Scan loose ref before packed in case gc about to remove the loose
Before this change, jgit used to read packed-refs before scanning loose refs. That was not a problem if gc didn't run concurrently. When gc did run concurrently with such refs reading, that order sometimes broke the latter. This lead to reading an older version of a ref's tip, which meant "losing" the real tip or commit. The specific read-Vs-gc concurrency scenario which broke reading that way follows: 1. let ref R be in packed-refs and R' be in loose 2. jgit starts reading packed-refs 3. gc also starts its business around that very time 4. jgit still has the time to read R from packed-refs 5. as gc is not done yet updating packed-refs with R' 6. jgit then starts scanning loose refs (or is about to) 7. gc quickly ends up being done moving loose R' to packed-refs 8. so gc (quickly) removes loose refs 9. -while jgit is scanning loose refs, now gone 10. so jgit assumes no loose to consider => packed-refs winning 11. so jgit wrongfully returns R (from 4.) as the tip, instead of R'. This fix switches the order so loose refs are scanned (secured) before taking the time to read packed-refs. This way, knowledge of the likelier tip is guaranteed for ref reading to return the true tip - despite concurrent gc. If there is no loose ref to scan, jgit reads packed-refs and lands on R' (or S), which it then returns, as expected. The gerrit issue [1] should be solved by this fix. [1] https://code.google.com/p/gerrit/issues/detail?id=2302 Change-Id: Ibd120120a361a3a6ed565f3836afc1db706fbcdd Signed-off-by: Marco Miller <marco.miller@ericsson.com> Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
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@ -312,11 +312,10 @@ public Ref getRef(final String needle) throws IOException {
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@Override
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public Map<String, Ref> getRefs(String prefix) throws IOException {
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final RefList<Ref> packed = getPackedRefs();
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final RefList<LooseRef> oldLoose = looseRefs.get();
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LooseScanner scan = new LooseScanner(oldLoose);
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scan.scan(prefix);
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final RefList<Ref> packed = getPackedRefs();
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RefList<LooseRef> loose;
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if (scan.newLoose != null) {
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