Corrupted snapshots for large datasets

[fbacall]

When creating a snapshot with large files (4+GB total), some files end up missing from the resultant Zip file.

It seems that the zip file is cleared whilst being written to:

finn@finn-p520c ~/workspace/seek $ zipinfo /tmp/20191127-17238-qn3pbq/ro-bundle.zip 
Archive:  /tmp/20191127-17238-qn3pbq/ro-bundle.zip
Zip file size: 2743 bytes, number of entries: 7
-rw-r--r--  5.2 unx       36 t- stor 19-Nov-27 14:37 mimetype
drwxr-xr-x  5.2 unx        0 t- stor 19-Nov-27 14:37 .ro/
-rw-r--r--  5.2 unx     1187 t- defN 19-Nov-27 14:37 .ro/manifest.json
-rw-------  5.2 unx     1719 t- defN 19-Nov-27 14:37 metadata.rdf
-rw-------  5.2 unx      642 t- defN 19-Nov-27 14:37 metadata.json
-rw-------  5.2 unx     1283 t- defN 19-Nov-27 14:37 data_files/733-a-third-large-file/metadata.rdf
-rw-------  5.2 unx      569 t- defN 19-Nov-27 14:37 data_files/733-a-third-large-file/metadata.json
7 files, 5436 bytes uncompressed, 1895 bytes compressed:  65.1%
finn@finn-p520c ~/workspace/seek $ zipinfo /tmp/20191127-17238-qn3pbq/ro-bundle.zip 
Archive:  /tmp/20191127-17238-qn3pbq/ro-bundle.zip
Zip file size: 1032 bytes, number of entries: 2
-rw-------  5.2 unx      566 t- defN 19-Nov-27 14:39 data_files/732-another-2gb-file/metadata.json
-rw-r--r--  5.2 unx     2833 t- defN 19-Nov-27 14:39 .ro/manifest.json
2 files, 3399 bytes uncompressed, 734 bytes compressed:  78.4%