move on enospc when writing feature. closes #141

This feature mimics the standard mhddfs behavior but is more thorough.
If a write fails and the errno is set to ENOSPC then mergerfs will (if
the feature is enabled) attempt to move the file to the drive with
the most free space but only if it has enough room for the file plus
the amount to be written. If that transfer is successful it will then
unlink the original file and attempt the previously failed write again.

The copy includes copying the path and file including the acls, owners,
attributes, extended attributes, and timestamps.

src/write_buf.cpp

@@ -25,11 +25,25 @@
 #if WRITE_BUF
 
 #include <fuse.h>
-
 #include <stdlib.h>
+#include <errno.h>
+#include <unistd.h>
 
+#include <string>
+#include <vector>
+
+#include "config.hpp"
+#include "fileinfo.hpp"
+#include "fs_movefile.hpp"
+#include "policy.hpp"
+#include "rwlock.hpp"
+#include "ugid.hpp"
 #include "write.hpp"
 
+using std::string;
+using std::vector;
+using namespace mergerfs;
+
 static
 int
 _write_buf(const int    fd,
@@ -58,9 +72,31 @@ namespace mergerfs
               off_t           offset,
               fuse_file_info *ffi)
     {
-      return _write_buf(ffi->fh,
-                        *src,
-                        offset);
+      int rv;
+      FileInfo *fi = reinterpret_cast<FileInfo*>(ffi->fh);
+
+      rv = _write_buf(fi->fd,*src,offset);
+      if(rv == -ENOSPC)
+       {
+          const fuse_context *fc     = fuse_get_context();
+          const Config       &config = Config::get(fc);
+
+          if(config.moveonenospc)
+            {
+              size_t extra;
+              const ugid::Set         ugid(0,0);
+              const rwlock::ReadGuard readlock(&config.srcmountslock);
+
+              extra = fuse_buf_size(src);
+              rv = fs::movefile(config.srcmounts,fusepath,extra,fi->fd);
+              if(rv == -1)
+                return -ENOSPC;
+
+              rv = _write_buf(fi->fd,*src,offset);
+            }
+        }
+
+      return rv;
     }
   }
 }