adityaanikam opened a new pull request, #2841:
URL: https://github.com/apache/karaf/pull/2841
Fixes #2808
## Problem
`SimpleDownloadTask.download()` stages `wrap:`/`blueprint:`/`spring:` bundle
URLs into a file named by hashing the URL, then does:
```java
if (file.exists() && !file.delete()) { throw ...; }
tmpFile.renameTo(file);
```
Two overlapping resolutions of the same URL (e.g. two feature installs, each
with their own `DownloadManager` -- dedup only happens within one instance) can
race: the second one's `delete()` removes the first one's just-written file
right as a third reader opens it, producing an intermittent
`FileNotFoundException`. This is generic to the staging code, not any one jar,
and shows up as needing multiple restarts before a container boots cleanly.
## Fix
Replace the delete-then-rename with a single `Files.move(..., ATOMIC_MOVE)`,
falling back to a plain move if the filesystem doesn't support atomic moves. A
concurrent reader then always sees either the old or the new file, never a
momentarily missing one.
## Testing
Added `SimpleDownloadTaskTest`, which runs 200 rounds of 6 concurrent
downloads racing on the same destination while a reader thread polls for the
file transiently disappearing after having already existed once. Verified with
a negative control: reverting only the fix (keeping the test) reproduces the
race decisively -- 221,147 missing-file observations across the 200 rounds;
restoring the fix brings that to zero.
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