Building the rentrez tutorial vignette

The vignette for rentrez makes many calls to the NCBI. Usually, rentrez limits the rate of these calls to match the NCBI's usage policy. However, the automated tests run on CRAN run the same code on multiple different platforms at the same time. The data returned by the NCBI doesn't allow us to manage rate-limiting in parallel or from multiple instances at the same IP. The only solution to this problem is to prevent the vignettes running on the CRAN server.

Following the approach described here, rentrez 1.2.3 (and likely all subsequent releases) will only build the vignette and test the vignette code as part of R CMD check if the environment variable BUILD_RENTREZ_VIGNETTE is set to true. The most simple way to do this is to edit (or create) an ~/.Renviron file to include the following:

BUILD_RENTREZ_VIGNETTE=true

This message is onyl really relevant to those of you who wish to develop rentrez. Most users will be able to read the vignette at the CRAN website as usual.