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Timestamp before outlink

Heinz Junkes requested to merge github/fork/bhill-slac/timestamp-before-outlink into 7.0

Created by: bhill-slac

We've been using this patch in SLAC photon controls since 2014. There was discussion re this in Core-Talk, Nov 26, 2014, subject "Incorrect Timestamp in aSubRecord." This issue has to do with whether a record should update its timestamp before or after processing output links. pro: Record processing chains can be created by output links and/or FLNK. Users shouldn't need to know which method was used when using TSEL links to fetch the timestamp which initiated the chain. con: Device driver code which relies on epicsTimeEventCurrentTime or epicsTimeEventBestTime for timestamping could see a slightly earlier timestamp depending on the time spent in the driver output routine and the resolution of their timestamp provider.

Note: Section 11.4.3 of the App. Dev. Guide has sample code in which recGblGetTimeStamp() is called after processing the output link. If we make this change, the Guide should be updated as well.

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