[Rd] Using rawConnection() + gzfile() to obtain a raw vector of compressed data does not appear to work.

[Pavel Krivitsky]

Dear All,

This was actually flagged by someone back in 2014
(https://stat.ethz.ch/pipermail/r-help/2014-November/423197.html),
though back then it caused a crash rather than an error.

Documentation for gzcon() says the following should be possible ("Use a
writable ‘rawConnection’ to compress data into a variable."), but when
I run:

   compressed <- rawConnection(raw(0), "wb")
   compressing <- gzcon(compressed)
   writeBin(paste(letters, collaplse = ""), compressing)
   rawConnectionValue(compressed)

I get:

   Error in rawConnectionValue(compressed) :
     internal connection is not a rawConnection

In principle, this should work: the rawConnection `compressed` in write
mode receives the data and stores them in a buffer. gzcon() then
creates a connection `compressing`, so that writing to it should gzip
the data and write the result to `compressed`. The compressed data
should then be accessible by rawConnectionValue(compressed).

In practice, the gzcon() actually modifies `compressed` in place, which
is fine if it's a file or similar, but apparently rawConnectionValue()
no longer works on it.

Am I using the wrong approach? Is there a workaround? memCompress()
uses an incompatible format.

                        Best Regards,
                        Pavel

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