fixes #12315 [backport]; refs #12314 (#12385)

(cherry picked from commit f30da2f266)

lib/system/io.nim

@@ -112,6 +112,8 @@ proc c_setvbuf(f: File, buf: pointer, mode: cint, size: csize): cint {.
 
 proc c_fprintf(f: File, frmt: cstring): cint {.
   importc: "fprintf", header: "<stdio.h>", varargs, discardable.}
+proc c_fputc(c: char, f: File): cint {.
+  importc: "fputc", header: "<stdio.h>".}
 
 ## When running nim in android app stdout goes no where, so echo gets ignored
 ## To redreict echo to the android logcat use -d:androidNDK
@@ -212,6 +214,10 @@ when defined(windows):
     var i = c_fprintf(f, "%s", s)
     while i < s.len:
       if s[i] == '\0':
+        let w = c_fputc('\0', f)
+        if w != 0:
+          if doRaise: raiseEIO("cannot write string to file")
+          break
         inc i
       else:
         let w = c_fprintf(f, "%s", unsafeAddr s[i])

tests/system/techo_unicode.nim

@@ -4,7 +4,7 @@ abasdfdsmÄhmaИ
 Иnastystring
 A你好
 ИnastystringA你好
-ÖÜhmabasdfdsmÄhmaИ'''
+ÖÜhmabasdfdsmÄhmaИOK'''
   disabled: "posix"
   joinable: "false"
 """
@@ -13,10 +13,10 @@ import winlean
 
 echo "ÄhmÖÜ"
 echo "abasdfdsmÄhmaИ"
-echo "И\0nasty\0\0\0\0string\0"
+echo "Иnastystring"
 echo "A你好"
 
-write stdout, "И\0nasty\0\0\0\0string\0"
+write stdout, "Иnastystring"
 writeLine stdout, "A你好"
 stdout.flushFile()
 
@@ -25,3 +25,17 @@ var a = "ÖÜhmabasdfdsmÄhmaИ"
 var ac = 0'i32
 discard writeFile(handle, addr a[0], int32(len(a)), addr ac, nil)
 stdout.flushFile()
+
+import os
+
+let str = "some nulls: \0\0\0 (three of them)"
+
+let fpath = getTempDir() / "file_with_nulls.bin"
+
+writeFile(fpath, str)
+
+doAssert(getFileSize(fpath) == 31)
+doAssert(readFile(fpath) == str)
+removeFile(fpath)
+
+echo "OK"