stdio-common: Introduce buffers for implementing printf

These buffers will eventually be used instead of FILE * objects
to implement printf functions.  The multibyte buffer is struct
__printf_buffer, the wide buffer is struct __wprintf_buffer.

To enable writing type-generic code, the header files
printf_buffer-char.h and printf_buffer-wchar_t.h define the
Xprintf macro differently, enabling Xprintf (buffer) to stand
for __printf_buffer and __wprintf_buffer as appropriate.  For
common cases, macros like Xprintf_buffer are provided as a more
syntactically convenient shortcut.

Buffer-specific flush callbacks are implemented with a switch
statement instead of a function pointer, to avoid hardening issues
similar to those of libio vtables.  struct __printf_buffer_as_file
is needed to support custom printf specifiers because the public
interface for that requires passing a FILE *, which is why there
is a trapdoor back from these buffers to FILE * streams.

Since the immediate user of these interfaces knows when processing
has finished, there is no flush callback for the end of processing,
only a flush callback for the intermediate buffer flush.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

stdio-common/printf_buffer_to_file.c

@@ -0,0 +1,122 @@
+/* Multibyte printf buffers writing data to a FILE * stream.
+   Copyright (C) 2022 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 <printf_buffer_to_file.h>
+
+#include <assert.h>
+#include <array_length.h>
+#include <libio/libioP.h>
+
+/* Switch to the file buffer if possible.  If the file has write_ptr
+   == write_end, use the stage buffer instead.  */
+void
+__printf_buffer_to_file_switch (struct __printf_buffer_to_file *buf)
+{
+  if (buf->fp->_IO_write_ptr < buf->fp->_IO_write_end)
+    {
+      /* buf->fp has a buffer associated with it, so write directly to
+         it from now on.  */
+      buf->base.write_ptr = buf->fp->_IO_write_ptr;
+      buf->base.write_end = buf->fp->_IO_write_end;
+    }
+  else
+    {
+      /* Use the staging area if no buffer is available in buf->fp.  */
+      buf->base.write_ptr = buf->stage;
+      buf->base.write_end = array_end (buf->stage);
+    }
+
+  buf->base.write_base = buf->base.write_ptr;
+}
+
+void
+__printf_buffer_flush_to_file (struct __printf_buffer_to_file *buf)
+{
+  /* The bytes in the buffer are always consumed.  */
+  buf->base.written += buf->base.write_ptr - buf->base.write_base;
+
+  if (buf->base.write_end == array_end (buf->stage))
+    {
+      /* If the stage buffer is used, make a copy into the file.  The
+         stage buffer is always consumed fully, even if just partially
+         written, to ensure that the file stream has all the data.  */
+      size_t count = buf->base.write_ptr - buf->stage;
+      if ((size_t) _IO_sputn (buf->fp, buf->stage, count) != count)
+        {
+          __printf_buffer_mark_failed (&buf->base);
+          return;
+        }
+      /* buf->fp may have a buffer now.  */
+      __printf_buffer_to_file_switch (buf);
+      return;
+    }
+  else if (buf->base.write_end == buf->stage + 1)
+    {
+      /* Special one-character buffer case.  This is used to avoid
+         flush-only overflow below.  */
+      if (buf->base.write_ptr == buf->base.write_end)
+        {
+          if (__overflow (buf->fp, (unsigned char) *buf->stage) == EOF)
+            {
+              __printf_buffer_mark_failed (&buf->base);
+              return;
+            }
+          __printf_buffer_to_file_switch (buf);
+        }
+      /* Else there is nothing to write.  */
+      return;
+    }
+
+  /* We have written directly into the buf->fp buffer.  */
+  assert (buf->base.write_end == buf->fp->_IO_write_end);
+
+  /* Mark the bytes as written.  */
+  buf->fp->_IO_write_ptr = buf->base.write_ptr;
+
+  if (buf->base.write_ptr == buf->base.write_end)
+    {
+      /* The buffer in buf->fp has been filled.  This should just call
+         __overflow (buf->fp, EOF), but flush-only overflow is obscure
+         and not always correctly implemented.  See bug 28949.  Be
+         conservative and switch to a one-character buffer instead, to
+         obtain one more character for a regular __overflow call.  */
+      buf->base.write_ptr = buf->stage;
+      buf->base.write_end = buf->stage + 1;
+    }
+  /* The bytes in the file stream were already marked as written above.  */
+
+  buf->base.write_base = buf->base.write_ptr;
+}
+
+void
+__printf_buffer_to_file_init (struct __printf_buffer_to_file *buf, FILE *fp)
+{
+  __printf_buffer_init (&buf->base, buf->stage, array_length (buf->stage),
+                        __printf_buffer_mode_to_file);
+  buf->fp = fp;
+  __printf_buffer_to_file_switch (buf);
+}
+
+int
+__printf_buffer_to_file_done (struct __printf_buffer_to_file *buf)
+{
+  if (__printf_buffer_has_failed (&buf->base))
+    return -1;
+  __printf_buffer_flush_to_file (buf);
+  return __printf_buffer_done (&buf->base);
+}