Mark the log_number file number used

Summary:
VersionSet::next_file_number_ is always assumed to be strictly greater than VersionSet::log_number_. In our new recovery code, we artificially set log_number_  to be (log_number + 1), so that once we flush, we don't recover from the same log file again (this is important because of merge operator non-idempotence)

When we set VersionSet::log_number_ to (log_number + 1), we also have to mark that file number used, such that next_file_number_ is increased to a legal level. Otherwise, VersionSet might assert.

This has not be a problem so far because here's what happens:
1. assume next_file_number is 5, we're recovering log_number 10
2. in DBImpl::Recover() we call MarkFileNumberUsed with 10. This will set VersionSet::next_file_number_ to 11.
3. If there are some updates, we will call WriteTable0ForRecovery(), which will use file number 11 as a new table file and advance VersionSet::next_file_number_ to 12.
4. When we LogAndApply() with log_number 11, assertion is true: assert(11 <= 12);

However, this was a lucky occurrence. Even though this diff doesn't cause a bug, I think the issue is important to fix.

Test Plan: In column families I have different recovery logic and this code path asserted. When adding MarkFileNumberUsed(log_number + 1) assert is gone.

Reviewers: dhruba, kailiu

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15783

db/db_impl.cc

@@ -1066,6 +1066,11 @@ Status DBImpl::RecoverLogFile(uint64_t log_number, SequenceNumber* max_sequence,
     // Since we already recovered log_number, we want all logs
     // with numbers `<= log_number` (includes this one) to be ignored
     edit.SetLogNumber(log_number + 1);
+    // we must mark the next log number as used, even though it's
+    // not actually used. that is because VersionSet assumes
+    // VersionSet::next_file_number_ always to be strictly greater than any log
+    // number
+    versions_->MarkFileNumberUsed(log_number + 1);
     status = versions_->LogAndApply(&edit, &mutex_);
   }