Fix the TODO portion of recently added posix_fallocate on OS X. (#21387)

lib/posix/posix.nim

@@ -198,9 +198,24 @@ proc posix_fadvise*(a1: cint, a2, a3: Off, a4: cint): cint {.
 
 proc ftruncate*(a1: cint, a2: Off): cint {.importc, header: "<unistd.h>".}
 when defined(osx):              # 2001 POSIX evidently does not concern Apple
+  type FStore {.importc: "fstore_t", header: "<fcntl.h>", bycopy.} = object
+    fst_flags: uint32           ## IN: flags word
+    fst_posmode: cint           ## IN: indicates offset field
+    fst_offset,                 ## IN: start of the region
+      fst_length,               ## IN: size of the region
+      fst_bytesalloc: Off       ## OUT: number of bytes allocated
+  var F_PEOFPOSMODE {.importc, header: "<fcntl.h>".}: cint
+  var F_ALLOCATEALL {.importc, header: "<fcntl.h>".}: uint32
+  var F_PREALLOCATE {.importc, header: "<fcntl.h>".}: cint
   proc posix_fallocate*(a1: cint, a2, a3: Off): cint =
-    ftruncate(a1, a2 + a3)      # Set size to off + len, max offset
+    var fst = FStore(fst_flags: F_ALLOCATEALL, fst_posmode: F_PEOFPOSMODE,
-else:                           # TODO: Use fcntl(fd, F_PREALLOCATE, ..) above
+                     fst_offset: a2, fst_length: a3)
+    # Must also call ftruncate to match what POSIX does. Unlike posix_fallocate,
+    # this can shrink files.  Could guard w/getFileSize, but caller likely knows
+    # present size & has no good reason to call this unless it is growing.
+    if fcntl(a1, F_PREALLOCATE, fst.addr) != cint(-1): ftruncate(a1, a2 + a3)
+    else: cint(-1)
+else:
   proc posix_fallocate*(a1: cint, a2, a3: Off): cint {.
     importc, header: "<fcntl.h>".}