MDEV-30836 MTR Cygwin subshell wrapper fix

See "Path-style conflict" in "MDEV-30836 MTR Cygwin fix" for explanation.

To install subshell fix use --cygwin-subshell-fix=do
To uninstall use --cygwin-subshell-fix=remove

This works only from Cygwin environment. As long as perl on PATH is
from Cygwin you are on Cygwin environment. Check it with

     perl --version

     This is perl 5, version 36, subversion 1 (v5.36.1) built for
     x86_64-cygwin-threads-multi

mysql-test/lib/My/Platform.pm

@@ -20,6 +20,7 @@ package My::Platform;
 use strict;
 use File::Basename;
 use File::Path;
+use Carp;
 
 use base qw(Exporter);
 our @EXPORT= qw(IS_CYGWIN IS_WINDOWS IS_WIN32PERL IS_AIX
@@ -219,4 +220,69 @@ sub open_for_append
 }
 
 
+sub check_cygwin_subshell
+{
+  # Only pipe (or sh-expansion) is fed to /bin/sh
+  my $out= `echo %comspec%|cat`;
+  return ($out =~ /\bcmd.exe\b/) ? 0 : 1;
+}
+
+sub install_shell_wrapper()
+{
+  system("rm -f /bin/sh.exe") and die $!;
+  my $wrapper= <<'EOF';
+#!/bin/bash
+if [[ -n "$MTR_PERL" && "$1" = "-c" ]]; then
+ shift
+ exec $(cygpath -m "$COMSPEC") /C "$@"
+fi
+exec /bin/bash "$@"
+EOF
+  open(OUT, '>', "/bin/sh") or die "/bin/sh: $!\n";
+  print OUT $wrapper;
+  close(OUT);
+  system("chmod +x /bin/sh") and die $!;
+  print "Cygwin subshell wrapper /bin/sh was installed, please restart MTR!\n";
+  exit(0);
+}
+
+sub uninstall_shell_wrapper()
+{
+  system("rm -f /bin/sh") and die $!;
+  system("cp /bin/bash.exe /bin/sh.exe") and die $!;
+}
+
+sub cygwin_subshell_fix
+{
+  my ($opt_name, $opt_value)= @_;
+  if ($opt_name ne "cygwin-subshell-fix") {
+    confess "Wrong option name: ${opt_name}";
+  }
+  if ($opt_value eq "do") {
+    if (check_cygwin_subshell()) {
+      install_shell_wrapper();
+    } else {
+      print "Cygwin subshell fix was already installed, skipping...\n";
+    }
+  } elsif ($opt_value eq "remove") {
+    if (check_cygwin_subshell()) {
+      print "Cygwin subshell fix was already uninstalled, skipping...\n";
+    } else {
+      uninstall_shell_wrapper();
+    }
+  } else {
+    die "Wrong --cygwin-subshell-fix value: ${opt_value} (expected do/remove)";
+  }
+}
+
+sub options
+{
+  if (IS_CYGWIN) {
+    return ('cygwin-subshell-fix=s'         => \&cygwin_subshell_fix);
+  } else {
+    return ();
+  }
+}
+
+
 1;

mysql-test/mysql-test-run.pl

@@ -1237,7 +1237,8 @@ sub command_line_setup {
              'xml-report=s'             => \$opt_xml_report,
 
              My::Debugger::options(),
-             My::CoreDump::options()
+             My::CoreDump::options(),
+             My::Platform::options()
            );
 
   # fix options (that take an optional argument and *only* after = sign
@@ -1273,6 +1274,9 @@ sub command_line_setup {
   }
   if (IS_CYGWIN)
   {
+    if (My::Platform::check_cygwin_subshell()) {
+      die("Cygwin /bin/sh subshell requires fix with --cygwin-subshell-fix=do\n");
+    }
     # Use mixed path format i.e c:/path/to/
     $glob_mysql_test_dir= mixed_path($glob_mysql_test_dir);
   }