Rolling log file writer: avoid truncating file when rotating #144
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There are several conditions under which the rotating log file writer can actually truncate the file it is rotating. Principally, this is caused by a lack of locking when writing to the log file. In rollingFileWriter.Write(), we test for whether the current file needs to be rotated, and if so then perform the rotation. This needs locking to ensure that we do not have two (or more) goroutines making the decision to rotate, then rotating. If this occurs then the second rotation of an empty or virtually-empty file will truncate the original file's contents. This commit resolves the problem by adding mutex locking to Write(). There is also a more convoluted path through the time-based file writer code which (previously) checked the modification time of the file on disk, meaning that a mutex in the Write() routine alone is not sufficient. This commit also changes the time-based tests to cache the time and filename, removing the need for a call to os.Stat() and removing the possibility of making an inconsistent decision to roll more than once. Fixes #143.
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@lwithers Many thanks for your contribution! No lock in rolling writing is really a dangerous bug. |
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There are several conditions under which the rotating log file writer can actually truncate the file it is rotating.
Principally, this is caused by a lack of locking when writing to the log file. In
rollingFileWriter.Write(), we test for whether the current file needs to be rotated, and if so then perform the rotation. This needs locking to ensure that we do not have two (or more) goroutines making the decision to rotate, then rotating. If this occurs then the second rotation of an empty or virtually-empty file will truncate the original file's contents. This commit resolves the problem by adding mutex locking toWrite().There is also a more convoluted path through the time-based file writer code which (previously) checked the modification time of the file on disk, meaning that a mutex in the
Write()routine alone is not sufficient. This commit also changes the time-based tests to cache the time and filename, removing the need for a call toos.Stat()and removing the possibility of making an inconsistent decision to roll more than once.Fixes #143. Supersedes #135.