libio: Implement internal function __libc_readline_unlocked

This is a variant of fgets which fails with ERANGE if the
buffer is too small, and the buffer length is given as an
argument of type size_t.

This function will be useful for implementing NSS file reading
operations.  Compared to a direct implementation using the public API,
it avoids an lseek system call in case the line terminator can be
found in the internal read buffer.

libio/readline.c

@@ -0,0 +1,170 @@
+/* fgets with ERANGE error reporting and size_t buffer length.
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "libioP.h"
+
+/* Return -1 and set errno to EINVAL if it is ERANGE.  */
+static ssize_t
+fail_no_erange (void)
+{
+  if (errno == ERANGE)
+    __set_errno (EINVAL);
+  return -1;
+}
+
+/* Slow path for reading the line.  Called with no data in the stream
+   read buffer.  Write data to [BUFFER, BUFFER_END).  */
+static ssize_t
+readline_slow (FILE *fp, char *buffer, char *buffer_end)
+{
+  char *start = buffer;
+
+  while (buffer < buffer_end)
+    {
+      if (__underflow (fp) == EOF)
+        {
+          if (_IO_ferror_unlocked (fp))
+            /* If the EOF was caused by a read error, report it.  */
+            return fail_no_erange ();
+          *buffer = '\0';
+          /* Do not include the null terminator.  */
+          return buffer - start;
+        }
+
+      /* __underflow has filled the buffer.  */
+      char *readptr = fp->_IO_read_ptr;
+      ssize_t readlen = fp->_IO_read_end - readptr;
+      /* Make sure that __underflow really has acquired some data.  */
+      assert (readlen > 0);
+      char *pnl = memchr (readptr, '\n', readlen);
+      if (pnl != NULL)
+        {
+          /* We found the terminator.  */
+          size_t line_length = pnl - readptr;
+          if (line_length + 2 > buffer_end - buffer)
+            /* Not enough room in the caller-supplied buffer.  */
+            break;
+          memcpy (buffer, readptr, line_length + 1);
+          buffer[line_length + 1] = '\0';
+          fp->_IO_read_ptr = pnl + 1;
+          /* Do not include the null terminator.  */
+          return buffer - start + line_length + 1;
+        }
+
+      if (readlen >= buffer_end - buffer)
+        /* Not enough room in the caller-supplied buffer.  */
+        break;
+
+      /* Save and consume the stream buffer.  */
+      memcpy (buffer, readptr, readlen);
+      fp->_IO_read_ptr = fp->_IO_read_end;
+      buffer += readlen;
+    }
+
+  /* The line does not fit into the buffer.  */
+  __set_errno (ERANGE);
+  return -1;
+}
+
+ssize_t
+__libc_readline_unlocked (FILE *fp, char *buffer, size_t buffer_length)
+{
+  char *buffer_end = buffer + buffer_length;
+
+  /* Orient the stream.  */
+  if (__builtin_expect (fp->_mode, -1) == 0)
+    _IO_fwide (fp, -1);
+
+  /* Fast path: The line terminator is found in the buffer.  */
+  char *readptr = fp->_IO_read_ptr;
+  ssize_t readlen = fp->_IO_read_end - readptr;
+  off64_t start_offset;         /* File offset before reading anything.  */
+  if (readlen > 0)
+    {
+      char *pnl = memchr (readptr, '\n', readlen);
+      if (pnl != NULL)
+        {
+          size_t line_length = pnl - readptr;
+          /* Account for line and null terminators.  */
+          if (line_length + 2 > buffer_length)
+            {
+              __set_errno (ERANGE);
+              return -1;
+            }
+          memcpy (buffer, readptr, line_length + 1);
+          buffer[line_length + 1] = '\0';
+          /* Consume the entire line.  */
+          fp->_IO_read_ptr = pnl + 1;
+          return line_length + 1;
+        }
+
+      /* If the buffer does not have enough space for what is pending
+         in the stream (plus a NUL terminator), the buffer is too
+         small.  */
+      if (readlen + 1 > buffer_length)
+        {
+          __set_errno (ERANGE);
+          return -1;
+        }
+
+      /* End of line not found.  We need all the buffered data.  Fall
+         through to the slow path.  */
+      memcpy (buffer, readptr, readlen);
+      buffer += readlen;
+      /* The original length is invalid after this point.  Use
+         buffer_end instead.  */
+#pragma GCC poison buffer_length
+      /* Read the old offset before updating the read pointer.  */
+      start_offset = __ftello64 (fp);
+      fp->_IO_read_ptr = fp->_IO_read_end;
+    }
+  else
+    {
+      readlen = 0;
+      start_offset = __ftello64 (fp);
+    }
+
+  /* Slow path: Read more data from the underlying file.  We need to
+     restore the file pointer if the buffer is too small.  First,
+     check if the __ftello64 call above failed.  */
+  if (start_offset < 0)
+    return fail_no_erange ();
+
+  ssize_t result = readline_slow (fp, buffer, buffer_end);
+  if (result < 0)
+    {
+      if (errno == ERANGE)
+        {
+          /* Restore the file pointer so that the caller may read the
+             same line again.  */
+          if (__fseeko64 (fp, start_offset, SEEK_SET) < 0)
+            return fail_no_erange ();
+          __set_errno (ERANGE);
+        }
+      /* Do not restore the file position on other errors; it is
+         likely that the __fseeko64 call would fail, too.  */
+      return -1;
+    }
+  return readlen + result;
+}
+libc_hidden_def (__libc_readline_unlocked)