Allow tarball URLs to redirect to a lockable immutable URL

Previously, for tarball flakes, we recorded the original URL of the
tarball flake, rather than the URL to which it ultimately
redirects. Thus, a flake URL like
http://example.org/patchelf-latest.tar that redirects to
http://example.org/patchelf-<revision>.tar was not really usable. We
couldn't record the redirected URL, because sites like GitHub redirect
to CDN URLs that we can't rely on to be stable.

So now we use the redirected URL only if the server returns the
`x-nix-is-immutable` or `x-amz-meta-nix-is-immutable` headers in its
response.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/libstore/filetransfer.hh b/src/libstore/filetransfer.hh
index 378c6ff78..a3b0dde1f 100644
--- a/src/libstore/filetransfer.hh
+++ b/src/libstore/filetransfer.hh
@@ -80,6 +80,10 @@ struct FileTransferResult
     std::string effectiveUri;
     std::string data;
     uint64_t bodySize = 0;
+    /* An "immutable" URL for this resource (i.e. one whose contents
+       will never change), as returned by the `Link: <url>;
+       rel="immutable"` header. */
+    std::optional<std::string> immutableUrl;
 };
 
 class Store;