Added std/oserrors for OS error reporting (nim-lang#19390)

* Added 'std/oserrors' for OS error reporting

* add a simple test

* New code should not support -d:useWinAnsi anymore

thanks to @Araq

changelog.md

@@ -74,6 +74,8 @@ becomes an alias for `addr`.
   for the right-hand side of type definitions in type sections. Previously
   they would error with "invalid indentation".
 
+- Added `std/oserrors` for OS error reporting.
+
 ## Compiler changes
 
 - `nim` can now compile version 1.4.0 as follows: `nim c --lib:lib --stylecheck:off compiler/nim`,

lib/std/oserrors.nim

@@ -0,0 +1,118 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2022 Nim contributors
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+
+## The `std/oserrors` module implements OS error reporting.
+
+type
+  OSErrorCode* = distinct int32 ## Specifies an OS Error Code.
+
+when not defined(nimscript):
+  when defined(windows):
+    import winlean
+  else:
+    var errno {.importc, header: "<errno.h>".}: cint
+
+    proc c_strerror(errnum: cint): cstring {.
+      importc: "strerror", header: "<string.h>".}
+
+proc `==`*(err1, err2: OSErrorCode): bool {.borrow.}
+proc `$`*(err: OSErrorCode): string {.borrow.}
+
+proc osErrorMsg*(errorCode: OSErrorCode): string =
+  ## Converts an OS error code into a human readable string.
+  ##
+  ## The error code can be retrieved using the `osLastError proc`_.
+  ##
+  ## If conversion fails, or `errorCode` is `0` then `""` will be
+  ## returned.
+  ##
+  ## See also:
+  ## * `raiseOSError proc`_
+  ## * `osLastError proc`_
+  runnableExamples:
+    when defined(linux):
+      assert osErrorMsg(OSErrorCode(0)) == ""
+      assert osErrorMsg(OSErrorCode(1)) == "Operation not permitted"
+      assert osErrorMsg(OSErrorCode(2)) == "No such file or directory"
+
+  result = ""
+  when defined(nimscript):
+    discard
+  elif defined(windows):
+    if errorCode != OSErrorCode(0'i32):
+      var msgbuf: WideCString
+      if formatMessageW(0x00000100 or 0x00001000 or 0x00000200,
+                      nil, errorCode.int32, 0, addr(msgbuf), 0, nil) != 0'i32:
+        result = $msgbuf
+        if msgbuf != nil: localFree(cast[pointer](msgbuf))
+  else:
+    if errorCode != OSErrorCode(0'i32):
+      result = $c_strerror(errorCode.int32)
+
+proc newOSError*(
+  errorCode: OSErrorCode, additionalInfo = ""
+): owned(ref OSError) {.noinline.} =
+  ## Creates a new `OSError exception <system.html#OSError>`_.
+  ##
+  ## The `errorCode` will determine the
+  ## message, `osErrorMsg proc`_ will be used
+  ## to get this message.
+  ##
+  ## The error code can be retrieved using the `osLastError proc`_.
+  ##
+  ## If the error code is `0` or an error message could not be retrieved,
+  ## the message `unknown OS error` will be used.
+  ##
+  ## See also:
+  ## * `osErrorMsg proc`_
+  ## * `osLastError proc`_
+  var e: owned(ref OSError); new(e)
+  e.errorCode = errorCode.int32
+  e.msg = osErrorMsg(errorCode)
+  if additionalInfo.len > 0:
+    if e.msg.len > 0 and e.msg[^1] != '\n': e.msg.add '\n'
+    e.msg.add "Additional info: "
+    e.msg.add additionalInfo
+      # don't add trailing `.` etc, which negatively impacts "jump to file" in IDEs.
+  if e.msg == "":
+    e.msg = "unknown OS error"
+  return e
+
+proc raiseOSError*(errorCode: OSErrorCode, additionalInfo = "") {.noinline.} =
+  ## Raises an `OSError exception <system.html#OSError>`_.
+  ##
+  ## Read the description of the `newOSError proc`_ to learn
+  ## how the exception object is created.
+  raise newOSError(errorCode, additionalInfo)
+
+{.push stackTrace:off.}
+proc osLastError*(): OSErrorCode {.sideEffect.} =
+  ## Retrieves the last operating system error code.
+  ##
+  ## This procedure is useful in the event when an OS call fails. In that case
+  ## this procedure will return the error code describing the reason why the
+  ## OS call failed. The `OSErrorMsg` procedure can then be used to convert
+  ## this code into a string.
+  ##
+  ## .. warning:: The behaviour of this procedure varies between Windows and POSIX systems.
+  ##   On Windows some OS calls can reset the error code to `0` causing this
+  ##   procedure to return `0`. It is therefore advised to call this procedure
+  ##   immediately after an OS call fails. On POSIX systems this is not a problem.
+  ##
+  ## See also:
+  ## * `osErrorMsg proc`_
+  ## * `raiseOSError proc`_
+  when defined(nimscript):
+    discard
+  elif defined(windows):
+    result = cast[OSErrorCode](getLastError())
+  else:
+    result = OSErrorCode(errno)
+{.pop.}

tests/stdlib/toserrors.nim

@@ -0,0 +1,9 @@
+discard """
+  action: compile
+"""
+
+import std/oserrors
+
+let x1 = osLastError()
+raiseOSError(x1)
+echo osErrorMsg(x1)