m3l0n-sq commented on code in PR #4051:
URL: https://github.com/apache/jena/pull/4051#discussion_r3611779280
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jena-db/jena-dboe-base/src/main/java/org/apache/jena/dboe/base/file/BlockAccessMapped.java:
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@@ -149,20 +149,19 @@ private ByteBuffer getByteBuffer(long _id) {
synchronized (this) {
try {
- // Need to put the alloc AND the slice/reset inside a sync.
+ // Need to put the alloc AND the slice inside a sync.
ByteBuffer segBuffer = allocSegment(seg);
- // Now slice the buffer to get the ByteBuffer to return
- segBuffer.position(segOff);
- segBuffer.limit(segOff+blockSize);
- ByteBuffer dst = segBuffer.slice();
-
- // And then reset limit to max for segment.
- segBuffer.limit(segBuffer.capacity());
+ // Slice from a cleared duplicate: never touches the shared
+ // segment buffer's position/limit, and does not depend on them
+ // either. The old position/limit/slice/reset sequence could
+ // leave a shrunken limit on the shared buffer if anything
threw
+ // before the reset, poisoning every later access to a higher
+ // block in the segment ("newPosition > limit").
+ ByteBuffer dst = segBuffer.duplicate().clear().slice(segOff,
blockSize);
Review Comment:
You're right that `slice(segOff, blockSize)` does not modify `segBuffer`.
Simplified this down to just using `slice(segOff, blockSize)` and removing
`duplicate`
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