builtins.fetchurl: Fix segfault on s3:// URLs

Also, add an activity to show that we're downloading an s3:// file.

Fixes #11674.

src/libstore/filetransfer.cc

@@ -759,12 +759,17 @@ struct curlFileTransfer : public FileTransfer
 
                 S3Helper s3Helper(profile, region, scheme, endpoint);
 
+                Activity act(*logger, lvlTalkative, actFileTransfer,
+                    fmt("downloading '%s'", request.uri),
+                    {request.uri}, request.parentAct);
+
                 // FIXME: implement ETag
                 auto s3Res = s3Helper.getObject(bucketName, key);
                 FileTransferResult res;
                 if (!s3Res.data)
                     throw FileTransferError(NotFound, "S3 object '%s' does not exist", request.uri);
                 res.data = std::move(*s3Res.data);
+                res.urls.push_back(request.uri);
                 callback(std::move(res));
 #else
                 throw nix::Error("cannot download '%s' because Nix is not built with S3 support", request.uri);

tests/nixos/s3-binary-cache-store.nix

@@ -51,6 +51,9 @@ in {
 
     server.succeed("${env} nix copy --to '${storeUrl}' ${pkgA}")
 
+    # Test fetchurl on s3:// URLs while we're at it.
+    client.succeed("${env} nix eval --impure --expr 'builtins.fetchurl { name = \"foo\"; url = \"s3://my-cache/nix-cache-info?endpoint=http://server:9000&region=eu-west-1\"; }'")
+
     # Copy a package from the binary cache.
     client.fail("nix path-info ${pkgA}")