add volflag sparse to force use of sparse files;

this improves performance on s3-backed volumes

noktuas reported on discord that the upload performance was
unexpectedly poor when writing to an s3 bucket through a JuiceFS
fuse-mount, only getting 1.5 MiB/s with copyparty, meanwhile a
regular filecopy averaged 30 MiB/s plus

the issue was that s3 does not support sparse files, so copyparty
would fall back to sequential uploading, and also disable fpool,
causing JuiceFS to repeatedly commit the same 5 MiB range to
the storage provider as each chunk arrived from the client

by forcing use of sparse files, s3 adapters such as JuiceFS and
geesefs will "only" write the entire file to s3 *twice*, initially
it writes the full filesize of zerobytes (depending on adapter,
hopefully using gzip compression to reduce the bandwidth necessary)
and then the actual file data in an adapter-specific chunksize

with this volflag, copyparty appears to reach the full expected speed

copyparty/cfg.py

@@ -116,6 +116,7 @@ flagcats = {
         "hardlink": "does dedup with hardlinks instead of symlinks",
         "neversymlink": "disables symlink fallback; full copy instead",
         "copydupes": "disables dedup, always saves full copies of dupes",
+        "sparse": "force use of sparse files, mainly for s3-backed storage",
         "daw": "enable full WebDAV write support (dangerous);\nPUT-operations will now \033[1;31mOVERWRITE\033[0;35m existing files",
         "nosub": "forces all uploads into the top folder of the vfs",
         "magic": "enables filetype detection for nameless uploads",

copyparty/up2k.py

@@ -3956,7 +3956,13 @@ class Up2k(object):
 
             if not ANYWIN and sprs and sz > 1024 * 1024:
                 fs = self.fstab.get(pdir)
-                if fs != "ok":
+                if fs == "ok":
+                    pass
+                elif "sparse" in self.flags[job["ptop"]]:
+                    t = "volflag 'sparse' is forcing use of sparse files for uploads to [%s]"
+                    self.log(t % (job["ptop"],))
+                    relabel = True
+                else:
                     relabel = True
                     f.seek(1024 * 1024 - 1)
                     f.write(b"e")