Add --update-input flag to update a specific flake input

Typical usage:

  $ nix flake update ~/Misc/eelco-configurations/hagbard --update-input nixpkgs

to update the 'nixpkgs' input of a flake while leaving every other
input unchanged.

The argument is an input path, so you can do e.g. '--update-input
dwarffs/nixpkgs' to update an input of an input.

Fixes #2928.

src/libexpr/flake/flake.cc

@@ -394,7 +394,13 @@ LockedFlake lockFlake(
                     continue;
                 }
 
-                auto oldLock = oldLocks.inputs.find(id);
+                /* Do we have an entry in the existing lock file? And
+                   we don't have a --update-input flag for this
+                   input? */
+                auto oldLock =
+                    lockFlags.inputUpdates.count(inputPath)
+                    ? oldLocks.inputs.end()
+                    : oldLocks.inputs.find(id);
 
                 if (oldLock != oldLocks.inputs.end() && oldLock->second.originalRef == input.ref && !hasOverride) {
                     /* Copy the input from the old lock file if its
@@ -402,22 +408,39 @@ LockedFlake lockFlake(
                        from a higher level flake. */
                     newLocks.inputs.insert_or_assign(id, oldLock->second);
 
-                    /* However there may be new overrides on the
-                       inputs of this flake, so we need to check those
-                       (without fetching this flake - we need to be
-                       lazy). */
+                    /* If we have an --update-input flag for an input
+                       of this input, then we must fetch the flake to
+                       to update it. */
+                    auto lb = lockFlags.inputUpdates.lower_bound(inputPath);
+
+                    auto hasChildUpdate =
+                        lb != lockFlags.inputUpdates.end()
+                        && lb->size() > inputPath.size()
+                        && std::equal(inputPath.begin(), inputPath.end(), lb->begin());
+
+                    if (hasChildUpdate) {
+                        auto inputFlake = getFlake(state, oldLock->second.ref, false, flakeCache);
+
+                        updateLocks(inputFlake.inputs,
+                            (const LockedInputs &) oldLock->second,
+                            newLocks.inputs.find(id)->second,
+                            inputPath);
+
+                    } else {
+                        /* No need to fetch this flake, we can be
+                           lazy. However there may be new overrides on
+                           the inputs of this flake, so we need to
+                           check those. */
                         FlakeInputs fakeInputs;
 
-                    for (auto & i : oldLock->second.inputs) {
-                        fakeInputs.emplace(i.first, FlakeInput {
-                            .ref = i.second.originalRef
-                        });
-                    }
+                        for (auto & i : oldLock->second.inputs)
+                            fakeInputs.emplace(i.first, FlakeInput { .ref = i.second.originalRef });
 
                         updateLocks(fakeInputs,
                             oldLock->second,
                             newLocks.inputs.find(id)->second,
                             inputPath);
+                    }
 
                 } else {
                     /* We need to update/create a new lock file

src/libexpr/flake/flake.hh

@@ -78,7 +78,12 @@ struct LockFlags
        allowed. */
     bool allowMutable = true;
 
+    /* Flake inputs to be overriden. */
     std::map<InputPath, FlakeRef> inputOverrides;
+
+    /* Flake inputs to be updated. This means that any existing lock
+       for those inputs will be ignored. */
+    std::set<InputPath> inputUpdates;
 };
 
 LockedFlake lockFlake(

src/libexpr/flake/lockfile.cc

@@ -72,6 +72,22 @@ std::optional<LockedInput *> LockedInputs::findInput(const InputPath & path)
     return (LockedInput *) pos;
 }
 
+void LockedInputs::removeInput(const InputPath & path)
+{
+    assert(!path.empty());
+
+    LockedInputs * pos = this;
+
+    for (size_t n = 0; n < path.size(); n++) {
+        auto i = pos->inputs.find(path[n]);
+        if (i == pos->inputs.end()) return;
+        if (n + 1 == path.size())
+            pos->inputs.erase(i);
+        else
+            pos = &i->second;
+    }
+}
+
 nlohmann::json LockFile::toJson() const
 {
     auto json = LockedInputs::toJson();

src/libexpr/flake/lockfile.hh

@@ -27,6 +27,8 @@ struct LockedInputs
     bool isImmutable() const;
 
     std::optional<LockedInput *> findInput(const InputPath & path);
+
+    void removeInput(const InputPath & path);
 };
 
 /* Lock file information about a flake input. */

src/nix/installables.cc

@@ -39,6 +39,14 @@ MixFlakeOptions::MixFlakeOptions()
         .description("don't use flake registries")
         .set(&lockFlags.useRegistries, false);
 
+    mkFlag()
+        .longName("update-input")
+        .description("update a specific flake input")
+        .label("input-path")
+        .handler([&](std::vector<std::string> ss) {
+            lockFlags.inputUpdates.insert(flake::parseInputPath(ss[0]));
+        });
+
     mkFlag()
         .longName("override-input")
         .description("override a specific flake input (e.g. 'dwarffs/nixpkgs')")

tests/flakes.sh

@@ -610,3 +610,10 @@ nix flake update $flake3Dir --override-input flake2/flake1 flake1
 
 nix flake update $flake3Dir --override-input flake2/flake1 flake1/master/$hash1
 [[ $(jq .inputs.flake2.inputs.flake1.url $flake3Dir/flake.lock) =~ flake1.*rev=$hash1 ]]
+
+# Test --update-input.
+nix flake update $flake3Dir
+[[ $(jq .inputs.flake2.inputs.flake1.url $flake3Dir/flake.lock) =~ flake1.*rev=$hash1 ]]
+
+nix flake update $flake3Dir --update-input flake2/flake1
+[[ $(jq .inputs.flake2.inputs.flake1.url $flake3Dir/flake.lock) =~ flake1.*rev=$hash2 ]]