zos: fix readlink for mounts with system variables

On z/OS, fs mounts can have variables starting with '$'. The problem is
that following symlinks that contain these variables can lead to a loop
because readlink($SYSVAR) will return $SYSVAR.  To work around this, we
must resolve the $SYSVAR to an actual path using realpath.

More information about z/OS symlink resolution is available here:

https://www.ibm.com/support/knowledgecenter/SSLTBW_2.1.0/
com.ibm.zos.v2r1.bpxb100/sym.htm

PR-URL: https://github.com/libuv/libuv/pull/1472
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/unix/fs.c

@@ -438,7 +438,12 @@ static ssize_t uv__fs_readlink(uv_fs_t* req) {
     return -1;
   }
 
+#if defined(__MVS__)
+  len = os390_readlink(req->path, buf, len);
+#else
   len = readlink(req->path, buf, len);
+#endif
+
 
   if (len == -1) {
     uv__free(buf);

src/unix/os390-syscalls.c

@@ -327,3 +327,72 @@ char* mkdtemp(char* path) {
 
   return path;
 }
+
+
+ssize_t os390_readlink(const char* path, char* buf, size_t len) {
+  ssize_t rlen;
+  ssize_t vlen;
+  ssize_t plen;
+  char* delimiter;
+  char old_delim;
+  char* tmpbuf;
+  char realpathstr[PATH_MAX + 1];
+
+  tmpbuf = uv__malloc(len + 1);
+  if (tmpbuf == NULL) {
+    errno = ENOMEM;
+    return -1;
+  }
+
+  rlen = readlink(path, tmpbuf, len);
+  if (rlen < 0) {
+    uv__free(tmpbuf);
+    return rlen;
+  }
+
+  if (rlen < 3 || strncmp("/$", tmpbuf, 2) != 0) {
+    /* Straightforward readlink. */
+    memcpy(buf, tmpbuf, rlen);
+    uv__free(tmpbuf);
+    return rlen;
+  }
+
+  /*
+   * There is a parmlib variable at the beginning
+   * which needs interpretation.
+   */
+  tmpbuf[rlen] = '\0';
+  delimiter = strchr(tmpbuf + 2, '/');
+  if (delimiter == NULL)
+    /* No slash at the end */
+    delimiter = strchr(tmpbuf + 2, '\0');
+
+  /* Read real path of the variable. */
+  old_delim = *delimiter;
+  *delimiter = '\0';
+  if (realpath(tmpbuf, realpathstr) == NULL) {
+    uv__free(tmpbuf);
+    return -1;
+  }
+
+  /* realpathstr is not guaranteed to end with null byte.*/
+  realpathstr[PATH_MAX] = '\0';
+
+  /* Reset the delimiter and fill up the buffer. */
+  *delimiter = old_delim;
+  plen = strlen(delimiter);
+  vlen = strlen(realpathstr);
+  rlen = plen + vlen;
+  if (rlen > len) {
+    uv__free(tmpbuf);
+    errno = ENAMETOOLONG;
+    return -1;
+  }
+  memcpy(buf, realpathstr, vlen);
+  memcpy(buf + vlen, delimiter, plen);
+
+  /* Done using temporary buffer. */
+  uv__free(tmpbuf);
+
+  return rlen;
+}

src/unix/os390-syscalls.h

@@ -65,5 +65,6 @@ int scandir(const char* maindir, struct dirent*** namelist,
             int (*compar)(const struct dirent **,
             const struct dirent **));
 char *mkdtemp(char* path);
+ssize_t os390_readlink(const char* path, char* buf, size_t len);
 
 #endif /* UV_OS390_SYSCALL_H_ */