Distinct queries can scan many index keys without yielding read lock

For certain distinct queries, the distinct scan query stage can scan many index keys between returning results. Distinct queries that generate inexact bounds and encounter non-matching index entries (and thus generate a call to skip() on the underlying index cursor) are affected. As a result, the distinct query can scan a large index range without yielding its read lock.

To reproduce, run the following shell snippet. The distinct query on the third line will scan the complete {a: 1, b: 1} index without yielding.

for (i=0; i<1000000; i++) { db.foo.insert({a: i, b: 1}); }

db.foo.ensureIndex({a: 1, b: 1});

db.foo.distinct("a", {a: {$ne: 0}, b: 0});

Regression introduced in 2.6.0.