mershad-manesh opened a new issue, #2808:
URL: https://github.com/apache/karaf/issues/2808
Seen this issue in Karaf 4.4.9, but suspect it will impact newer releases.
`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);
```
If two overlapping resolutions of the same URL race (e.g. two feature
installs, each with their own `DownloadManager` — dedup only happens within one
instance), the second one can `delete()` the file the first one just wrote,
right as something else opens it:
```
java.io.FileNotFoundException:
/..../data/tmp/9c73e02c-failureaccess-1.0.1.jar (No such file or directory)
at java.base/java.io.FileInputStream.open0(Native Method)
at java.base/java.io.FileInputStream.open(FileInputStream.java:216)
```
Seen for multiple different artifacts, so it's generic to the staging code,
not any one jar. Shows up as needing multiple restarts before a container boots
cleanly.
## Fix
Replace delete-then-rename with one atomic move:
```java
try {
Files.move(tmpFile.toPath(), file.toPath(),
StandardCopyOption.REPLACE_EXISTING,
StandardCopyOption.ATOMIC_MOVE);
} catch (AtomicMoveNotSupportedException e) {
Files.move(tmpFile.toPath(), file.toPath(),
StandardCopyOption.REPLACE_EXISTING);
}
```
A concurrent reader then always sees either the old or new file — never a
missing one.
## Proof
Stress test (attached `RaceDemo.java`, 6 writers + 6 readers, 3s):
```
original delete+renameTo -> FileNotFoundException count: 18733
patched Files.move -> FileNotFoundException count: 0
```
## Attached
-
[SimpleDownloadTask.patch](https://github.com/user-attachments/files/31228683/SimpleDownloadTask.patch)
- the fix as a diff
-
[RaceDemo.java](https://github.com/user-attachments/files/31228704/RaceDemo.java)
- standalone reproducer (`javac RaceDemo.java && java RaceDemo`)
Related: #2805 (different bug, same "startup race" theme — fileinstall
config corruption).
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