Data.table: SHM size exceeded Error

Feature request, check the size of /dev/shm to see if it matches the mmap size. In my case I was using R inside a docker container and the default size for /dev/shm is 64M. When the code does a mmap on a piped stream (such as with hadoop fs -cat /myfile.csv) it will only read the shm size bytes from the pipe into mmap. It does not report an error via the C api which I suspect is normal. However, debugging why fread complained about the file format resulted in a deep dive into the R and C code of data.table to discover it uses this mechanism. The error reported (random message based on where my pipe happened to be cut off):

 (ERROR):Expected sep (',') but ' ' ends field 0 when detecting types from point 10: 14183667

This can be reproduced by doing the following:

• Build a file that is ~5 meg large than /dev/shm
• Adjust the /dev/shm to something like 64M (this is the default for a Docker container)
• Run fread on "cat ~/myfile.csv" <-- cat creates the pipe
• Docker V1.12+
• Centos latest image from docker hub
• R-open v3.4.0 (microsoft)

In the code: https://github.com/Rdatatable/data.table/blob/master/src/fread.c
Around line 788 perhaps it should check the size of /dev/shm to see if it matches the file it just read into memory. In my case in docker here is the verbose output of the failed test condition:

 > dat.df3<-fread("/opt/cloudera/parcels/CDH/bin/hadoop fs -cat /user/tcederquist/tim_pop_comm_14_5 | head -3668850" ,sep=",", header=TRUE, verbose=TRUE)
 Input contains no \n. Taking this to be a filename to open
 File opened, filesize is 0.062500 GB.
 Memory mapping ... ok
 ....basic output
 Type codes (point  8): 1111
 Type codes (point  9): 1111
 (ERROR):Expected sep (',') but ' ' ends field 0 when detecting types from point 10: 14183667

Expected results:

 File opened, filesize is 0.373524 GB.

In addition, when it fails it knows internally the line # and other useful information when it failed. I had to zero in on the value by hand before I discovered the verbose flag. Would be nice if normal error messages indicated the row #. vebose=T shows the location when it calculates the # of delimiters such as for this test case would have be useful output on error (since I knew the file had 20M records):

 Count of eol: 3668839 (including 0 at the end)
 nrow = MIN( nsep [11006514] / (ncol [4] -1), neol [3668839] - endblanks [0] ) = 3668838