[GitHub] [couchdb] chewbranca commented on issue #2003: Don't reset_index if read_header fails

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chewbranca commented on issue #2003: Don't reset_index if read_header fails
URL: https://github.com/apache/couchdb/pull/2003#issuecomment-482241405
 
 
   > @rnewson which process would crash and why that would cause read_header() to throw?
   
   I suspect there are multiple ways in which this could happen through the code path, but one highly plausible scenario is when the `couch_file:read_header` call is made, that request goes through an IOQ pid [1], and if that IOQ pid exits, we'll get a `gen_server:call` error that falls into the `_` codepath here and triggers an index reset. When that happens, the `couch_file:truncate` call (along with sync) go directly to the Fd pid and skip going through IOQ [2]. So a read_header requests sitting in IOQ that then exits could trigger a faulty index reset.
   
   One approach is we could look for `gen_server:call` errors and skip resetting in those scenarios, but I think we would be better off explicitly resetting _only_ when we get the `no_valid_header` error. Also, the `couch_mrview_util:reset_index` call both truncates the index and writes a new base header [3], so it's quite possible that this code path is used to initialize empty indexes, and as such, I think we should keep the reset_index logic in here unless we can verify that indexes are bootstrapped always elsewhere and that this logic isn't needed.
   
   
   [1] https://github.com/apache/couchdb/blob/17aec1f3339f864eb08869d6235753f0fdd28259/src/couch/src/couch_file.erl#L326-L332
   [2] https://github.com/apache/couchdb/blob/17aec1f3339f864eb08869d6235753f0fdd28259/src/couch/src/couch_file.erl#L206-L223
   [3] https://github.com/apache/couchdb/blob/17aec1f3339f864eb08869d6235753f0fdd28259/src/couch_mrview/src/couch_mrview_util.erl#L679

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