libstore: collect effective url and cachedness earlier

this will let us return metadata for a transfer without having to wait
for the entire transfer to complete. more importantly for current uses
though is that we could now send retries to the effective url directly
instead of retreading the entire redirect path. this improves latency,
and in such cases where redirects change while we're downloading it'll
also improve correctness (previously we'd silently download bad data).

Change-Id: I6fcd920eb96fbdb2e960b73773c0b854e0300e99

src/libstore/filetransfer.cc

@@ -50,7 +50,17 @@ struct curlFileTransfer : public FileTransfer
         std::optional<std::string> uploadData;
         std::string downloadData;
         bool noBody = false; // \equiv HTTP HEAD, don't download data
-        bool done = false; // whether either the success or failure function has been called
+        enum {
+            /// nothing has been transferred yet
+            initialSetup,
+            /// at least some metadata has already been transferred,
+            /// but the transfer did not succeed and is now retrying
+            retrySetup,
+            /// data transfer in progress
+            transferring,
+            /// transfer complete, result or failure reported
+            transferComplete,
+        } phase = initialSetup;
         std::packaged_task<
             std::pair<FileTransferResult, std::string>(std::exception_ptr, FileTransferResult)>
             callback;
@@ -129,7 +139,7 @@ struct curlFileTransfer : public FileTransfer
             curl_easy_cleanup(req);
             if (requestHeaders) curl_slist_free_all(requestHeaders);
             try {
-                if (!done)
+                if (phase != transferComplete)
                     fail(FileTransferError(Interrupted, {}, "download of '%s' was interrupted", uri));
             } catch (...) {
                 ignoreExceptionInDestructor();
@@ -138,8 +148,8 @@ struct curlFileTransfer : public FileTransfer
 
         void failEx(std::exception_ptr ex)
         {
-            assert(!done);
+            assert(phase != transferComplete);
-            done = true;
+            phase = transferComplete;
             callback(ex, std::move(result));
         }
 
@@ -149,6 +159,22 @@ struct curlFileTransfer : public FileTransfer
             failEx(std::make_exception_ptr(std::forward<T>(e)));
         }
 
+        void maybeFinishSetup()
+        {
+            if (phase > retrySetup) {
+                return;
+            }
+
+            char * effectiveUriCStr = nullptr;
+            curl_easy_getinfo(req, CURLINFO_EFFECTIVE_URL, &effectiveUriCStr);
+            if (effectiveUriCStr)
+                result.effectiveUri = effectiveUriCStr;
+
+            result.cached = getHTTPStatus() == 304;
+
+            phase = transferring;
+        }
+
         std::exception_ptr writeException;
 
         size_t writeCallback(void * contents, size_t size, size_t nmemb)
@@ -156,6 +182,8 @@ struct curlFileTransfer : public FileTransfer
             const size_t realSize = size * nmemb;
 
             try {
+                maybeFinishSetup();
+
                 bodySize += realSize;
 
                 if (successfulStatuses.count(getHTTPStatus()) && this->dataCallback) {
@@ -268,6 +296,10 @@ struct curlFileTransfer : public FileTransfer
 
         void init()
         {
+            if (phase > initialSetup) {
+                phase = retrySetup;
+            }
+
             curl_easy_reset(req);
 
             if (verbosity >= lvlVomit) {
@@ -337,10 +369,7 @@ struct curlFileTransfer : public FileTransfer
         {
             auto httpStatus = getHTTPStatus();
 
-            char * effectiveUriCStr = nullptr;
+            maybeFinishSetup();
-            curl_easy_getinfo(req, CURLINFO_EFFECTIVE_URL, &effectiveUriCStr);
-            if (effectiveUriCStr)
-                result.effectiveUri = effectiveUriCStr;
 
             debug("finished %s of '%s'; curl status = %d, HTTP status = %d, body = %d bytes",
                 verb(), uri, code, httpStatus, bodySize);
@@ -358,9 +387,8 @@ struct curlFileTransfer : public FileTransfer
 
             else if (code == CURLE_OK && successfulStatuses.count(httpStatus))
             {
-                result.cached = httpStatus == 304;
                 act.progress(bodySize, bodySize);
-                done = true;
+                phase = transferComplete;
                 callback(nullptr, std::move(result));
             }