Fix nested flake input overrides

src/libexpr/flake/flake.cc

@@ -90,11 +90,11 @@ static void expectType(EvalState & state, ValueType type,
 
 static std::map<FlakeId, FlakeInput> parseFlakeInputs(
     EvalState & state, Value * value, const PosIdx pos,
-    const std::optional<Path> & baseDir, InputPath lockRootPath);
+    const std::optional<Path> & baseDir, InputPath lockRootPath, unsigned depth);
 
 static FlakeInput parseFlakeInput(EvalState & state,
     const std::string & inputName, Value * value, const PosIdx pos,
-    const std::optional<Path> & baseDir, InputPath lockRootPath)
+    const std::optional<Path> & baseDir, InputPath lockRootPath, unsigned depth)
 {
     expectType(state, nAttrs, *value, pos);
 
@@ -118,7 +118,7 @@ static FlakeInput parseFlakeInput(EvalState & state,
                 expectType(state, nBool, *attr.value, attr.pos);
                 input.isFlake = attr.value->boolean;
             } else if (attr.name == sInputs) {
-                input.overrides = parseFlakeInputs(state, attr.value, attr.pos, baseDir, lockRootPath);
+                input.overrides = parseFlakeInputs(state, attr.value, attr.pos, baseDir, lockRootPath, depth + 1);
             } else if (attr.name == sFollows) {
                 expectType(state, nString, *attr.value, attr.pos);
                 auto follows(parseInputPath(attr.value->string.s));
@@ -163,15 +163,19 @@ static FlakeInput parseFlakeInput(EvalState & state,
             input.ref = parseFlakeRef(*url, baseDir, true, input.isFlake);
     }
 
-    if (!input.follows && !input.ref)
+    if (!input.follows && !input.ref && depth == 0)
+        // in `input.nixops.inputs.nixpkgs.url = ...`, we assume `nixops` is from
+        // the flake registry absent `ref`/`follows`, but we should not assume so
+        // about `nixpkgs` (where `depth == 1`) as the `nixops` flake should
+        // determine its default source
         input.ref = FlakeRef::fromAttrs({{"type", "indirect"}, {"id", inputName}});
 
     return input;
 }
 
 static std::map<FlakeId, FlakeInput> parseFlakeInputs(
     EvalState & state, Value * value, const PosIdx pos,
-    const std::optional<Path> & baseDir, InputPath lockRootPath)
+    const std::optional<Path> & baseDir, InputPath lockRootPath, unsigned depth)
 {
     std::map<FlakeId, FlakeInput> inputs;
 
@@ -184,7 +188,8 @@ static std::map<FlakeId, FlakeInput> parseFlakeInputs(
                 inputAttr.value,
                 inputAttr.pos,
                 baseDir,
-                lockRootPath));
+                lockRootPath,
+                depth));
     }
 
     return inputs;
@@ -230,7 +235,7 @@ static Flake getFlake(
     auto sInputs = state.symbols.create("inputs");
 
     if (auto inputs = vInfo.attrs->get(sInputs))
-        flake.inputs = parseFlakeInputs(state, inputs->value, inputs->pos, flakeDir, lockRootPath);
+        flake.inputs = parseFlakeInputs(state, inputs->value, inputs->pos, flakeDir, lockRootPath, 0);
 
     auto sOutputs = state.symbols.create("outputs");
 
@@ -313,6 +318,19 @@ Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup
     return getFlake(state, originalRef, allowLookup, flakeCache);
 }
 
+/* Recursively merge `overrides` into `overrideMap` */
+static void updateOverrides(std::map<InputPath, FlakeInput> & overrideMap, const FlakeInputs & overrides,
+                       const InputPath & inputPathPrefix)
+{
+    for (auto & [id, input] : overrides) {
+        auto inputPath(inputPathPrefix);
+        inputPath.push_back(id);
+        // Do not override existing assignment from outer flake
+        overrideMap.insert({inputPath, input});
+        updateOverrides(overrideMap, input.overrides, inputPath);
+    }
+}
+
 /* Compute an in-memory lock file for the specified top-level flake,
    and optionally write it to file, if the flake is writable. */
 LockedFlake lockFlake(
@@ -375,12 +393,9 @@ LockedFlake lockFlake(
             /* Get the overrides (i.e. attributes of the form
                'inputs.nixops.inputs.nixpkgs.url = ...'). */
             for (auto & [id, input] : flakeInputs) {
-                for (auto & [idOverride, inputOverride] : input.overrides) {
-                    auto inputPath(inputPathPrefix);
-                    inputPath.push_back(id);
-                    inputPath.push_back(idOverride);
-                    overrides.insert_or_assign(inputPath, inputOverride);
-                }
+                auto inputPath(inputPathPrefix);
+                inputPath.push_back(id);
+                updateOverrides(overrides, input.overrides, inputPath);
             }
 
             /* Check whether this input has overrides for a
@@ -415,6 +430,12 @@ LockedFlake lockFlake(
                         // Respect the “flakeness” of the input even if we
                         // override it
                         i->second.isFlake = input2.isFlake;
+                        if (!i->second.ref)
+                            i->second.ref = input2.ref;
+                        if (!i->second.follows)
+                            i->second.follows = input2.follows;
+                        // Note that `input.overrides` is not used in the following,
+                        // so no need to merge it here (already done by `updateOverrides`)
                     }
                     auto & input = hasOverride ? i->second : input2;
 

src/libexpr/flake/lockfile.cc

@@ -338,7 +338,7 @@ void LockFile::check()
 
     for (auto & [inputPath, input] : inputs) {
         if (auto follows = std::get_if<1>(&input)) {
-            if (!follows->empty() && !get(inputs, *follows))
+            if (!follows->empty() && !findInput(*follows))
                 throw Error("input '%s' follows a non-existent input '%s'",
                     printInputPath(inputPath),
                     printInputPath(*follows));

tests/flakes/follow-paths.sh

@@ -148,3 +148,63 @@ git -C $flakeFollowsA add flake.nix
 
 nix flake lock $flakeFollowsA 2>&1 | grep "warning: input 'B' has an override for a non-existent input 'invalid'"
 nix flake lock $flakeFollowsA 2>&1 | grep "warning: input 'B' has an override for a non-existent input 'invalid2'"
+
+# Test nested flake overrides: A overrides B/C/D
+
+cat <<EOF > $flakeFollowsD/flake.nix
+{ outputs = _: {}; }
+EOF
+cat <<EOF > $flakeFollowsC/flake.nix
+{
+  inputs.D.url = "path:nosuchflake";
+  outputs = _: {};
+}
+EOF
+cat <<EOF > $flakeFollowsB/flake.nix
+{
+  inputs.C.url = "path:$flakeFollowsC";
+  outputs = _: {};
+}
+EOF
+cat <<EOF > $flakeFollowsA/flake.nix
+{
+  inputs.B.url = "path:$flakeFollowsB";
+  inputs.D.url = "path:$flakeFollowsD";
+  inputs.B.inputs.C.inputs.D.follows = "D";
+  outputs = _: {};
+}
+EOF
+
+nix flake lock $flakeFollowsA
+
+[[ $(jq -c .nodes.C.inputs.D $flakeFollowsA/flake.lock) = '["D"]' ]]
+
+# Test overlapping flake follows: B has D follow C/D, while A has B/C follow C
+
+cat <<EOF > $flakeFollowsC/flake.nix
+{
+  inputs.D.url = "path:$flakeFollowsD";
+  outputs = _: {};
+}
+EOF
+cat <<EOF > $flakeFollowsB/flake.nix
+{
+  inputs.C.url = "path:nosuchflake";
+  inputs.D.url = "path:nosuchflake";
+  inputs.D.follows = "C/D";
+  outputs = _: {};
+}
+EOF
+cat <<EOF > $flakeFollowsA/flake.nix
+{
+  inputs.B.url = "path:$flakeFollowsB";
+  inputs.C.url = "path:$flakeFollowsC";
+  inputs.B.inputs.C.follows = "C";
+  outputs = _: {};
+}
+EOF
+
+# bug was not triggered without recreating the lockfile
+nix flake lock $flakeFollowsA --recreate-lock-file
+
+[[ $(jq -c .nodes.B.inputs.D $flakeFollowsA/flake.lock) = '["B","C","D"]' ]]