Add updateFilePos() primitive and tests.

stew/io2.nim

@@ -1356,6 +1356,49 @@ proc setFilePos*(handle: IoHandle, offset: int64,
     else:
       ok()
 
+proc updateFilePos*(handle: IoHandle, offset: int64,
+                    whence: SeekPosition): IoResult[int64] =
+  ## Procedure shall set the file offset for the open file associated with the
+  ## file descriptor ``handle``, as follows:
+  ##   * If whence is ``SeekPosition.SeekBegin``, the file offset shall be set
+  ##     to ``offset`` bytes.
+  ##   * If whence is ``SeekPosition.SeekCur``, the file offset shall be set to
+  ##     its current location plus ``offset``.
+  ##   * If whence is ``SeekPosition.SeekEnd``, the file offset shall be set to
+  ##     the size of the file plus ``offset``.
+  ##
+  ## Returns the resulting offset location as measured in bytes from the
+  ## beginning of the file.
+  when defined(windows):
+    var noffset = 0'i64
+    let pos =
+      case whence
+      of SeekBegin:
+        FILE_BEGIN
+      of SeekCurrent:
+        FILE_CURRENT
+      of SeekEnd:
+        FILE_END
+    let res = setFilePointerEx(uint(handle), offset, addr noffset, pos)
+    if res == 0:
+      err(ioLastError())
+    else:
+      ok(noffset)
+  else:
+    let pos =
+      case whence
+      of SeekBegin:
+        posix.SEEK_SET
+      of SeekCurrent:
+        posix.SEEK_CUR
+      of SeekEnd:
+        posix.SEEK_END
+    let res = int64(posix.lseek(cint(handle), Off(offset), pos))
+    if res == -1'i64:
+      err(ioLastError())
+    else:
+      ok(res)
+
 proc truncate*(handle: IoHandle, length: int64): IoResult[void] =
   ## Procedure cause the regular file referenced by handle ``handle`` to be
   ## truncated to a size of precisely ``length`` bytes.

tests/test_io2.nim

@@ -629,6 +629,31 @@ suite "OS Input/Output procedures test suite":
       positions[3] == 10'i64
       positions[4] == 20'i64
 
+  test "updateFilePos(handle) test":
+    proc performTest(path: string): IoResult[tuple[s0, s1, s2, s3, s4: int64]] =
+      let flags = {OpenFlags.Write, OpenFlags.Truncate, OpenFlags.Create}
+      let handle = ? openFile(path, flags)
+      let msg = "AAAAABBBBBCCCCCDDDDD"
+      discard ? io2.writeFile(handle, msg)
+      let
+        pos0 = ? updateFilePos(handle, 0'i64, SeekBegin)
+        pos1 = ? updateFilePos(handle, 5'i64, SeekCurrent)
+        pos2 = ? updateFilePos(handle, 5'i64, SeekCurrent)
+        pos3 = ? updateFilePos(handle, 0'i64, SeekEnd)
+        pos4 = ? updateFilePos(handle, -5'i64, SeekEnd)
+      ? closeFile(handle)
+      ? removeFile(path)
+      ok((pos0, pos1, pos2, pos3, pos4))
+    let res = performTest("testblob4")
+    check res.isOk()
+    let positions = res.get()
+    check:
+      positions[0] == 0'i64
+      positions[1] == 5'i64
+      positions[2] == 10'i64
+      positions[3] == 20'i64
+      positions[4] == 15'i64
+
   test "lockFile(handle)/unlockFile(handle) test":
     type
       TestResult = object