D23008: [baloo_file_extractor] Be more resistant to multiple QSocketNotifier events

[Igor Poboiko]

poboiko created this revision.
poboiko added reviewers: Baloo, bruns, ngraham.
Herald added projects: Frameworks, Baloo.
poboiko requested review of this revision.

REVISION SUMMARY
  When a new batch arrives at stdin (as notified by QSocketNotifier), extractor
  creates a new transaction. If code, which reacts to notification, gets executed
  multiple times, we end up having two write transaction, which is a bad idea.
  
  Current implementation disables `QSocketNotifier`, so that we won't receive new
  notifications. It blocks further notifications, but it does not eliminate the
  possibility when several notifications already appeared in the event queue
  (which can happen due to race condition).
  It shouldn't happen, normally, but the only guard for that case currently is a
  single `Q_ASSERT`, which gets silently ignored in the non-debug build.
  
  This patch replaces assert with proper check, which instead commits & deletes
  the transaction if there is any (just in case).
  It also removes attempts to open the database multiple times (for each batch),
  which is a bad idea, as LMDB documentation suggests. These attempts will be
  ignored anyway (because of the check inside Database::open), yet semantically
  it's better to move the code outside, to the main(), as it is in `baloo_file`.

TEST PLAN
  I didn't manage to cause such race condition "in the laboratory environment", 
  so I've only checked it does not break anything

REPOSITORY
  R293 Baloo

BRANCH
  extractor-transaction (branched from master)

REVISION DETAIL
  https://phabricator.kde.org/D23008

AFFECTED FILES
  src/file/extractor/app.cpp
  src/file/extractor/app.h
  src/file/extractor/main.cpp

To: poboiko, #baloo, bruns, ngraham
Cc: kde-frameworks-devel, LeGast00n, fbampaloukas, domson, ashaposhnikov, michaelh, astippich, spoorun, ngraham, bruns, abrahams
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