalIndexState,
     getActiveRepos      :: Flag ActiveRepos,
@@ -1307,6 +1308,7 @@ data GetFlags = GetFlags {
 defaultGetFlags :: GetFlags
 defaultGetFlags = GetFlags {
     getDestDir          = mempty,
+    getOnlyPkgDescr     = mempty,
     getPristine         = mempty,
     getIndexState       = mempty,
     getActiveRepos      = mempty,
@@ -1352,6 +1354,16 @@ getCommand = CommandUI {
                                       (toFlag `fmap` parsec))
                           (flagToList . fmap prettyShow))
 
+       , option [] ["only-package-description"]
+           "Unpack only the package description file."
+           getOnlyPkgDescr (\v flags -> flags { getOnlyPkgDescr = v })
+           trueArg
+
+       , option [] ["package-description-only"]
+           "A synonym for --only-package-description."
+           getOnlyPkgDescr (\v flags -> flags { getOnlyPkgDescr = v })
+           trueArg
+
        , option [] ["pristine"]
            ("Unpack the original pristine tarball, rather than updating the "
            ++ ".cabal file with the latest revision from the package archive.")

cabal-testsuite/PackageTests/Get/OnlyDescription/cabal.out

@@ -0,0 +1,6 @@
+# cabal v2-update
+Downloading the latest package list from test-local-repo
+# cabal update
+Downloading the latest package list from test-local-repo
+# cabal get
+Writing package description to criterion-1.1.4.0.cabal

cabal-testsuite/PackageTests/Get/OnlyDescription/cabal.test.hs

@@ -0,0 +1,11 @@
+-- 2022-07-03, issue #1954
+--
+-- Purpose of this test:
+-- Make sure that `cabal get --only-package-description` works
+
+import Test.Cabal.Prelude
+main = cabalTest $ withRepo "repo" $ do
+  cabal "update" []
+  cabal
+    "get"
+    [ "criterion", "--only-package-description" ]

cabal-testsuite/PackageTests/Get/OnlyDescription/repo/criterion-1.1.4.0/criterion.cabal

@@ -0,0 +1,8 @@
+name: criterion
+version: 1.1.4.0
+build-type: Simple
+cabal-version: >= 1.10
+
+library
+    build-depends: base, ghc-prim
+    default-language: Haskell2010

changelog.d/issue-1954

@@ -0,0 +1,4 @@
+synopsis: Add `cabal get --only-package-description`
+packages: cabal-install
+prs: #1977 #5162 #8263
+issues: #1954

doc/cabal-package.rst

@@ -2865,6 +2865,9 @@ The ``get`` command supports the following options:
     ``2016-09-24T17:47:48Z``), or ``HEAD`` (default).
     This determines which package versions are available as well as which
     ``.cabal`` file revision is selected (unless ``--pristine`` is used).
+``--only-package-description``
+    Unpack only the package description file. A synonym,
+    ``--package-description-only``, is provided for convenience.
 ``--pristine``
     Unpack the original pristine tarball, rather than updating the
     ``.cabal`` file with the latest revision from the package archive.