MDEV-10907 MTR and server writes can interleave in the error log

Ensure atomic appends to the error log by using CreateFile with
FILE_APPEND_DATA flag to open error log file (both MTR and server)

mysql-test/lib/My/Platform.pm

@@ -24,7 +24,7 @@ use File::Path;
 use base qw(Exporter);
 our @EXPORT= qw(IS_CYGWIN IS_WINDOWS IS_WIN32PERL
 		native_path posix_path mixed_path
-                check_socket_path_length process_alive);
+                check_socket_path_length process_alive open_for_append);
 
 BEGIN {
   if ($^O eq "cygwin") {
@@ -161,4 +161,51 @@ sub process_alive {
 }
 
 
+
+use Symbol qw( gensym );
+
+use if $^O eq 'MSWin32', 'Win32API::File', qw( CloseHandle CreateFile GetOsFHandle OsFHandleOpen  OPEN_ALWAYS FILE_APPEND_DATA 
+  FILE_SHARE_READ FILE_SHARE_WRITE FILE_SHARE_DELETE );
+use if $^O eq 'MSWin32', 'Win32::API';
+
+use constant WIN32API_FILE_NULL => [];
+
+# Open a file for append
+# On Windows we use CreateFile with FILE_APPEND_DATA
+# to insure that writes are atomic, not interleaved
+# with writes by another processes. 
+sub open_for_append
+{
+  my ($file) = @_;
+  my $fh = gensym();
+
+  if (IS_WIN32PERL)
+  {
+    my $handle;
+    if (!($handle = CreateFile(
+        $file,
+        FILE_APPEND_DATA(),
+        FILE_SHARE_READ()|FILE_SHARE_WRITE()|FILE_SHARE_DELETE(),
+        WIN32API_FILE_NULL,
+        OPEN_ALWAYS(),# Create if doesn't exist.
+        0,
+        WIN32API_FILE_NULL,
+      )))
+    {
+      return undef;
+    }
+
+    if (!OsFHandleOpen($fh, $handle, 'wat'))
+    {
+      CloseHandle($handle);
+      return undef;
+    }
+    return $fh;
+  }
+  
+  open($fh,">>",$file) or return undef;
+  return $fh;
+}
+
+
 1;

mysql-test/lib/mtr_io.pl

@@ -21,6 +21,7 @@
 
 use strict;
 use Carp;
+use My::Platform;
 
 sub mtr_fromfile ($);
 sub mtr_tofile ($@);
@@ -45,10 +46,10 @@ sub mtr_fromfile ($) {
 
 sub mtr_tofile ($@) {
   my $file=  shift;
-
-  open(FILE,">>",$file) or mtr_error("can't open file \"$file\": $!");
-  print FILE join("", @_);
-  close FILE;
+  my $fh= open_for_append $file;
+  mtr_error("can't open file \"$file\": $!") unless defined($fh);
+  print $fh join("", @_);
+  close $fh;
 }
 
 

mysys/my_fopen.c

@@ -102,6 +102,7 @@ static FILE *my_win_freopen(const char *path, const char *mode, FILE *stream)
   HANDLE osfh;
 
   DBUG_ASSERT(path && stream);
+  DBUG_ASSERT(strchr(mode, 'a')); /* We use FILE_APPEND_DATA below */
 
   /* Services don't have stdout/stderr on Windows, so _fileno returns -1. */
   if (fd < 0)
@@ -112,15 +113,14 @@ static FILE *my_win_freopen(const char *path, const char *mode, FILE *stream)
     fd= _fileno(stream);
   }
 
-  if ((osfh= CreateFile(path, GENERIC_READ | GENERIC_WRITE,
+  if ((osfh= CreateFile(path, GENERIC_READ | FILE_APPEND_DATA,
                         FILE_SHARE_READ | FILE_SHARE_WRITE |
                         FILE_SHARE_DELETE, NULL,
                         OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL,
                         NULL)) == INVALID_HANDLE_VALUE)
     return NULL;
 
-  if ((handle_fd= _open_osfhandle((intptr_t)osfh,
-                                  _O_APPEND | _O_TEXT)) == -1)
+  if ((handle_fd= _open_osfhandle((intptr_t)osfh, _O_TEXT)) == -1)
   {
     CloseHandle(osfh);
     return NULL;