// env.c -- environment variable access

#include <assert.h>

#include <uv.h>

// vim.h must be included before charset.h (and possibly others) or things
// blow up
#include "nvim/vim.h"
#include "nvim/ascii.h"
#include "nvim/charset.h"
#include "nvim/os/os.h"
#include "nvim/memory.h"
#include "nvim/message.h"
#include "nvim/path.h"
#include "nvim/strings.h"
#include "nvim/eval.h"
#include "nvim/ex_getln.h"
#include "nvim/version.h"

#ifdef HAVE__NSGETENVIRON
#include <crt_externs.h>
#endif

#ifdef HAVE_SYS_UTSNAME_H
#include <sys/utsname.h>
#endif

/// Like getenv(), but returns NULL if the variable is empty.
const char *os_getenv(const char *name)
  FUNC_ATTR_NONNULL_ALL
{
  const char *e = getenv(name);
  return e == NULL || *e == NUL ? NULL : e;
}

/// Returns `true` if the environment variable, `name`, has been defined
/// (even if empty).
bool os_env_exists(const char *name)
  FUNC_ATTR_NONNULL_ALL
{
  return getenv(name) != NULL;
}

int os_setenv(const char *name, const char *value, int overwrite)
  FUNC_ATTR_NONNULL_ALL
{
#ifdef HAVE_SETENV
  return setenv(name, value, overwrite);
#elif defined(HAVE_PUTENV_S)
  if (!overwrite && os_getenv(name) != NULL) {
    return 0;
  }
  if (_putenv_s(name, value) == 0) {
    return 0;
  }
  return -1;
#else
# error "This system has no implementation available for os_setenv()"
#endif
}

/// Unset environment variable
///
/// For systems where unsetenv() is not available the value will be set as an
/// empty string
int os_unsetenv(const char *name)
{
#ifdef HAVE_UNSETENV
  return unsetenv(name);
#else
  return os_setenv(name, "", 1);
#endif
}

char *os_getenvname_at_index(size_t index)
{
# if defined(HAVE__NSGETENVIRON)
  char **environ = *_NSGetEnviron();
# elif !defined(__WIN32__)
  // Borland C++ 5.2 has this in a header file.
  extern char         **environ;
# endif
  // check if index is inside the environ array
  for (size_t i = 0; i < index; i++) {
    if (environ[i] == NULL) {
      return NULL;
    }
  }
  char *str = environ[index];
  if (str == NULL) {
    return NULL;
  }
  size_t namesize = 0;
  while (str[namesize] != '=' && str[namesize] != NUL) {
    namesize++;
  }
  char *name = (char *)vim_strnsave((char_u *)str, namesize);
  return name;
}


/// Get the process ID of the Neovim process.
///
/// @return the process ID.
int64_t os_get_pid(void)
{
#ifdef _WIN32
  return (int64_t)GetCurrentProcessId();
#else
  return (int64_t)getpid();
#endif
}

/// Get the hostname of the machine running Neovim.
///
/// @param hostname Buffer to store the hostname.
/// @param len Length of `hostname`.
void os_get_hostname(char *hostname, size_t len)
{
#ifdef HAVE_SYS_UTSNAME_H
  struct utsname vutsname;

  if (uname(&vutsname) < 0) {
    *hostname = '\0';
  } else {
    strncpy(hostname, vutsname.nodename, len - 1);
    hostname[len - 1] = '\0';
  }
#else
  // TODO(unknown): Implement this for windows.
  // See the implementation used in vim:
  // https://code.google.com/p/vim/source/browse/src/os_win32.c?r=6b69d8dde19e32909f4ee3a6337e6a2ecfbb6f72#2899
  *hostname = '\0';
#endif
}

/// To get the "real" home directory:
///   - get value of $HOME
/// For Unix:
///   - go to that directory
///   - do os_dirname() to get the real name of that directory.
/// This also works with mounts and links.
/// Don't do this for Windows, it will change the "current dir" for a drive.
static char_u   *homedir = NULL;

void init_homedir(void)
{
  // In case we are called a second time (when 'encoding' changes).
  xfree(homedir);
  homedir = NULL;

  char_u *var = (char_u *)os_getenv("HOME");

#ifdef WIN32
  // Typically, $HOME is not defined on Windows, unless the user has
  // specifically defined it for Vim's sake. However, on Windows NT
  // platforms, $HOMEDRIVE and $HOMEPATH are automatically defined for
  // each user. Try constructing $HOME from these.
  if (var == NULL) {
    const char *homedrive = os_getenv("HOMEDRIVE");
    const char *homepath = os_getenv("HOMEPATH");
    if (homepath == NULL) {
        homepath = "\\";
    }
    if (homedrive != NULL && strlen(homedrive) + strlen(homepath) < MAXPATHL) {
      snprintf((char *)NameBuff, MAXPATHL, "%s%s", homedrive, homepath);
      if (NameBuff[0] != NUL) {
        var = NameBuff;
        vim_setenv("HOME", (char *)NameBuff);
      }
    }
  }
#endif

  if (var != NULL) {
#ifdef UNIX
    // Change to the directory and get the actual path.  This resolves
    // links.  Don't do it when we can't return.
    if (os_dirname(NameBuff, MAXPATHL) == OK
        && os_chdir((char *)NameBuff) == 0) {
      if (!os_chdir((char *)var) && os_dirname(IObuff, IOSIZE) == OK) {
        var = IObuff;
      }
      if (os_chdir((char *)NameBuff) != 0) {
        EMSG(_(e_prev_dir));
      }
    }
#endif
    homedir = vim_strsave(var);
  }
}

#if defined(EXITFREE)

void free_homedir(void)
{
  xfree(homedir);
}

#endif

/// Call expand_env() and store the result in an allocated string.
/// This is not very memory efficient, this expects the result to be freed
/// again soon.
/// @param src String containing environment variables to expand
/// @see {expand_env}
char_u *expand_env_save(char_u *src)
{
  return expand_env_save_opt(src, false);
}

/// Similar to expand_env_save() but when "one" is `true` handle the string as
/// one file name, i.e. only expand "~" at the start.
/// @param src String containing environment variables to expand
/// @param one Should treat as only one file name
/// @see {expand_env}
char_u *expand_env_save_opt(char_u *src, bool one)
{
  char_u *p = xmalloc(MAXPATHL);
  expand_env_esc(src, p, MAXPATHL, false, one, NULL);
  return p;
}

/// Expand environment variable with path name.
/// "~/" is also expanded, using $HOME. For Unix "~user/" is expanded.
/// Skips over "\ ", "\~" and "\$" (not for Win32 though).
/// If anything fails no expansion is done and dst equals src.
/// @param src Input string e.g. "$HOME/vim.hlp"
/// @param dst Where to put the result
/// @param dstlen Maximum length of the result
void expand_env(char_u *src, char_u *dst, int dstlen)
{
  expand_env_esc(src, dst, dstlen, false, false, NULL);
}

/// Expand environment variable with path name and escaping.
/// "~/" is also expanded, using $HOME. For Unix "~user/" is expanded.
/// Skips over "\ ", "\~" and "\$" (not for Win32 though).
/// If anything fails no expansion is done and dst equals src.
/// prefix recognize the start of a new name, for '~' expansion.
/// @param srcp Input string e.g. "$HOME/vim.hlp"
/// @param dst Where to put the result
/// @param dstlen Maximum length of the result
/// @param esc Should we escape spaces in expanded variables?
/// @param one Should we expand more than one '~'?
/// @param prefix Common prefix for paths, can be NULL
void expand_env_esc(char_u *restrict srcp,
                    char_u *restrict dst,
                    int dstlen,
                    bool esc,
                    bool one,
                    char_u *prefix)
{
  char_u      *tail;
  char_u      *var;
  bool copy_char;
  bool mustfree;  // var was allocated, need to free it later
  bool at_start = true;  // at start of a name

  int prefix_len = (prefix == NULL) ? 0 : (int)STRLEN(prefix);

  char_u *src = skipwhite(srcp);
  dstlen--;  // leave one char space for "\,"
  while (*src && dstlen > 0) {
    // Skip over `=expr`.
    if (src[0] == '`' && src[1] == '=') {
      var = src;
      src += 2;
      (void)skip_expr(&src);
      if (*src == '`') {
        src++;
      }
      size_t len = (size_t)(src - var);
      if (len > (size_t)dstlen) {
        len = (size_t)dstlen;
      }
      memcpy((char *)dst, (char *)var, len);
      dst += len;
      dstlen -= (int)len;
      continue;
    }

    copy_char = true;
    if ((*src == '$') || (*src == '~' && at_start)) {
      mustfree = false;

      // The variable name is copied into dst temporarily, because it may
      // be a string in read-only memory and a NUL needs to be appended.
      if (*src != '~') {  // environment var
        tail = src + 1;
        var = dst;
        int c = dstlen - 1;

#ifdef UNIX
        // Unix has ${var-name} type environment vars
        if (*tail == '{' && !vim_isIDc('{')) {
          tail++;               // ignore '{'
          while (c-- > 0 && *tail != NUL && *tail != '}') {
            *var++ = *tail++;
          }
        } else // NOLINT
#endif
        {
          while (c-- > 0 && *tail != NUL && vim_isIDc(*tail)) {
            *var++ = *tail++;
          }
        }

#if defined(UNIX)
        // Verify that we have found the end of a Unix ${VAR} style variable
        if (src[1] == '{' && *tail != '}') {
          var = NULL;
        } else {
          if (src[1] == '{') {
            ++tail;
          }
#endif
        *var = NUL;
        var = (char_u *)vim_getenv((char *)dst);
        mustfree = true;
#if defined(UNIX)
        }
#endif
      } else if (src[1] == NUL  // home directory
                 || vim_ispathsep(src[1])
                 || vim_strchr((char_u *)" ,\t\n", src[1]) != NULL) {
        var = homedir;
        tail = src + 1;
      } else {  // user directory
#if defined(UNIX)
        // Copy ~user to dst[], so we can put a NUL after it.
        tail = src;
        var = dst;
        int c = dstlen - 1;
        while (c-- > 0
               && *tail
               && vim_isfilec(*tail)
               && !vim_ispathsep(*tail)) {
          *var++ = *tail++;
        }
        *var = NUL;
        // Use os_get_user_directory() to get the user directory.
        // If this function fails, the shell is used to
        // expand ~user. This is slower and may fail if the shell
        // does not support ~user (old versions of /bin/sh).
        var = (char_u *)os_get_user_directory((char *)dst + 1);
        mustfree = true;
        if (var == NULL) {
          expand_T xpc;

          ExpandInit(&xpc);
          xpc.xp_context = EXPAND_FILES;
          var = ExpandOne(&xpc, dst, NULL,
              WILD_ADD_SLASH|WILD_SILENT, WILD_EXPAND_FREE);
          mustfree = true;
        }
#else
        // cannot expand user's home directory, so don't try
        var = NULL;
        tail = (char_u *)"";  // for gcc
#endif  // UNIX
      }

#ifdef BACKSLASH_IN_FILENAME
      // If 'shellslash' is set change backslashes to forward slashes.
      // Can't use slash_adjust(), p_ssl may be set temporarily.
      if (p_ssl && var != NULL && vim_strchr(var, '\\') != NULL) {
        char_u  *p = vim_strsave(var);

        if (mustfree) {
          xfree(var);
        }
        var = p;
        mustfree = true;
        forward_slash(var);
      }
#endif

      // If "var" contains white space, escape it with a backslash.
      // Required for ":e ~/tt" when $HOME includes a space.
      if (esc && var != NULL && vim_strpbrk(var, (char_u *)" \t") != NULL) {
        char_u  *p = vim_strsave_escaped(var, (char_u *)" \t");

        if (mustfree) {
          xfree(var);
        }
        var = p;
        mustfree = true;
      }

      if (var != NULL && *var != NUL
          && (STRLEN(var) + STRLEN(tail) + 1 < (unsigned)dstlen)) {
        STRCPY(dst, var);
        dstlen -= (int)STRLEN(var);
        int c = (int)STRLEN(var);
        // if var[] ends in a path separator and tail[] starts
        // with it, skip a character
        if (*var != NUL && after_pathsep((char *)dst, (char *)dst + c)
#if defined(BACKSLASH_IN_FILENAME)
            && dst[-1] != ':'
#endif
            && vim_ispathsep(*tail))
          ++tail;
        dst += c;
        src = tail;
        copy_char = false;
      }
      if (mustfree) {
        xfree(var);
      }
    }

    if (copy_char) {  // copy at least one char
      // Recognize the start of a new name, for '~'.
      // Don't do this when "one" is true, to avoid expanding "~" in
      // ":edit foo ~ foo".
      at_start = false;
      if (src[0] == '\\' && src[1] != NUL) {
        *dst++ = *src++;
        --dstlen;
      } else if ((src[0] == ' ' || src[0] == ',') && !one) {
        at_start = true;
      }
      *dst++ = *src++;
      --dstlen;

      if (prefix != NULL && src - prefix_len >= srcp
          && STRNCMP(src - prefix_len, prefix, prefix_len) == 0) {
        at_start = true;
      }
    }
  }
  *dst = NUL;
}

/// Check if the directory "vimdir/<version>" or "vimdir/runtime" exists.
/// Return NULL if not, return its name in allocated memory otherwise.
/// @param vimdir directory to test
static char *vim_version_dir(const char *vimdir)
{
  if (vimdir == NULL || *vimdir == NUL) {
    return NULL;
  }
  char *p = concat_fnames(vimdir, VIM_VERSION_NODOT, true);
  if (os_isdir((char_u *)p)) {
    return p;
  }
  xfree(p);
  p = concat_fnames(vimdir, RUNTIME_DIRNAME, true);
  if (os_isdir((char_u *)p)) {
    return p;
  }
  xfree(p);
  return NULL;
}

/// If `dirname + "/"` precedes `pend` in the path, return the pointer to
/// `dirname + "/" + pend`.  Otherwise return `pend`.
///
/// Examples (path = /usr/local/share/nvim/runtime/doc/help.txt):
///
///   pend    = help.txt
///   dirname = doc
///   -> doc/help.txt
///
///   pend    = doc/help.txt
///   dirname = runtime
///   -> runtime/doc/help.txt
///
///   pend    = runtime/doc/help.txt
///   dirname = vim74
///   -> runtime/doc/help.txt
///
/// @param path    Path to a file
/// @param pend    A suffix of the path
/// @param dirname The immediate path fragment before the pend
/// @return The new pend including dirname or just pend
static char *remove_tail(char *path, char *pend, char *dirname)
{
  size_t len = STRLEN(dirname);
  char *new_tail = pend - len - 1;

  if (new_tail >= path
      && fnamencmp((char_u *)new_tail, (char_u *)dirname, len) == 0
      && (new_tail == path || after_pathsep(path, new_tail))) {
    return new_tail;
  }
  return pend;
}

/// Iterate over colon-separated list
///
/// @note Environment variables must not be modified during iteration.
///
/// @param[in]   val   Value of the environment variable to iterate over.
/// @param[in]   iter  Pointer used for iteration. Must be NULL on first
///                    iteration.
/// @param[out]  dir   Location where pointer to the start of the current
///                    directory name should be saved. May be set to NULL.
/// @param[out]  len   Location where current directory length should be saved.
///
/// @return Next iter argument value or NULL when iteration should stop.
const void *vim_colon_env_iter(const char *const val,
                               const void *const iter,
                               const char **const dir,
                               size_t *const len)
  FUNC_ATTR_NONNULL_ARG(1, 3, 4) FUNC_ATTR_WARN_UNUSED_RESULT
{
  const char *varval = (const char *) iter;
  if (varval == NULL) {
    varval = val;
  }
  *dir = varval;
  const char *const dirend = strchr(varval, ':');
  if (dirend == NULL) {
    *len = strlen(varval);
    return NULL;
  } else {
    *len = (size_t) (dirend - varval);
    return dirend + 1;
  }
}

/// Iterate over colon-separated list in reverse order
///
/// @note Environment variables must not be modified during iteration.
///
/// @param[in]   val   Value of the environment variable to iterate over.
/// @param[in]   iter  Pointer used for iteration. Must be NULL on first
///                    iteration.
/// @param[out]  dir   Location where pointer to the start of the current
///                    directory name should be saved. May be set to NULL.
/// @param[out]  len   Location where current directory length should be saved.
///
/// @return Next iter argument value or NULL when iteration should stop.
const void *vim_colon_env_iter_rev(const char *const val,
                                   const void *const iter,
                                   const char **const dir,
                                   size_t *const len)
  FUNC_ATTR_NONNULL_ARG(1, 3, 4) FUNC_ATTR_WARN_UNUSED_RESULT
{
  const char *varend = (const char *) iter;
  if (varend == NULL) {
    varend = val + strlen(val) - 1;
  }
  const size_t varlen = (size_t) (varend - val) + 1;
  const char *const colon = xmemrchr(val, ':', varlen);
  if (colon == NULL) {
    *len = varlen;
    *dir = val;
    return NULL;
  } else {
    *dir = colon + 1;
    *len = (size_t) (varend - colon);
    return colon - 1;
  }
}

/// Vim's version of getenv().
/// Special handling of $HOME, $VIM and $VIMRUNTIME, allowing the user to
/// override the vim runtime directory at runtime.  Also does ACP to 'enc'
/// conversion for Win32.  Results must be freed by the calling function.
/// @param name Name of environment variable to expand
char *vim_getenv(const char *name)
{
  const char *kos_env_path = os_getenv(name);
  if (kos_env_path != NULL) {
    return xstrdup(kos_env_path);
  }

  bool vimruntime = (strcmp(name, "VIMRUNTIME") == 0);
  if (!vimruntime && strcmp(name, "VIM") != 0) {
    return NULL;
  }

  // When expanding $VIMRUNTIME fails, try using $VIM/vim<version> or $VIM.
  // Don't do this when default_vimruntime_dir is non-empty.
  char *vim_path = NULL;
  if (vimruntime
#ifdef HAVE_PATHDEF
      && *default_vimruntime_dir == NUL
#endif
      ) {
    kos_env_path = os_getenv("VIM");
    if (kos_env_path != NULL) {
      vim_path = vim_version_dir(kos_env_path);
      if (vim_path == NULL) {
        vim_path = xstrdup(kos_env_path);
      }
    }
  }

  // When expanding $VIM or $VIMRUNTIME fails, try using:
  // - the directory name from 'helpfile' (unless it contains '$')
  // - the executable name from argv[0]
  if (vim_path == NULL) {
    if (p_hf != NULL && vim_strchr(p_hf, '$') == NULL) {
      vim_path = (char *)p_hf;
    }
    if (vim_path != NULL) {
      // remove the file name
      char *vim_path_end = (char *)path_tail((char_u *)vim_path);

      // remove "doc/" from 'helpfile', if present
      if (vim_path == (char *)p_hf) {
        vim_path_end = remove_tail(vim_path, vim_path_end, "doc");
      }

      // for $VIM, remove "runtime/" or "vim54/", if present
      if (!vimruntime) {
        vim_path_end = remove_tail(vim_path, vim_path_end, RUNTIME_DIRNAME);
        vim_path_end = remove_tail(vim_path, vim_path_end, VIM_VERSION_NODOT);
      }

      // remove trailing path separator
      if (vim_path_end > vim_path && after_pathsep(vim_path, vim_path_end)) {
        vim_path_end--;
      }

      // check that the result is a directory name
      assert(vim_path_end >= vim_path);
      vim_path = xstrndup(vim_path, (size_t)(vim_path_end - vim_path));

      if (!os_isdir((char_u *)vim_path)) {
        xfree(vim_path);
        vim_path = NULL;
      }
    }
  }

#ifdef HAVE_PATHDEF
  // When there is a pathdef.c file we can use default_vim_dir and
  // default_vimruntime_dir
  if (vim_path == NULL) {
    // Only use default_vimruntime_dir when it is not empty
    if (vimruntime && *default_vimruntime_dir != NUL) {
      vim_path = xstrdup(default_vimruntime_dir);
    } else if (*default_vim_dir != NUL) {
      if (vimruntime
          && (vim_path = vim_version_dir(default_vim_dir)) == NULL) {
        vim_path = xstrdup(default_vim_dir);
      }
    }
  }
#endif

  // Set the environment variable, so that the new value can be found fast
  // next time, and others can also use it (e.g. Perl).
  if (vim_path != NULL) {
    if (vimruntime) {
      vim_setenv("VIMRUNTIME", vim_path);
      didset_vimruntime = true;
    } else {
      vim_setenv("VIM", vim_path);
      didset_vim = true;
    }
  }
  return vim_path;
}

/// Replace home directory by "~" in each space or comma separated file name in
/// 'src'.
/// If anything fails (except when out of space) dst equals src.
/// @param buf When not NULL, check for help files
/// @param src Input file name
/// @param dst Where to put the result
/// @param dstlen Maximum length of the result
/// @param one If true, only replace one file name, including spaces and commas
///            in the file name
void home_replace(buf_T *buf, char_u *src, char_u *dst, int dstlen, bool one)
{
  size_t dirlen = 0, envlen = 0;
  size_t len;

  if (src == NULL) {
    *dst = NUL;
    return;
  }

  /*
   * If the file is a help file, remove the path completely.
   */
  if (buf != NULL && buf->b_help) {
    STRCPY(dst, path_tail(src));
    return;
  }

  /*
   * We check both the value of the $HOME environment variable and the
   * "real" home directory.
   */
  if (homedir != NULL)
    dirlen = STRLEN(homedir);

  char_u *homedir_env = (char_u *)os_getenv("HOME");
  bool must_free = false;

  if (homedir_env != NULL && vim_strchr(homedir_env, '~') != NULL) {
    must_free = true;
    size_t usedlen = 0;
    size_t flen = STRLEN(homedir_env);
    char_u *fbuf = NULL;
    (void)modify_fname((char_u *)":p", &usedlen, &homedir_env, &fbuf, &flen);
    flen = STRLEN(homedir_env);
    if (flen > 0 && vim_ispathsep(homedir_env[flen - 1]))
      /* Remove the trailing / that is added to a directory. */
      homedir_env[flen - 1] = NUL;
  }

  if (homedir_env != NULL)
    envlen = STRLEN(homedir_env);

  if (!one)
    src = skipwhite(src);
  while (*src && dstlen > 0) {
    /*
     * Here we are at the beginning of a file name.
     * First, check to see if the beginning of the file name matches
     * $HOME or the "real" home directory. Check that there is a '/'
     * after the match (so that if e.g. the file is "/home/pieter/bla",
     * and the home directory is "/home/piet", the file does not end up
     * as "~er/bla" (which would seem to indicate the file "bla" in user
     * er's home directory)).
     */
    char_u *p = homedir;
    len = dirlen;
    for (;; ) {
      if (   len
             && fnamencmp(src, p, len) == 0
             && (vim_ispathsep(src[len])
                 || (!one && (src[len] == ',' || src[len] == ' '))
                 || src[len] == NUL)) {
        src += len;
        if (--dstlen > 0)
          *dst++ = '~';

        /*
         * If it's just the home directory, add  "/".
         */
        if (!vim_ispathsep(src[0]) && --dstlen > 0)
          *dst++ = '/';
        break;
      }
      if (p == homedir_env)
        break;
      p = homedir_env;
      len = envlen;
    }

    /* if (!one) skip to separator: space or comma */
    while (*src && (one || (*src != ',' && *src != ' ')) && --dstlen > 0)
      *dst++ = *src++;
    /* skip separator */
    while ((*src == ' ' || *src == ',') && --dstlen > 0)
      *dst++ = *src++;
  }
  /* if (dstlen == 0) out of space, what to do??? */

  *dst = NUL;

  if (must_free) {
    xfree(homedir_env);
  }
}

/// Like home_replace, store the replaced string in allocated memory.
/// @param buf When not NULL, check for help files
/// @param src Input file name
char_u * home_replace_save(buf_T *buf, char_u *src) FUNC_ATTR_NONNULL_RET
{
  size_t len = 3;             // space for "~/" and trailing NUL
  if (src != NULL) {          // just in case
    len += STRLEN(src);
  }
  char_u *dst = xmalloc(len);
  home_replace(buf, src, dst, (int)len, true);
  return dst;
}

/// Our portable version of setenv.
/// Has special handling for $VIMRUNTIME to keep the localization machinery
/// sane.
void vim_setenv(const char *name, const char *val)
{
  os_setenv(name, val, 1);
#ifndef LOCALE_INSTALL_DIR
  // When setting $VIMRUNTIME adjust the directory to find message
  // translations to $VIMRUNTIME/lang.
  if (*val != NUL && STRICMP(name, "VIMRUNTIME") == 0) {
    char *buf = (char *)concat_str((char_u *)val, (char_u *)"/lang");
    bindtextdomain(PROJECT_NAME, buf);
    xfree(buf);
  }
#endif
}


/// Function given to ExpandGeneric() to obtain an environment variable name.
char_u *get_env_name(expand_T *xp, int idx)
{
# define ENVNAMELEN 100
  // this static buffer is needed to avoid a memory leak in ExpandGeneric
  static char_u name[ENVNAMELEN];
  assert(idx >= 0);
  char *envname = os_getenvname_at_index((size_t)idx);
  if (envname) {
    STRLCPY(name, envname, ENVNAMELEN);
    xfree(envname);
    return name;
  }
  return NULL;
}