getFileSize and setFileSize procedures for File and AsyncFile

Platform independent procedure to set a file length. Useful when
replacing file content.

lib/system/sysio.nim

@@ -52,6 +52,8 @@ proc c_ferror(f: File): cint {.
   importc: "ferror", header: "<stdio.h>", tags: [].}
 proc c_setvbuf(f: File, buf: pointer, mode: cint, size: csize): cint {.
   importc: "setvbuf", header: "<stdio.h>", tags: [].}
+proc c_ftruncate(f: FileHandle, len: clong): cint {.
+  importc: "ftruncate", header: "<unistd.h>".}
 
 proc raiseEIO(msg: string) {.noinline, noreturn.} =
   sysFatal(IOError, msg)
@@ -373,6 +375,11 @@ proc getFileSize(f: File): int64 =
   result = getFilePos(f)
   setFilePos(f, oldPos)
 
+proc setFileSize(f: File, size: int64) =
+  ## Set a file length.
+  if c_ftruncate(getFileHandle f, size.clong) == -1:
+    raiseEIO("cannot truncate file")
+
 proc readFile(filename: string): TaintedString =
   var f: File
   if open(f, filename):