util: fix brotli decompression of empty input

This caused an infinite loop before since it would just keep asking the
underlying source for more data.

In practice this happened because an HTTP server served a
response to a HEAD request (for which curl will not retrieve any body or
call our write callback function) with Content-Encoding: br, leading to
decompressing nothing at all and going into an infinite loop.

This adds a test to make sure none of our compression methods do that
again, as well as just patching the HTTP client to never feed empty data
into a compression algorithm (since they absolutely have the right to
throw CompressionError on unexpectedly-short streams!).

Reported on Matrix: https://matrix.to/#/!lymvtcwDJ7ZA9Npq:lix.systems/$8BWQR_zKxCQDJ40C5NnDo4bQPId3pZ_aoDj2ANP7Itc?via=lix.systems&via=matrix.org&via=tchncs.de

Change-Id: I027566e280f0f569fdb8df40e5ecbf46c211dad1

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index f3e8a5312..10c810e49 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -337,7 +337,7 @@ struct curlFileTransfer : public FileTransfer
             // wrapping user `callback`s instead is not possible because the
             // Callback api expects std::functions, and copying Callbacks is
             // not possible due the promises they hold.
-            if (code == CURLE_OK && !dataCallback) {
+            if (code == CURLE_OK && !dataCallback && result.data.length() > 0) {
                 result.data = decompress(encoding, result.data);
             }