Automatically do git/hg add on flake.lock

src/libstore/fetchers/fetchers.hh

@@ -62,6 +62,9 @@ struct Input : std::enable_shared_from_this<Input>
 
     virtual std::optional<Path> getSourcePath() const { return {}; }
 
+    // FIXME: should merge with getSourcePath().
+    virtual void markChangedFile(std::string_view file) const { assert(false); }
+
     virtual void clone(const Path & destDir) const
     {
         throw Error("do not know how to clone input '%s'", to_string());

src/libstore/fetchers/git.cc

@@ -165,6 +165,18 @@ struct GitInput : Input
         return {};
     }
 
+    void markChangedFile(std::string_view file) const override
+    {
+        auto sourcePath = getSourcePath();
+        assert(sourcePath);
+        runProgram("git", true,
+            { "-C", *sourcePath, "add",
+              "--force",
+              "--intent-to-add",
+              std::string(file)
+            });
+    }
+
     std::pair<bool, std::string> getActualUrl() const
     {
         // Don't clone file:// URIs (but otherwise treat them the

src/libstore/fetchers/mercurial.cc

@@ -84,6 +84,17 @@ struct MercurialInput : Input
         return {};
     }
 
+    void markChangedFile(std::string_view file) const override
+    {
+        auto sourcePath = getSourcePath();
+        assert(sourcePath);
+        // FIXME: shut up if file is already tracked.
+        runProgram("hg", true,
+            { "add",
+              *sourcePath + "/" + std::string(file)
+            });
+    }
+
     std::pair<bool, std::string> getActualUrl() const
     {
         bool isLocal = url.scheme == "file";