From 4ed2187377bb915c0eac7f93f20afa3d16f79a5d Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Thu, 9 Apr 2015 12:49:13 +0200
Subject: [PATCH] Use cached result if there is a network error

---
 src/libexpr/download.cc |  2 +-
 src/libexpr/download.hh |  2 ++
 src/libexpr/primops.cc  | 19 ++++++++++++-------
 3 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/src/libexpr/download.cc b/src/libexpr/download.cc
index 050e8d69c..b23e31967 100644
--- a/src/libexpr/download.cc
+++ b/src/libexpr/download.cc
@@ -100,7 +100,7 @@ struct Curl
         CURLcode res = curl_easy_perform(curl);
         if (res == CURLE_WRITE_ERROR && etag == expectedETag) return false;
         if (res != CURLE_OK)
-            throw Error(format("unable to download ‘%1%’: %2% (%3%)")
+            throw DownloadError(format("unable to download ‘%1%’: %2% (%3%)")
                 % url % curl_easy_strerror(res) % res);
 
         long httpStatus = 0;
diff --git a/src/libexpr/download.hh b/src/libexpr/download.hh
index aa4fd5083..65396e5de 100644
--- a/src/libexpr/download.hh
+++ b/src/libexpr/download.hh
@@ -13,4 +13,6 @@ struct DownloadResult
 
 DownloadResult downloadFile(string url, string expectedETag = "");
 
+MakeError(DownloadError, Error)
+
 }
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 33ec26d26..8823efe82 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1551,20 +1551,25 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
 
     if (!skip) {
 
-        if (expectedETag.empty())
+        if (storePath.empty())
             printMsg(lvlInfo, format("downloading ‘%1%’...") % url);
         else
             printMsg(lvlInfo, format("checking ‘%1%’...") % url);
 
-        auto res = downloadFile(url, expectedETag);
+        try {
+            auto res = downloadFile(url, expectedETag);
 
-        if (!res.cached)
-            storePath = store->addTextToStore(name, res.data, PathSet(), state.repair);
+            if (!res.cached)
+                storePath = store->addTextToStore(name, res.data, PathSet(), state.repair);
 
-        assert(!storePath.empty());
-        replaceSymlink(storePath, fileLink);
+            assert(!storePath.empty());
+            replaceSymlink(storePath, fileLink);
 
-        writeFile(dataFile, url + "\n" + res.etag + "\n" + int2String(time(0)) + "\n");
+            writeFile(dataFile, url + "\n" + res.etag + "\n" + int2String(time(0)) + "\n");
+        } catch (DownloadError & e) {
+            if (storePath.empty()) throw;
+            printMsg(lvlError, format("warning: %1%; using cached result") % e.msg());
+        }
     }
 
     if (unpack) {