TerraSync: stronger fix for handling 0-byte files

Change logic so we create an empty file for such cases, i.e exactly
matching the repository. This simplifies logic in downstream code,
compared with not creating a local file.

Add a test-case to cover this

Modify TerraSync to detect a failure of Airports_archive downloading,
and fall back to file-by-file updating.

simgear/io/HTTPRepository.cxx

@@ -979,17 +979,14 @@ HTTPRepository::failure() const
         {
             pathInRepo = _directory->absolutePath();
             pathInRepo.append(fileName);
-
-            sha1_init(&hashContext);
         }
 
     protected:
-      void gotBodyData(const char *s, int n) override {
+        void gotBodyData(const char* s, int n) override
+        {
             if (!file.get()) {
-          file.reset(new SGBinaryFile(pathInRepo));
+                const bool ok = createOutputFile();
-          if (!file->open(SG_IO_OUT)) {
+                if (!ok) {
-            SG_LOG(SG_TERRASYNC, SG_WARN,
-                   "unable to create file " << pathInRepo);
                     _directory->repository()->http->cancelRequest(
                         this, "Unable to create output file:" + pathInRepo.utf8Str());
                 }
@@ -999,12 +996,34 @@ HTTPRepository::failure() const
             file->write(s, n);
         }
 
-      void onDone() override {
+        bool createOutputFile()
+        {
+            file.reset(new SGBinaryFile(pathInRepo));
+            if (!file->open(SG_IO_OUT)) {
+                SG_LOG(SG_TERRASYNC, SG_WARN,
+                       "unable to create file " << pathInRepo);
+                return false;
+            }
+
+            sha1_init(&hashContext);
+            return true;
+        }
+
+        void onDone() override
+        {
+            const bool is200Response = (responseCode() == 200);
+            if (!file && is200Response) {
+                // if the server defines a zero-byte file, we will never call
+                // gotBodyData, so create the file here
+                // this ensures all the logic below works as expected
+                createOutputFile();
+            }
+
             if (file) {
                 file->close();
             }
 
-        if (responseCode() == 200) {
+            if (is200Response) {
                 std::string hash =
                     strutils::encodeHex(sha1_result(&hashContext), HASH_LENGTH);
                 _directory->didUpdateFile(fileName, hash, contentSize());

simgear/io/test_repository.cxx

@@ -31,6 +31,10 @@ using TestApi = simgear::HTTP::TestApi;
 
 std::string dataForFile(const std::string& parentName, const std::string& name, int revision)
 {
+    if (name == "zeroByteFile") {
+        return {};
+    }
+
     std::ostringstream os;
     // random content but which definitely depends on our tree location
     // and revision.
@@ -446,6 +450,7 @@ void testBasicClone(HTTP::Client* cl)
     verifyFileState(p, "fileA");
     verifyFileState(p, "dirA/subdirA/fileAAA");
     verifyFileState(p, "dirC/subdirA/subsubA/fileCAAA");
+    verifyFileState(p, "dirA/subdirA/zeroByteFile");
 
     global_repo->findEntry("fileA")->revision++;
     global_repo->findEntry("dirB/subdirA/fileBAA")->revision++;
@@ -911,6 +916,8 @@ int main(int argc, char* argv[])
     global_repo->defineFile("dirA/fileAC");
     global_repo->defineFile("dirA/subdirA/fileAAA");
     global_repo->defineFile("dirA/subdirA/fileAAB");
+    global_repo->defineFile("dirA/subdirA/zeroByteFile");
+
     global_repo->defineFile("dirB/subdirA/fileBAA");
     global_repo->defineFile("dirB/subdirA/fileBAB");
     global_repo->defineFile("dirB/subdirA/fileBAC");

simgear/scene/tsync/terrasync.cxx

@@ -571,6 +571,15 @@ void SGTerraSync::WorkerThread::updateSyncSlot(SyncSlot &slot)
             notFound(slot.currentItem);
         } else if (res != HTTPRepository::REPO_NO_ERROR) {
             fail(slot.currentItem);
+
+            // in case the Airports_archive download fails, create the
+            // directory, so that next sync, we do a manual sync
+            if ((slot.currentItem._type == SyncItem::AirportData) && slot.isNewDirectory) {
+                SG_LOG(SG_TERRASYNC, SG_ALERT, "Failed to download Airports_archive, will download discrete files next time");
+                simgear::Dir d(_local_dir + "/Airports");
+                d.create(0755);
+                _completedTiles.erase(slot.currentItem._dir);
+            }
         } else {
             updated(slot.currentItem, slot.isNewDirectory);
             SG_LOG(SG_TERRASYNC, SG_DEBUG, "sync of " << slot.repository->baseUrl() << " finished ("