From bf693319f63601c0e72107012328c298cd78e88b Mon Sep 17 00:00:00 2001
From: Bernardo Meurer <bernardo@meurer.org>
Date: Tue, 14 Mar 2023 11:54:04 -0300
Subject: [PATCH] feat: add always-allow-substitutes

This adds a new configuration option to Nix, `always-allow-substitutes`,
whose effect is simple: it causes the `allowSubstitutes` attribute in
derivations to be ignored, and for substituters to always be used.

This is extremely valuable for users of Nix in CI, where usually
`nix-build-uncached` is used. There, derivations which disallow
substitutes cause headaches as the inputs for building already-cached
derivations need to be fetched to spuriously rebuild some simple text
file.

This option should be a good middle-ground, since it doesn't imply
rebuilding the world, such as the approach I took in
https://github.com/NixOS/nixpkgs/pull/221048
---
 doc/manual/src/language/advanced-attributes.md | 3 +++
 src/libstore/globals.hh                        | 8 ++++++++
 src/libstore/parsed-derivations.cc             | 2 +-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/doc/manual/src/language/advanced-attributes.md b/doc/manual/src/language/advanced-attributes.md
index 3079714347c..211d195b2e8 100644
--- a/doc/manual/src/language/advanced-attributes.md
+++ b/doc/manual/src/language/advanced-attributes.md
@@ -261,6 +261,9 @@ Derivations can declare some infrequently used optional attributes.
     useful for very trivial derivations (such as `writeText` in Nixpkgs)
     that are cheaper to build than to substitute from a binary cache.
 
+    You may disable the effects of this attibute by enabling the
+    `always-allow-substitutes` configuration option in Nix.
+
     > **Note**
     >
     > You need to have a builder configured which satisfies the
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index 31dfe5b4ee5..8b0e3fc7db8 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -255,6 +255,14 @@ public:
           For the exact format and examples, see [the manual chapter on remote builds](../advanced-topics/distributed-builds.md)
         )"};
 
+    Setting<bool> alwaysAllowSubstitutes{
+        this, false, "always-allow-substitutes",
+        R"(
+          If set to `true`, Nix will ignore the `allowSubstitutes` attribute in
+          derivations and always attempt to use available substituters.
+          For more information on `allowSubstitutes`, see [the manual chapter on advanced attributes](../language/advanced-attributes.md).
+        )"};
+
     Setting<bool> buildersUseSubstitutes{
         this, false, "builders-use-substitutes",
         R"(
diff --git a/src/libstore/parsed-derivations.cc b/src/libstore/parsed-derivations.cc
index cc4a94fab8d..1d900c272df 100644
--- a/src/libstore/parsed-derivations.cc
+++ b/src/libstore/parsed-derivations.cc
@@ -122,7 +122,7 @@ bool ParsedDerivation::willBuildLocally(Store & localStore) const
 
 bool ParsedDerivation::substitutesAllowed() const
 {
-    return getBoolAttr("allowSubstitutes", true);
+    return settings.alwaysAllowSubstitutes ? true : getBoolAttr("allowSubstitutes", true);
 }
 
 bool ParsedDerivation::useUidRange() const