Merge pull request #9985 from alois31/symlink-resolution

Restore `builtins.pathExists` behavior on broken symlinks

(cherry picked from commit d53c8901ef7f2033855dd99063522e3d56a19dab)

===

note that this variant differs markedly from the source commit because
we haven't endured quite as much lazy trees.

Change-Id: I0facf282f21fe0db4134be5c65a8368c1b3a06fc

src/libexpr/primops.cc

@@ -1540,11 +1540,12 @@ static void prim_pathExists(EvalState & state, const PosIdx pos, Value * * args,
             || arg.str().ends_with("/."));
 
     try {
-        auto checked = state.checkSourcePath(path);
-        auto exists = checked.pathExists();
-        if (exists && mustBeDir) {
-            exists = checked.lstat().type == InputAccessor::tDirectory;
-        }
+        auto checked = state
+            .checkSourcePath(path)
+            .resolveSymlinks(mustBeDir ? SymlinkResolution::Full : SymlinkResolution::Ancestors);
+
+        auto st = checked.maybeLstat();
+        auto exists = st && (!mustBeDir || st->type == InputAccessor::tDirectory);
         v.mkBool(exists);
     } catch (SysError & e) {
         /* Don't give away info from errors while canonicalising