Show progress indicator for builtin fetchurl

src/libstore/builtins.cc

@@ -7,14 +7,16 @@ void builtinFetchurl(const BasicDerivation & drv)
 {
     auto url = drv.env.find("url");
     if (url == drv.env.end()) throw Error("attribute ‘url’ missing");
-    printMsg(lvlInfo, format("downloading ‘%1%’...") % url->second);
 
     /* No need to do TLS verification, because we check the hash of
        the result anyway. */
     DownloadOptions options;
     options.verifyTLS = false;
 
-    auto data = downloadFile(url->second, options); // FIXME: show progress
+    /* Show a progress indicator, even though stderr is not a tty. */
+    options.forceProgress = true;
+
+    auto data = downloadFile(url->second, options);
 
     auto out = drv.env.find("out");
     if (out == drv.env.end()) throw Error("attribute ‘url’ missing");

src/libstore/download.cc

@@ -114,8 +114,6 @@ struct Curl
         curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, progressCallback_);
         curl_easy_setopt(curl, CURLOPT_PROGRESSDATA, (void *) &curl);
         curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0);
-
-        showProgress = isatty(STDERR_FILENO);
     }
 
     ~Curl()
@@ -126,6 +124,8 @@ struct Curl
 
     bool fetch(const string & url, const DownloadOptions & options)
     {
+        showProgress = options.forceProgress || isatty(STDERR_FILENO);
+
         curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
 
         if (options.verifyTLS)

src/libstore/download.hh

@@ -9,6 +9,7 @@ struct DownloadOptions
 {
     string expectedETag;
     bool verifyTLS{true};
+    bool forceProgress{false};
 };
 
 struct DownloadResult