Handle absolute paths to build roots in Hadrian.

Fixes #16187.

This patch fixes various path concatenation issues to allow functioning
builds with hadrian when the build root location is specified with an
absolute path.

Remarks:

- The path concatenation operator (-/-) now handles absolute second operands
  appropriately. Its behavior should match that of POSIX (</>) in this
  regard.

- The `getDirectoryFiles*` family of functions only searches for matches
  under the directory tree rooted by its first argument; all of the
  results are also relative to this root. If the first argument is the
  empty string, the current working directory is used.

  This patch passes the appropriate directory (almost always either `top`
  or `root`), and subsequently attaches that directory prefix so that
  the paths refer to the appropriate files.

- Windows `tar` does not like colons (':') in paths to archive files, it
  tries to resolve them as remote paths. The `--force-local` option
  remedies this, and is applied on windows builds.

1 files changed, 14 insertions, 0 deletions

diff --git a/hadrian/src/Rules/Selftest.hs b/hadrian/src/Rules/Selftest.hs
index 68aa6e3889..b931f85ef6 100644
--- a/hadrian/src/Rules/Selftest.hs
+++ b/hadrian/src/Rules/Selftest.hs
@@ -13,6 +13,8 @@ import Settings
 import Target
 import Utilities
 
+import qualified System.FilePath.Posix as Posix ((</>))
+
 instance Arbitrary Way where
     arbitrary = wayFromUnits <$> arbitrary
 
@@ -31,6 +33,7 @@ selftestRules =
         testLookupAll
         testModuleName
         testPackages
+        testPaths
         testWay
 
 testBuilder :: Action ()
@@ -111,3 +114,14 @@ testWay :: Action ()
 testWay = do
     putBuild "==== Read Way, Show Way"
     test $ \(x :: Way) -> read (show x) == x
+
+testPaths :: Action ()
+testPaths = do
+    putBuild "==== Absolute, Relative Path Concatenation"
+    test $ forAll paths $ \(path1, path2) ->
+      path1 -/- path2 == path1 Posix.</> path2
+  where
+    paths = (,) <$> path <*> path
+    path = frequency [(1, relativePath), (1, absolutePath)]
+    relativePath = intercalate "/" <$> listOf1 (elements ["a"])
+    absolutePath = ('/':) <$> relativePath