seek(con, 0, "end", rw="r") does not always work correctly (PR#7899)

I've noticed that seek(con, 0, "end", rw="r") on a file connection does 
not always work correctly after a write (R 2.1.0 on Windows).

[Is a call to fflush() needed inside file_seek() in main/connections.c?]
If you have an idea where to fflush() precisely and your patch works, 
please tell it! I'll happily run some test cases where seeking matters.

Note that ?seek currently tells us "The value returned by seek(where=NA) 
appears to be unreliable on Windows systems, at least for text files."
It would be nice if this comment could be removed, of course ....

Uwe Ligges
Example (see the lines with the "***WRONG***" comment)

 > # seek(, rw="r") on a file does not always work correctly after a write
 > f <- file("tmp3.txt", "w+b")
 > # Write something earlier in the file
 > seek(f, 10, rw="w")
[1] 0
 > writeLines(c("ghi", "jkl"), f)
 > seek(f, 20, rw="w")
[1] 18
 > writeLines(c("abc"), f)
 > seek(f, 0, "end", rw="w")
[1] 24
 > # Try to read at the end of the file
 > seek(f, 0, "end", rw="r")
[1] 0
 > readLines(f, -1)
character(0)
 > seek(f, 0, "end", rw="w")
[1] 18
 > # write something at the end of the file
 > writeLines(c("def"), f)
 > # Try to read at the end of the file
 > # flush(f) # flushing here makes the seek work correctly
 > seek(f, 0, "end", rw="r")
[1] 24
 > seek(f, NA, rw="r") # ***WRONG*** (should return 28)
[1] 24
 > readLines(f, -1) # ***WRONG*** (should return character(0))
[1] "def"
 > seek(f, 20, rw="r")
[1] 28
 > readLines(f, -1)
[1] "abc" "def"
 > seek(f, 0, "end", rw="r") # now it works correctly
[1] 28
 > seek(f, NA, rw="r")
[1] 28
 > readLines(f, -1)
character(0)
 > close(f)
 >
 > version
          _
platform i386-pc-mingw32
arch     i386
os       mingw32
system   i386, mingw32
status
major    2
minor    1.0
year     2005
month    04
day      18
language R
 >
-- Tony Plate

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