nss: Add __nss_fgetent_r

And helper functions __nss_readline, __nss_readline_seek,
 __nss_parse_line_result.

This consolidates common code for handling overlong lines and
parse files.  Use the new functionality in internal_getent
in nss/nss_files/files-XXX.c.

Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

nss/nss_readline.c

@@ -0,0 +1,99 @@
+/* Read a line from an nss_files database file.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <nss_files.h>
+
+#include <ctype.h>
+#include <errno.h>
+#include <string.h>
+
+int
+__nss_readline (FILE *fp, char *buf, size_t len, off64_t *poffset)
+{
+  /* We need space for at least one character, the line terminator,
+     and the NUL byte.  */
+  if (len < 3)
+    {
+      *poffset = -1;
+      __set_errno (ERANGE);
+      return ERANGE;
+    }
+
+  while (true)
+    {
+      /* Keep original offset for retries.  */
+      *poffset = __ftello64 (fp);
+
+      buf[len - 1] = '\xff';        /* Marker to recognize truncation.  */
+      if (fgets_unlocked (buf, len, fp) == NULL)
+        {
+          if (feof_unlocked (fp))
+            {
+              __set_errno (ENOENT);
+              return ENOENT;
+            }
+          else
+            {
+              /* Any other error.  Do not return ERANGE in this case
+                 because the caller would retry.  */
+              if (errno == ERANGE)
+                __set_errno (EINVAL);
+              return errno;
+            }
+        }
+      else if (buf[len - 1] != '\xff')
+        /* The buffer is too small.  Arrange for re-reading the same
+           line on the next call.  */
+        return __nss_readline_seek (fp, *poffset);
+
+      /* fgets_unlocked succeeded.  */
+
+      /* Remove leading whitespace.  */
+      char *p = buf;
+      while (isspace (*p))
+        ++p;
+      if (*p == '\0' || *p == '#')
+        /* Skip empty lines and comments.  */
+        continue;
+      if (p != buf)
+        memmove (buf, p, strlen (p));
+
+      /* Return line to the caller.  */
+      return 0;
+    }
+}
+libc_hidden_def (__nss_readline)
+
+int
+__nss_readline_seek (FILE *fp, off64_t offset)
+{
+  if (offset < 0 /* __ftello64 failed.  */
+      || __fseeko64 (fp, offset, SEEK_SET) < 0)
+    {
+      /* Without seeking support, it is not possible to
+         re-read the same line, so this is a hard failure.  */
+      fseterr_unlocked (fp);
+      __set_errno (ESPIPE);
+      return ESPIPE;
+    }
+  else
+    {
+      __set_errno (ERANGE);
+      return ERANGE;
+    }
+}