Cygwin: add very simple newgrp(1) tool

This tool allows to change the primary group for a child process.
The new primary group MUST be part of the supplementary group list
of newgrp's user token.

The command started as child process is specified on the command line.
If it's missing, start the user's default shell with the new primary
group.

TODO: Implement '-' option.
      Add command description to documentation.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/utils/Makefile.am

@@ -31,6 +31,7 @@ bin_PROGRAMS = \
 	mkgroup \
 	mkpasswd \
 	mount \
+	newgrp \
 	passwd \
 	pldd \
 	profiler \

winsup/utils/newgrp.c

@@ -0,0 +1,101 @@
+/* newgrp.c
+
+This file is part of Cygwin.
+
+This software is a copyrighted work licensed under the terms of the
+Cygwin license.  Please consult the file "CYGWIN_LICENSE" for
+details. */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <ctype.h>
+#include <string.h>
+#include <grp.h>
+#include <pwd.h>
+
+int
+main (int argc, const char **argv)
+{
+  struct group *gr;
+  gid_t gid;
+  const char *cmd;
+  const char **cmd_av;
+  const char *fake_av[2];
+
+  /* TODO: Implement '-' option */
+  /* TODO: Add command description to documentation */
+
+  if (argc < 2 || argv[1][0] == '-')
+    {
+      fprintf (stderr,
+	       "Usage: %1$s group [command [args...]]\n"
+	       "\n"
+	       "%1$s changes the current primary group for a command.\n"
+	       "The primary group must be member of the supplementary group\n"
+	       "list of the user.\n"
+	       "The command and its arguments are specified on the command\n"
+	       "line.  Default is the user's standard shell.\n",
+	       program_invocation_short_name);
+      return 1;
+    }
+  if (isdigit ((int) argv[1][0]))
+    {
+      char *e = NULL;
+
+      gid = strtol (argv[1], &e, 10);
+      if (e && *e != '\0')
+	{
+	  fprintf (stderr, "%s: invalid gid `%s'\n",
+		   program_invocation_short_name, argv[1]);
+	  return 2;
+	}
+      gr = getgrgid (gid);
+      if (!gr)
+	{
+	  fprintf (stderr, "%s: unknown group gid `%u'\n",
+		   program_invocation_short_name, gid);
+	  return 2;
+	}
+    }
+  else
+    {
+      gr = getgrnam (argv[1]);
+      if (!gr)
+	{
+	  fprintf (stderr, "%s: unknown group name `%s'\n",
+		   program_invocation_short_name, argv[1]);
+	  return 2;
+	}
+      gid = gr->gr_gid;
+    }
+  if (setgid (gid) != 0)
+    {
+      fprintf (stderr, "%s: can't switch primary group to `%s'\n",
+	       program_invocation_short_name, argv[1]);
+      return 2;
+    }
+  argc -= 2;
+  argv += 2;
+  if (argc < 1)
+    {
+      struct passwd *pw = getpwuid (getuid ());
+      if (!pw)
+	cmd = "/usr/bin/bash";
+      else
+	cmd = pw->pw_shell;
+      fake_av[0] = cmd;
+      fake_av[1] = NULL;
+      cmd_av = fake_av;
+    }
+  else
+    {
+      cmd = argv[0];
+      cmd_av = argv;
+    }
+  execvp (cmd, (char **) cmd_av);
+  fprintf (stderr, "%s: failed to start `%s': %s\n",
+	   program_invocation_short_name, cmd, strerror (errno));
+  return 3;
+}