[IMPALA-5113] Buffer pool unpinned invariant does not take into account multiply-pinned bytes

• Type: Sub-task
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Impala 2.9.0
• Fix Version/s: Impala 2.9.0
• Component/s: Backend
• Epic Color: ghx-label-6
• Assignee: Tim Armstrong
• Reporter: Tim Armstrong
• Created: 24/Mar/17 16:09
• Updated: 27/Mar/17 23:13
• Resolved: 27/Mar/17 23:13

Description

While working on converting the BufferedBlockMgr test I discovered that the "dirty unpinned bytes <= unused reservation" invariant is too strict. It should be "dirty unpinned bytes <= unused reservation + multiply pinned bytes", since pinning a page multiple times does not require additional buffers.

I was able to prove the bug by adding a DCHECK for the previous invariant. Pinning a page multiple times could violate the invariant (harmlessly, aside from waiting for some extra I/O).