From ca1262a4832b290d9f35c4b9c5f67f93144c7f2a Mon Sep 17 00:00:00 2001
From: Shea Levy <shea@shealevy.com>
Date: Tue, 30 Jan 2024 17:29:47 -0500
Subject: [PATCH] lib: Add optionalDrvAttr to conditionally set drv attributes.

This allows for adding new, conditionally set, derivation attributes
to an existing derivation without changing any output paths in the
case where the condition is not met.
---
 doc/default.nix     |  1 +
 lib/default.nix     |  2 +-
 lib/derivations.nix | 26 ++++++++++++++++++++++++++
 lib/tests/misc.nix  | 20 +++++++++++++++++++-
 4 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/doc/default.nix b/doc/default.nix
index 26aae9efa573..bcbc20b9f983 100644
--- a/doc/default.nix
+++ b/doc/default.nix
@@ -25,6 +25,7 @@ let
       { name = "gvariant"; description = "GVariant formatted string serialization functions"; }
       { name = "customisation"; description = "Functions to customise (derivation-related) functions, derivatons, or attribute sets"; }
       { name = "meta"; description = "functions for derivation metadata"; }
+      { name = "derivations"; description = "miscellaneous derivation-specific functions"; }
     ];
   };
 
diff --git a/lib/default.nix b/lib/default.nix
index f6c94ae91634..a17307be6e07 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -116,7 +116,7 @@ let
     inherit (self.customisation) overrideDerivation makeOverridable
       callPackageWith callPackagesWith extendDerivation hydraJob
       makeScope makeScopeWithSplicing makeScopeWithSplicing';
-    inherit (self.derivations) lazyDerivation;
+    inherit (self.derivations) lazyDerivation optionalDrvAttr;
     inherit (self.meta) addMetaAttrs dontDistribute setName updateName
       appendToName mapDerivationAttrset setPrio lowPrio lowPrioSet hiPrio
       hiPrioSet getLicenseFromSpdxId getExe getExe';
diff --git a/lib/derivations.nix b/lib/derivations.nix
index 5b7ed1868e86..44b727ee31cc 100644
--- a/lib/derivations.nix
+++ b/lib/derivations.nix
@@ -98,4 +98,30 @@ in
       # `lazyDerivation` caller knew a shortcut, be taken from there.
       meta = args.meta or checked.meta;
     } // passthru;
+
+  /* Conditionally set a derivation attribute.
+
+     Because `mkDerivation` sets `__ignoreNulls = true`, a derivation
+     attribute set to `null` will not impact the derivation output hash.
+     Thus, this function passes through its `value` argument if the `cond`
+     is `true`, but returns `null` if not.
+
+     Type: optionalDrvAttr :: Bool -> a -> a | Null
+
+     Example:
+       (stdenv.mkDerivation {
+         name = "foo";
+         x = optionalDrvAttr true 1;
+         y = optionalDrvAttr false 1;
+       }).drvPath == (stdenv.mkDerivation {
+         name = "foo";
+         x = 1;
+       }).drvPath
+       => true
+  */
+  optionalDrvAttr =
+    # Condition
+    cond:
+    # Attribute value
+    value: if cond then value else null;
 }
diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index 3059878ba069..193e68a96933 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -1902,7 +1902,7 @@ runTests {
     expected = true;
   };
 
-  # lazyDerivation
+  # DERIVATIONS
 
   testLazyDerivationIsLazyInDerivationForAttrNames = {
     expr = attrNames (lazyDerivation {
@@ -1955,6 +1955,24 @@ runTests {
     expected = derivation;
   };
 
+  testOptionalDrvAttr = let
+    mkDerivation = args: derivation (args // {
+      builder = "builder";
+      system = "system";
+      __ignoreNulls = true;
+    });
+  in {
+    expr = (mkDerivation {
+      name = "foo";
+      x = optionalDrvAttr true 1;
+      y = optionalDrvAttr false 1;
+    }).drvPath;
+    expected = (mkDerivation {
+      name = "foo";
+      x = 1;
+    }).drvPath;
+  };
+
   testTypeDescriptionInt = {
     expr = (with types; int).description;
     expected = "signed integer";