downloadCached: Return ETag

(cherry picked from commit 529add316c5356a8060c35f987643b7bf5c796dc)

2 files changed, 23 insertions, 6 deletions

diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index 22382ab1d..cb77cdc77 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -790,7 +790,7 @@ void Downloader::download(DownloadRequest && request, Sink & sink)
     }
 }
 
-Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpack, string name, const Hash & expectedHash, string * effectiveUrl, int ttl)
+CachedDownloadResult Downloader::downloadCached(ref<Store> store, const string & url_, bool unpack, string name, const Hash & expectedHash, string * effectiveUrl, int ttl)
 {
     auto url = resolveUri(url_);
 
@@ -802,8 +802,11 @@ Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpa
     Path expectedStorePath;
     if (expectedHash) {
         expectedStorePath = store->makeFixedOutputPath(unpack, expectedHash, name);
-        if (store->isValidPath(expectedStorePath))
-            return store->toRealPath(expectedStorePath);
+        if (store->isValidPath(expectedStorePath)) {
+            CachedDownloadResult result;
+            result.path = store->toRealPath(expectedStorePath);
+            return result;
+        }
     }
 
     Path cacheDir = getCacheDir() + "/nix/tarballs";
@@ -822,6 +825,8 @@ Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpa
 
     bool skip = false;
 
+    CachedDownloadResult result;
+
     if (pathExists(fileLink) && pathExists(dataFile)) {
         storePath = readLink(fileLink);
         store->addTempRoot(storePath);
@@ -833,6 +838,7 @@ Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpa
                     skip = true;
                     if (effectiveUrl)
                         *effectiveUrl = url_;
+                    result.etag = ss[1];
                 } else if (!ss[1].empty()) {
                     debug(format("verifying previous ETag '%1%'") % ss[1]);
                     expectedETag = ss[1];
@@ -850,6 +856,7 @@ Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpa
             auto res = download(request);
             if (effectiveUrl)
                 *effectiveUrl = res.effectiveUrl;
+            result.etag = res.etag;
 
             if (!res.cached) {
                 ValidPathInfo info;
@@ -871,6 +878,7 @@ Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpa
         } catch (DownloadError & e) {
             if (storePath.empty()) throw;
             printError(format("warning: %1%; using cached result") % e.msg());
+            result.etag = expectedETag;
         }
     }
 
@@ -904,7 +912,8 @@ Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpa
             url, expectedHash.to_string(), gotHash.to_string());
     }
 
-    return store->toRealPath(storePath);
+    result.path = store->toRealPath(storePath);
+    return result;
 }
 
 
diff --git a/src/libstore/download.hh b/src/libstore/download.hh
index f0228f7d0..8acfe4e1a 100644
--- a/src/libstore/download.hh
+++ b/src/libstore/download.hh
@@ -41,6 +41,12 @@ struct DownloadResult
     uint64_t bodySize = 0;
 };
 
+struct CachedDownloadResult
+{
+    Path path;
+    std::optional<std::string> etag;
+};
+
 class Store;
 
 struct Downloader
@@ -64,8 +70,10 @@ struct Downloader
        and is more recent than ‘tarball-ttl’ seconds. Otherwise,
        use the recorded ETag to verify if the server has a more
        recent version, and if so, download it to the Nix store. */
-    Path downloadCached(ref<Store> store, const string & uri, bool unpack, string name = "",
-        const Hash & expectedHash = Hash(), string * effectiveUri = nullptr, int ttl = settings.tarballTtl);
+    CachedDownloadResult downloadCached(
+        ref<Store> store, const string & uri, bool unpack, string name = "",
+        const Hash & expectedHash = Hash(), string * effectiveUri = nullptr,
+        int ttl = settings.tarballTtl);
 
     enum Error { NotFound, Forbidden, Misc, Transient, Interrupted };
 };