Add flag --override-input to override specific lock file entries

E.g.

  $ nix flake update ~/Misc/eelco-configurations/hagbard \
    --override-input 'dwarffs/nixpkgs' ../my-nixpkgs

overrides the 'nixpkgs' input of the 'dwarffs' input of the top-level
flake.

Fixes #2837.

3 files changed, 17 insertions, 5 deletions

diff --git a/src/nix/command.hh b/src/nix/command.hh
index 08fa0c5fa..eb44caf05 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -4,6 +4,7 @@
 #include "args.hh"
 #include "common-eval-args.hh"
 #include "path.hh"
+#include "flake/lockfile.hh"
 
 #include <optional>
 
@@ -42,11 +43,11 @@ struct EvalCommand : virtual StoreCommand, MixEvalArgs
 struct MixFlakeOptions : virtual Args
 {
     bool recreateLockFile = false;
-
     bool saveLockFile = true;
-
     bool useRegistries = true;
 
+    flake::LockFlags lockFlags;
+
     MixFlakeOptions();
 
     flake::LockFileMode getLockFileMode();
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index f7e329b49..2852a627d 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -43,7 +43,7 @@ public:
 
     LockedFlake lockFlake()
     {
-        return flake::lockFlake(*getEvalState(), getFlakeRef(), getLockFileMode());
+        return flake::lockFlake(*getEvalState(), getFlakeRef(), getLockFileMode(), lockFlags);
     }
 };
 
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index 8e4b53308..7d59a25ee 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -33,6 +33,17 @@ MixFlakeOptions::MixFlakeOptions()
         .longName("no-registries")
         .description("don't use flake registries")
         .set(&useRegistries, false);
+
+    mkFlag()
+        .longName("override-input")
+        .description("override a specific flake input (e.g. 'dwarffs/nixpkgs')")
+        .arity(2)
+        .labels({"input-path", "flake-url"})
+        .handler([&](std::vector<std::string> ss) {
+            lockFlags.inputOverrides.insert_or_assign(
+                flake::parseInputPath(ss[0]),
+                parseFlakeRef(ss[1], absPath(".")));
+        });
 }
 
 flake::LockFileMode MixFlakeOptions::getLockFileMode()
@@ -321,7 +332,7 @@ std::tuple<std::string, FlakeRef, flake::EvalCache::Derivation> InstallableFlake
 {
     auto state = cmd.getEvalState();
 
-    auto lockedFlake = lockFlake(*state, flakeRef, cmd.getLockFileMode());
+    auto lockedFlake = lockFlake(*state, flakeRef, cmd.getLockFileMode(), cmd.lockFlags);
 
     Value * vOutputs = nullptr;
 
@@ -375,7 +386,7 @@ std::vector<flake::EvalCache::Derivation> InstallableFlake::toDerivations()
 
 Value * InstallableFlake::toValue(EvalState & state)
 {
-    auto lockedFlake = lockFlake(state, flakeRef, cmd.getLockFileMode());
+    auto lockedFlake = lockFlake(state, flakeRef, cmd.getLockFileMode(), cmd.lockFlags);
 
     auto vOutputs = getFlakeOutputs(state, lockedFlake);