fix(downloads): let interrupted content downloads resume (#1202)

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

admin/app/services/collection_update_service.ts

@@ -120,7 +120,6 @@ export class CollectionUpdateService {
       timeout: 30000,
       allowedMimeTypes:
         update.resource_type === 'zim' ? ZIM_MIME_TYPES : PMTILES_MIME_TYPES,
-      forceNew: true,
       filetype: update.resource_type,
       title: update.resource_id,
       totalBytes: update.size_bytes,

admin/app/services/creator_pack_service.ts

@@ -117,7 +117,6 @@ export class CreatorPackService {
       filepath,
       timeout: 30000,
       allowedMimeTypes: ZIM_MIME_TYPES,
-      forceNew: true,
       filetype: 'zim',
       title: pack.name,
       totalBytes: pack.size_mb ? pack.size_mb * 1024 * 1024 : undefined,

admin/app/services/zim_service.ts

@@ -222,7 +222,6 @@ export class ZimService {
       filepath,
       timeout: 30000,
       allowedMimeTypes: ZIM_MIME_TYPES,
-      forceNew: true,
       filetype: 'zim',
       title: metadata?.title,
       totalBytes: metadata?.size_bytes,
@@ -331,7 +330,6 @@ export class ZimService {
         filepath,
         timeout: 30000,
         allowedMimeTypes: ZIM_MIME_TYPES,
-        forceNew: true,
         filetype: 'zim',
         title: (resource as any).title || undefined,
         totalBytes: resolved.sizeBytes,
@@ -830,7 +828,6 @@ export class ZimService {
       filepath,
       timeout: 30000,
       allowedMimeTypes: ZIM_MIME_TYPES,
-      forceNew: true,
       filetype: 'zim',
       title: selectedOption.name,
       totalBytes: selectedOption.size_mb ? selectedOption.size_mb * 1024 * 1024 : undefined,

admin/app/utils/downloads.ts

@@ -8,6 +8,7 @@ import { deleteFileIfExists, ensureDirectoryExists, getFileStatsIfExists } from
 import { createWriteStream } from 'fs'
 import { rename } from 'fs/promises'
 import path from 'path'
+import logger from '@adonisjs/core/services/logger'
 
 // Some upstream mirrors reject requests with a missing or generic User-Agent.
 // Notably, download.kiwix.org routes the large Wikimedia-family ZIMs (Wikipedia,
@@ -121,6 +122,20 @@ export async function doResumableDownload({
     appendMode = false
   }
 
+  // A .tmp bigger than the file now on the server cannot be a prefix of it — the
+  // publisher replaced the file under the same name (openZIM rolls builds forward,
+  // see #1189/#1187). Resuming would ask for a range past the end and get a 416 on
+  // every attempt, with nothing deleting the .tmp, so the download could never
+  // recover on its own. Discard and start clean.
+  if (startByte > totalBytes && totalBytes > 0) {
+    logger.warn(
+      `[Download] Discarding stale partial for ${filepath}: .tmp is ${startByte}B but the server reports ${totalBytes}B`
+    )
+    await deleteFileIfExists(tempPath)
+    startByte = 0
+    appendMode = false
+  }
+
   // Add Range header if resuming
   if (supportsRangeRequests && startByte > 0) {
     headers.Range = `bytes=${startByte}-`