symlinkJoin: support __structuredAttrs

Use the $paths bash array from __structuredAttrs.
Populate it from $pathsPath if passAsFile is used instead.

pkgs/build-support/trivial-builders/default.nix

@@ -559,7 +559,10 @@ rec {
         paths = mapPaths (path: "${path}${stripPrefix}") paths;
         buildCommand = ''
           mkdir -p $out
-          for i in $(cat $pathsPath); do
+          if [ -n "''${pathsPath:-}" ] && [ -f "$pathsPath" ]; then
+            mapfile -d " " -t paths < "$pathsPath"
+          fi
+          for i in "''${paths[@]}"; do
             ${optionalString (!failOnMissing) "if test -d $i; then "}${lndir}/bin/lndir -silent $i $out${
               optionalString (!failOnMissing) "; fi"
             }

pkgs/build-support/trivial-builders/test/symlink-join.nix

@@ -55,6 +55,24 @@ in
     '';
   };
 
+  symlinkJoin-structured-attrs = testEqualContents {
+    assertion = "symlinkJoin-structured-attrs";
+    actual = symlinkJoin {
+      __structuredAttrs = true;
+      name = "symlinkJoin-structured-attrs";
+      paths = [
+        foo
+        bar
+        baz
+      ];
+    };
+    expected = runCommand "symlinkJoin-foo-bar-baz" { } ''
+      mkdir -p $out/{var/lib/arbitrary,etc/test.d}
+      ln -s {${foo},${bar}}/etc/test.d/* $out/etc/test.d
+      ln -s ${baz}/var/lib/arbitrary/baz $out/var/lib/arbitrary/
+    '';
+  };
+
   symlinkJoin-strip-paths = testEqualContents {
     assertion = "symlinkJoin-strip-paths";
     actual = symlinkJoin {