From 1c127e6a82dbc128602aa4451dfa8f4c2fe4a751 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 19 Mar 2020 11:42:50 +0100
Subject: [PATCH] downloadFile(): Use expired file if the download fails

---
 src/libstore/fetchers/tarball.cc | 18 ++++++++++++++++--
 tests/flakes.sh                  |  7 +++----
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/src/libstore/fetchers/tarball.cc b/src/libstore/fetchers/tarball.cc
index 54b59babc..62d43ca36 100644
--- a/src/libstore/fetchers/tarball.cc
+++ b/src/libstore/fetchers/tarball.cc
@@ -25,17 +25,31 @@ DownloadFileResult downloadFile(
 
     auto cached = getCache()->lookupExpired(store, inAttrs);
 
-    if (cached && !cached->expired)
+    auto useCached = [&]() -> DownloadFileResult
+    {
         return {
             .storePath = std::move(cached->storePath),
             .etag = getStrAttr(cached->infoAttrs, "etag"),
             .effectiveUrl = getStrAttr(cached->infoAttrs, "url")
         };
+    };
+
+    if (cached && !cached->expired)
+        return useCached();
 
     DownloadRequest request(url);
     if (cached)
         request.expectedETag = getStrAttr(cached->infoAttrs, "etag");
-    auto res = getDownloader()->download(request);
+    DownloadResult res;
+    try {
+        res = getDownloader()->download(request);
+    } catch (DownloadError & e) {
+        if (cached) {
+            warn("%s; using cached version", e.msg());
+            return useCached();
+        } else
+            throw;
+    }
 
     // FIXME: write to temporary file.
 
diff --git a/tests/flakes.sh b/tests/flakes.sh
index fc1c23fe9..52f5fabc0 100644
--- a/tests/flakes.sh
+++ b/tests/flakes.sh
@@ -268,10 +268,9 @@ nix build -o $TEST_ROOT/result $flake3Dir#sth 2>&1 | grep 'unsupported edition'
 
 # Test whether registry caching works.
 nix flake list --flake-registry file://$registry | grep -q flake3
-# FIXME
-#mv $registry $registry.tmp
-#nix flake list --flake-registry file://$registry --refresh | grep -q flake3
-#mv $registry.tmp $registry
+mv $registry $registry.tmp
+nix flake list --flake-registry file://$registry --refresh | grep -q flake3
+mv $registry.tmp $registry
 
 # Test whether flakes are registered as GC roots for offline use.
 # FIXME: use tarballs rather than git.