Flakes: refetch the input when a follows disappears

When an input follows disappears, we can't just reuse the old lock
file entries since we may be missing some required ones. Refetch the
input when this happens.

Closes #5289

src/libexpr/flake/flake.cc

@@ -462,19 +462,33 @@ LockedFlake lockFlake(
                                those. */
                             FlakeInputs fakeInputs;
 
+                            bool refetch = false;
+
                             for (auto & i : oldLock->inputs) {
                                 if (auto lockedNode = std::get_if<0>(&i.second)) {
                                     fakeInputs.emplace(i.first, FlakeInput {
                                         .ref = (*lockedNode)->originalRef,
                                         .isFlake = (*lockedNode)->isFlake,
                                     });
                                 } else if (auto follows = std::get_if<1>(&i.second)) {
+                                    auto o = input.overrides.find(i.first);
+                                    // If the override disappeared, we have to refetch the flake,
+                                    // since some of the inputs may not be present in the lockfile.
+                                    if (o == input.overrides.end()) {
+                                        refetch = true;
+                                        // There's no point populating the rest of the fake inputs,
+                                        // since we'll refetch the flake anyways.
+                                        break;
+                                    }
                                     fakeInputs.emplace(i.first, FlakeInput {
                                         .follows = *follows,
                                     });
                                 }
                             }
 
+                            if (refetch)
+                                fakeInputs = getFlake(state, oldLock->lockedRef, false, flakeCache).inputs;
+
                             computeLocks(fakeInputs, childNode, inputPath, oldLock, parent, parentPath);
                         }
 

tests/flakes.sh

@@ -707,10 +707,10 @@ cat > $flakeFollowsA/flake.nix <<EOF
         B = {
             url = "path:./flakeB";
             inputs.foobar.follows = "D";
+            inputs.nonFlake.follows = "D";
         };
 
         D.url = "path:./flakeD";
-        foobar.url = "path:./flakeE";
     };
     outputs = { ... }: {};
 }
@@ -720,7 +720,8 @@ cat > $flakeFollowsB/flake.nix <<EOF
 {
     description = "Flake B";
     inputs = {
-        foobar.url = "path:./../flakeE";
+        foobar.url = "path:$flakeFollowsA/flakeE";
+        nonFlake.url = "path:$nonFlakeDir";
         C = {
             url = "path:./flakeC";
             inputs.foobar.follows = "foobar";
@@ -734,7 +735,7 @@ cat > $flakeFollowsC/flake.nix <<EOF
 {
     description = "Flake C";
     inputs = {
-        foobar.url = "path:./../../flakeE";
+        foobar.url = "path:$flakeFollowsA/flakeE";
     };
     outputs = { ... }: {};
 }
@@ -765,6 +766,27 @@ nix flake lock $flakeFollowsA
 [[ $(jq -c .nodes.B.inputs.foobar $flakeFollowsA/flake.lock) = '["D"]' ]]
 [[ $(jq -c .nodes.C.inputs.foobar $flakeFollowsA/flake.lock) = '["B","foobar"]' ]]
 
+# Ensure removing follows from flake.nix removes them from the lockfile
+
+cat > $flakeFollowsA/flake.nix <<EOF
+{
+    description = "Flake A";
+    inputs = {
+        B = {
+            url = "path:./flakeB";
+            inputs.nonFlake.follows = "D";
+        };
+        D.url = "path:./flakeD";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+nix flake lock $flakeFollowsA
+
+[[ $(jq -c .nodes.B.inputs.foobar $flakeFollowsA/flake.lock) = '"foobar"' ]]
+jq -r -c '.nodes | keys | .[]' $flakeFollowsA/flake.lock | grep "^foobar$"
+
 # Ensure a relative path is not allowed to go outside the store path
 cat > $flakeFollowsA/flake.nix <<EOF
 {