Program: 91262
Project: 91262
Buggy source:
/** 
 * Describes the specified commit. Target defaults to HEAD if no commit was set explicitly.
 * @return if there's a tag that points to the commit being described, thistag name is returned. Otherwise additional suffix is added to the nearest tag, just like git-describe(1). <p> If none of the ancestors of the commit being described has any tags at all, then this method returns null, indicating that there's no way to describe this tag.
 */
@Override public String call() throws GitAPIException {
  try {
    checkCallable();
    if (target == null)     setTarget(Constants.HEAD);
    Map<ObjectId,Ref> tags=new HashMap<>();
    for (    Ref r : repo.getRefDatabase().getRefs(R_TAGS).values()) {
      ObjectId key=repo.peel(r).getPeeledObjectId();
      if (key == null)       key=r.getObjectId();
      tags.put(key,r);
    }
    final RevFlagSet allFlags=new RevFlagSet();
    /** 
 * Tracks the depth of each tag as we find them.
 */
class Candidate {
      final Ref tag;
      final RevFlag flag;
      /** 
 * This field counts number of commits that are reachable from the tip but not reachable from the tag.
 */
      int depth;
      Candidate(      RevCommit commit,      Ref tag){
        this.tag=tag;
        this.flag=w.newFlag(tag.getName());
        allFlags.add(flag);
        w.carry(flag);
        commit.add(flag);
        commit.carry(flag);
      }
      /** 
 * Does this tag contain the given commit?
 */
      boolean reaches(      RevCommit c){
        return c.has(flag);
      }
      String describe(      ObjectId tip) throws IOException {
        return longDescription(tag,depth,tip);
      }
    }
    List<Candidate> candidates=new ArrayList<>();
    Ref tagOnTarget=tags.get(target);
    if (tagMatches(tagOnTarget)) {
      return longDesc ? longDescription(tagOnTarget,0,target) : tagOnTarget.getName().substring(R_TAGS.length());
    }
    w.markStart(target);
    int seen=0;
    RevCommit c;
    while ((c=w.next()) != null) {
      if (!c.hasAny(allFlags)) {
        Ref t=tags.get(c);
        if (tagMatches(t)) {
          Candidate cd=new Candidate(c,t);
          candidates.add(cd);
          cd.depth=seen;
        }
      }
      for (      Candidate cd : candidates) {
        if (!cd.reaches(c))         cd.depth++;
      }
      if (candidates.size() >= maxCandidates)       break;
      seen++;
    }
    while ((c=w.next()) != null) {
      if (c.hasAll(allFlags)) {
        for (        RevCommit p : c.getParents())         p.add(RevFlag.SEEN);
      }
 else {
        for (        Candidate cd : candidates) {
          if (!cd.reaches(c))           cd.depth++;
        }
      }
    }
    if (candidates.isEmpty())     return null;
    Candidate best=Collections.min(candidates,new Comparator<Candidate>(){
      @Override public int compare(      Candidate o1,      Candidate o2){
        return o1.depth - o2.depth;
      }
    }
);
    return best.describe(target);
  }
 catch (  IOException e) {
    throw new JGitInternalException(e.getMessage(),e);
  }
 finally {
    setCallable(false);
    w.close();
  }
}

Fixed version (ground truth):
/** 
 * Describes the specified commit. Target defaults to HEAD if no commit was set explicitly.
 * @return if there's a tag that points to the commit being described, thistag name is returned. Otherwise additional suffix is added to the nearest tag, just like git-describe(1). <p> If none of the ancestors of the commit being described has any tags at all, then this method returns null, indicating that there's no way to describe this tag.
 */
@Override public String call() throws GitAPIException {
  try {
    checkCallable();
    if (target == null)     setTarget(Constants.HEAD);
    Collection<Ref> tagList=repo.getRefDatabase().getRefs(R_TAGS).values();
    Map<ObjectId,List<Ref>> tags=tagList.stream().collect(Collectors.groupingBy(this::getObjectIdFromRef));
    final RevFlagSet allFlags=new RevFlagSet();
    /** 
 * Tracks the depth of each tag as we find them.
 */
class Candidate {
      final Ref tag;
      final RevFlag flag;
      /** 
 * This field counts number of commits that are reachable from the tip but not reachable from the tag.
 */
      int depth;
      Candidate(      RevCommit commit,      Ref tag){
        this.tag=tag;
        this.flag=w.newFlag(tag.getName());
        allFlags.add(flag);
        w.carry(flag);
        commit.add(flag);
        commit.carry(flag);
      }
      /** 
 * Does this tag contain the given commit?
 */
      boolean reaches(      RevCommit c){
        return c.has(flag);
      }
      String describe(      ObjectId tip) throws IOException {
        return longDescription(tag,depth,tip);
      }
    }
    List<Candidate> candidates=new ArrayList<>();
    Optional<Ref> bestMatch=getBestMatch(tags.get(target));
    if (bestMatch.isPresent()) {
      return longDesc ? longDescription(bestMatch.get(),0,target) : bestMatch.get().getName().substring(R_TAGS.length());
    }
    w.markStart(target);
    int seen=0;
    RevCommit c;
    while ((c=w.next()) != null) {
      if (!c.hasAny(allFlags)) {
        bestMatch=getBestMatch(tags.get(c));
        if (bestMatch.isPresent()) {
          Candidate cd=new Candidate(c,bestMatch.get());
          candidates.add(cd);
          cd.depth=seen;
        }
      }
      for (      Candidate cd : candidates) {
        if (!cd.reaches(c))         cd.depth++;
      }
      if (candidates.size() >= maxCandidates)       break;
      seen++;
    }
    while ((c=w.next()) != null) {
      if (c.hasAll(allFlags)) {
        for (        RevCommit p : c.getParents())         p.add(RevFlag.SEEN);
      }
 else {
        for (        Candidate cd : candidates) {
          if (!cd.reaches(c))           cd.depth++;
        }
      }
    }
    if (candidates.isEmpty())     return null;
    Candidate best=Collections.min(candidates,new Comparator<Candidate>(){
      @Override public int compare(      Candidate o1,      Candidate o2){
        return o1.depth - o2.depth;
      }
    }
);
    return best.describe(target);
  }
 catch (  IOException e) {
    throw new JGitInternalException(e.getMessage(),e);
  }
 finally {
    setCallable(false);
    w.close();
  }
}