From e7e070d36b8f58cb48c95ae3ca6dabc0df7b79b9 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Fri, 5 Jul 2024 16:29:16 +0200
Subject: [PATCH] Document

---
 src/libutil/tarfile.cc         | 23 +++++++++++++++--------
 tests/nixos/tarball-flakes.nix |  2 +-
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/src/libutil/tarfile.cc b/src/libutil/tarfile.cc
index 445968b57..d985e5c2a 100644
--- a/src/libutil/tarfile.cc
+++ b/src/libutil/tarfile.cc
@@ -67,6 +67,17 @@ int getArchiveFilterCodeByName(const std::string & method)
     return code;
 }
 
+static void enableSupportedFormats(struct archive * archive)
+{
+    archive_read_support_format_tar(archive);
+    archive_read_support_format_zip(archive);
+
+    /* Enable support for empty files so we don't throw an exception
+       for empty HTTP 304 "Not modified" responses. See
+       downloadTarball(). */
+    archive_read_support_format_empty(archive);
+}
+
 TarArchive::TarArchive(Source & source, bool raw, std::optional<std::string> compression_method)
     : archive{archive_read_new()}
     , source{&source}
@@ -78,11 +89,9 @@ TarArchive::TarArchive(Source & source, bool raw, std::optional<std::string> com
         archive_read_support_filter_by_code(archive, getArchiveFilterCodeByName(*compression_method));
     }
 
-    if (!raw) {
-        archive_read_support_format_tar(archive);
-        archive_read_support_format_zip(archive);
-        archive_read_support_format_empty(archive);
-    } else {
+    if (!raw)
+        enableSupportedFormats(archive);
+    else {
         archive_read_support_format_raw(archive);
         archive_read_support_format_empty(archive);
     }
@@ -98,9 +107,7 @@ TarArchive::TarArchive(const Path & path)
     , buffer(defaultBufferSize)
 {
     archive_read_support_filter_all(archive);
-    archive_read_support_format_tar(archive);
-    archive_read_support_format_zip(archive);
-    archive_read_support_format_empty(archive);
+    enableSupportedFormats(archive);
     archive_read_set_option(archive, NULL, "mac-ext", NULL);
     check(archive_read_open_filename(archive, path.c_str(), 16384), "failed to open archive: %s");
 }
diff --git a/tests/nixos/tarball-flakes.nix b/tests/nixos/tarball-flakes.nix
index e20b853f7..3945bd8b0 100644
--- a/tests/nixos/tarball-flakes.nix
+++ b/tests/nixos/tarball-flakes.nix
@@ -74,7 +74,7 @@ in
     assert info["revision"] == "${nixpkgs.rev}"
     assert info["revCount"] == 1234
 
-    # Check that a 0-byte cached (304) result works.
+    # Check that a 0-byte HTTP 304 "Not modified" result works.
     machine.succeed("nix flake metadata --refresh --json http://localhost/tags/latest.tar.gz")
 
     # Check that fetching with rev/revCount/narHash succeeds.