FS: Fix performance on high latency systems

fseek() in File::append() was invalidating the file buffer for each call
to append().  Better handling improves "import" performance test by 9%,
45% in "commit", on a system with a spinning disk.

This performance problem affects all operations where
{pending,completed}.data are rewritten.  During normal operation a
garbage collection can be enough to trigger it.  On storage with high
latency, e.g. networked, this previously took 20 seconds and more.

src/FS.cpp

@@ -505,7 +505,19 @@ void File::append (const std::vector <std::string>& lines)
   {
     fseek (_fh, 0, SEEK_END);
     for (auto& line : lines)
-      fputs ((line + "\n").c_str (), _fh);
+      fputs (line.c_str (), _fh);
+  }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+void File::write_raw (const std::string& line)
+{
+  if (!_fh)
+    open ();
+
+  if (_fh)
+  {
+    fputs (line.c_str (), _fh);
   }
 }
 

src/FS.h

@@ -91,6 +91,7 @@ public:
 
   void append (const std::string&);
   void append (const std::vector <std::string>&);
+  void write_raw (const std::string&);
 
   void truncate ();
 

src/TDB2.cpp

@@ -257,14 +257,14 @@ void TF2::commit ()
           _file.lock ();
 
         // Write out all the added tasks.
+        _file.append (std::string(""));  // Seek to end of file
         for (auto& task : _added_tasks)
-          _file.append (task.composeF4 () + "\n");
+          _file.write_raw (task.composeF4 () + "\n");
 
         _added_tasks.clear ();
 
         // Write out all the added lines.
-        for (auto& line : _added_lines)
-          _file.append (line);
+        _file.append (_added_lines);
 
         _added_lines.clear ();
         _file.close ();
@@ -282,12 +282,12 @@ void TF2::commit ()
         _file.truncate ();
 
         // Only write out _tasks, because any deltas have already been applied.
+        _file.append (std::string(""));  // Seek to end of file
         for (auto& task : _tasks)
-          _file.append (task.composeF4 () + "\n");
+          _file.write_raw (task.composeF4 () + "\n");
 
         // Write out all the added lines.
-        for (auto& line : _added_lines)
-          _file.append (line);
+        _file.append (_added_lines);
 
         _added_lines.clear ();
         _file.close ();