SimpleDownloadTask race causes intermittent FileNotFoundException

[mershad-manesh]

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:

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:

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 - the fix as a diff
• RaceDemo.java - standalone reproducer (javac RaceDemo.java && java RaceDemo)

Related: #2805 (different bug, same "startup race" theme — fileinstall config corruption).