doResumableDownload already implements resume - stats the .tmp, sends a
Range header, handles a server that ignores it. The code was unreachable
for ZIMs because every dispatch site passed forceNew: true, which skips
the partial-file check and opens the stream with 'w' instead of 'a', so
an interrupted 12.5 GB Wikipedia download truncated and restarted at
byte 0. Maps already do this correctly (map_service.ts:681, "so retries
resume partial downloads"); ZIMs never got the same treatment.
Worse in combination with attempts: 10 - every retry also restarted from
zero, so a flaky connection re-downloaded the whole file up to ten times.
Drop forceNew from the content-download dispatch sites (it already
defaults to false) and add the guard that enabling resume requires: a
.tmp larger than the file now on the server cannot be a prefix of it,
because openZIM re-publishes builds under the same name. Resuming would
request a range past the end, 416 on every attempt, and never delete the
.tmp - so the download could never recover on its own.
Verified on a test appliance: a 470 MB partial survived a container
restart and continued rather than truncating, and a planted oversized
.tmp was discarded with the file then downloading to its exact size.
Refs #1201
2159c9dec6