Implement a TTL on cached fetchurl/fetchTarball results

This is because we don't want to do HTTP requests on every evaluation,
even though we can prevent a full redownload via the cached ETag. The
default is one hour.

src/libexpr/primops.cc

@@ -1631,15 +1631,23 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
 
     string expectedETag;
 
+    int ttl = settings.get("tarball-ttl", 60 * 60);
+    bool skip = false;
+
     if (pathExists(fileLink) && pathExists(dataFile)) {
         storePath = readLink(fileLink);
         store->addTempRoot(storePath);
         if (store->isValidPath(storePath)) {
             auto ss = tokenizeString<vector<string>>(readFile(dataFile), "\n");
-            if (ss.size() >= 2 && ss[0] == url) {
+            if (ss.size() >= 3 && ss[0] == url) {
+                time_t lastChecked;
+                if (string2Int(ss[2], lastChecked) && lastChecked + ttl >= time(0))
+                    skip = true;
+                else if (!ss[1].empty()) {
                     printMsg(lvlDebug, format("verifying previous ETag ‘%1%’") % ss[1]);
                     expectedETag = ss[1];
                 }
+            }
         } else
             storePath = "";
     }
@@ -1648,6 +1656,8 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
     auto p = url.rfind('/');
     if (p != string::npos) name = string(url, p + 1);
 
+    if (!skip) {
+
         if (expectedETag.empty())
             printMsg(lvlInfo, format("downloading ‘%1%’...") % url);
         else
@@ -1660,7 +1670,8 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
         assert(!storePath.empty());
         replaceSymlink(storePath, fileLink);
 
-    writeFile(dataFile, url + "\n" + curl.etag + "\n");
+        writeFile(dataFile, url + "\n" + curl.etag + "\n" + int2String(time(0)) + "\n");
+    }
 
     if (unpack) {
         Path unpackedLink = cacheDir + "/" + baseNameOf(storePath) + "-unpacked";

src/libstore/globals.cc

@@ -143,6 +143,14 @@ bool Settings::get(const string & name, bool def)
 }
 
 
+int Settings::get(const string & name, int def)
+{
+    int res = def;
+    _get(res, name);
+    return res;
+}
+
+
 void Settings::update()
 {
     _get(tryFallback, "build-fallback");

src/libstore/globals.hh

@@ -27,6 +27,8 @@ struct Settings {
 
     bool get(const string & name, bool def);
 
+    int get(const string & name, int def);
+
     void update();
 
     string pack();