[giggio]

Symptom

Some files can never be read through the mount. Every open() fails with EREMOTEIO ("Remote I/O error"), and the log shows the whole file downloading successfully and then one request too many:

INF Downloading bytes 136314880-146800639/146800640. name=VID_20231129_132554_10_036.insv
INF Downloading bytes 146800640-157286399/146800640. name=VID_20231129_132554_10_036.insv
ERR Failed to fetch remote content. error="HTTP 416 - : " op=Open

It looks intermittent, because it only happens when onedriver actually takes the download path — a cache miss, or Not using cached item due to file hash mismatch. Clearing the cache makes it worse, since a miss is exactly what forces the failing path.

Cause

GetItemContentStream in fs/graph/drive_item.go runs the multipart loop floor(size/chunk) + 1 times. That equals ceil(size/chunk) for every size except an exact multiple of the chunk size, where it is one trip too many. The final request then asks for bytes=<size>-<size+chunk-1> — a range starting at EOF — which the Graph API correctly rejects with 416 Range Not Satisfiable. The error is returned as fatal, so the item is unreadable for good.

So the condition is exactly: size % 10485760 == 0 and size > 10485760. Sizes at or below one chunk take the one-shot path and are unaffected.

Fix

One line — use ceil instead of floor + 1:

for i := 0; i < int((item.Size+downloadChunkSize-1)/downloadChunkSize); i++ {

Verification

In a tree of ~88,600 files, 32 had a size that was an exact multiple of 10485760. Of those, 24 were re-fetched during a verification pass and all 24 failed; the remaining 8 were served from cache and never hit the download path. Of the ~88,570 files whose size was not an exact multiple, none failed for this reason — including 179 files of the same type and similar sizes.

After the fix, all 24 read and hash correctly, and no other file changed behaviour.

The arithmetic on its own is decisive:

size	trips before	last range starts at		trips after
146800640 (14 chunks)	15	146800640	= EOF → 416	14
20800304	2	10485760	ok	2
15728640	2	10485760	ok	2
10485761	2	10485760	ok	2

Notes

• Unrelated to onedriver crashes with message "slice bounds out of range" #255. That issue is about cache/database corruption; this is a plain off-by-one in the range loop, and the two only look connected because fighting the cache is the natural (and counterproductive) reaction to the symptom.
• I did not add a test: the loop is inline in GetItemContentStream, which needs an authenticated Graph client, so covering it would mean extracting the trip count into its own function first. Happy to do that if you'd prefer it tested.