nix repl: properly deal with interruptions

When I stop a download with Ctrl-C in a `nix repl` of a flake, the REPL
refuses to do any other downloads:

    nix-repl> builtins.getFlake "nix-serve"
    [0.0 MiB DL] downloading 'https://api.github.com/repos/edolstra/nix-serve/tarball/e9828a9e01a14297d15ca41 error: download of 'e9828a9e01' was interrupted
    [0.0 MiB DL]
    nix-repl> builtins.getFlake "nix-serve"
    error: interrupted by the user
    [0.0 MiB DL]

To fix this issue, two changes were necessary:

* Reset the global `_isInterrupted` variable: only because a single
  operation was aborted, it should still be possible to continue the
  session.
* Recreate a `fileTransfer`-instance if the current one was shut down by
  an abort.

src/libstore/filetransfer.cc

@@ -716,15 +716,32 @@ struct curlFileTransfer : public FileTransfer
     }
 };
 
+ref<curlFileTransfer> makeCurlFileTransfer()
+{
+    return make_ref<curlFileTransfer>();
+}
+
 ref<FileTransfer> getFileTransfer()
 {
-    static ref<FileTransfer> fileTransfer = makeFileTransfer();
+    static ref<curlFileTransfer> fileTransfer = makeCurlFileTransfer();
+
+    // this has to be done in its own scope to make sure that the lock is released
+    // before creating a new fileTransfer instance.
+    auto needsRecreation = [&]() -> bool {
+        auto state = fileTransfer->state_.lock();
+        return state->quit;
+    };
+
+    if (needsRecreation()) {
+        fileTransfer = makeCurlFileTransfer();
+    }
+
     return fileTransfer;
 }
 
 ref<FileTransfer> makeFileTransfer()
 {
-    return make_ref<curlFileTransfer>();
+    return makeCurlFileTransfer();
 }
 
 std::future<FileTransferResult> FileTransfer::enqueueFileTransfer(const FileTransferRequest & request)

src/nix/repl.cc

@@ -396,6 +396,8 @@ bool NixRepl::processLine(string line)
 {
     if (line == "") return true;
 
+    _isInterrupted = false;
+
     string command, arg;
 
     if (line[0] == ':') {