symlinkJoin: ability to strip prefix from tree

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

@@ -3,7 +3,11 @@
 let
   inherit (lib)
     optionalAttrs
+    optionalString
+    hasPrefix
     warn
+    map
+    isList
     ;
 in
 
@@ -462,6 +466,29 @@ rec {
     ...
 
 
+    To create a directory structure from a specific subdirectory of input `paths` instead of their full trees,
+    you can either append the subdirectory path to each input path, or use the `stripPrefix` argument to
+    remove the common prefix during linking.
+
+    Example:
+
+
+    # create symlinks of tmpfiles.d rules from multiple packages
+    symlinkJoin { name = "tmpfiles.d"; paths = [ pkgs.lvm2 pkgs.nix ]; stripPrefix = "/lib/tmpfiles.d"; }
+
+
+    This creates a derivation with a directory structure like the following:
+
+
+    /nix/store/m5s775yicb763hfa133jwml5hwmwzv14-tmpfiles.d
+    |-- lvm2.conf -> /nix/store/k6js0l5f0zpvrhay49579fj939j77p2w-lvm2-2.03.29/lib/tmpfiles.d/lvm2.conf
+    `-- nix-daemon.conf -> /nix/store/z4v2s3s3y79fmabhps5hakb3c5dwaj5a-nix-1.33.7/lib/tmpfiles.d/nix-daemon.conf
+
+
+    By default, packages that don't contain the specified subdirectory are silently skipped.
+    Set `failOnMissing = true` to make the build fail if any input package is missing the subdirectory
+    (this is the default behavior when not using stripPrefix).
+
     symlinkJoin and linkFarm are similar functions, but they output
     derivations with different structure.
 
@@ -484,23 +511,37 @@ rec {
           "symlinkJoin requires either a `name` OR `pname` and `version`";
         "${args_.pname}-${args_.version}"
     , paths
+    , stripPrefix ? ""
     , preferLocalBuild ? true
     , allowSubstitutes ? false
     , postBuild ? ""
+    , failOnMissing ? stripPrefix == ""
     , ...
     }:
+    assert lib.assertMsg (stripPrefix != "" -> (hasPrefix "/" stripPrefix && stripPrefix != "/")) ''
+      stripPrefix must be either an empty string (disable stripping behavior), or relative path prefixed with /.
+
+      Ensure that the path starts with / and specifies path to the subdirectory.
+    '';
+
     let
-      args = removeAttrs args_ [ "name" "postBuild" ]
+      mapPaths = f: paths: map (path:
+        if path == null then null
+        else if isList path then mapPaths f path
+        else f path
+      ) paths;
+      args = removeAttrs args_ [ "name" "postBuild" "stripPrefix" "paths" "failOnMissing" ]
         // {
         inherit preferLocalBuild allowSubstitutes;
+        paths = mapPaths (path: "${path}${stripPrefix}") paths;
         passAsFile = [ "paths" ];
       }; # pass the defaults
     in
     runCommand name args
       ''
         mkdir -p $out
         for i in $(cat $pathsPath); do
-          ${lndir}/bin/lndir -silent $i $out
+          ${optionalString (!failOnMissing) "if test -d $i; then "}${lndir}/bin/lndir -silent $i $out${optionalString (!failOnMissing) "; fi"}
         done
         ${postBuild}
       '';

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

@@ -19,6 +19,7 @@ in
 recurseIntoAttrs {
   concat = callPackage ./concat-test.nix {};
   linkFarm = callPackage ./link-farm.nix {};
+  symlinkJoin = recurseIntoAttrs (callPackage ./symlink-join.nix {});
   overriding = callPackage ../test-overriding.nix {};
   inherit references;
   writeCBin = callPackage ./writeCBin.nix {};

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

@@ -0,0 +1,136 @@
+{
+  symlinkJoin,
+  writeTextFile,
+  runCommand,
+  testers,
+}:
+
+let
+  inherit (testers) testEqualContents testBuildFailure;
+
+  foo = writeTextFile {
+    name = "foo";
+    text = "foo";
+    destination = "/etc/test.d/foo";
+  };
+
+  bar = writeTextFile {
+    name = "bar";
+    text = "bar";
+    destination = "/etc/test.d/bar";
+  };
+
+  baz = writeTextFile {
+    name = "baz";
+    text = "baz";
+    destination = "/var/lib/arbitrary/baz";
+  };
+
+  qux = writeTextFile {
+    name = "qux";
+    text = "qux";
+  };
+
+  emulatedSymlinkJoinFooBarStrip = runCommand "symlinkJoin-strip-foo-bar" { } ''
+    mkdir $out
+    ln -s ${foo}/etc/test.d/foo $out/
+    ln -s ${bar}/etc/test.d/bar $out/
+  '';
+in
+{
+  symlinkJoin = testEqualContents {
+    assertion = "symlinkJoin";
+    actual = symlinkJoin {
+      name = "symlinkJoin";
+      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 {
+      name = "symlinkJoinPrefix";
+      paths = [
+        foo
+        bar
+      ];
+      stripPrefix = "/etc/test.d";
+    };
+    expected = emulatedSymlinkJoinFooBarStrip;
+  };
+
+  symlinkJoin-strip-paths-skip-missing = testEqualContents {
+    assertion = "symlinkJoin-strip-paths-skip-missing";
+    actual = symlinkJoin {
+      name = "symlinkJoinPrefix";
+      paths = [
+        foo
+        bar
+        baz
+      ];
+      stripPrefix = "/etc/test.d";
+    };
+    expected = emulatedSymlinkJoinFooBarStrip;
+  };
+
+  symlinkJoin-strip-paths-skip-not-directories = testEqualContents {
+    assertion = "symlinkJoin-strip-paths-skip-not-directories";
+    actual = symlinkJoin {
+      name = "symlinkJoinPrefix";
+      paths = [
+        foo
+        bar
+        qux
+      ];
+      stripPrefix = "/etc/test.d";
+    };
+    expected = emulatedSymlinkJoinFooBarStrip;
+  };
+
+  symlinkJoin-fails-on-missing =
+    runCommand "symlinkJoin-fails-on-missing"
+      {
+        failed = testBuildFailure (symlinkJoin {
+          name = "symlinkJoin-fail";
+          paths = [
+            foo
+            bar
+            baz
+          ];
+          stripPrefix = "/etc/test.d";
+          failOnMissing = true;
+        });
+      }
+      ''
+        grep -e "-baz/etc/test.d: No such file or directory" $failed/testBuildFailure.log
+        touch $out
+      '';
+
+  symlinkJoin-fails-on-file =
+    runCommand "symlinkJoin-fails-on-file"
+      {
+        failed = testBuildFailure (symlinkJoin {
+          name = "symlinkJoin-fail";
+          paths = [
+            foo
+            bar
+            qux
+          ];
+          stripPrefix = "/etc/test.d";
+          failOnMissing = true;
+        });
+      }
+      ''
+        grep -e "-qux/etc/test.d: Not a directory" $failed/testBuildFailure.log
+        touch $out
+      '';
+}