[PATCH 2/3] scratch_buffer: new module

[Paul Eggert]

* lib/scratch_buffer.h, lib/scratch_buffer_grow.c:
* lib/scratch_buffer_grow_preserve.c:
* lib/scratch_buffer_set_array_size.c:
New files, copied from glibc with very minor changes that can be
copied back.
* modules/scratch_buffer: New file.
---
 ChangeLog                           |   8 +++
 lib/scratch_buffer.h                | 135 ++++++++++++++++++++++++++++++++++++
 lib/scratch_buffer_grow.c           |  56 +++++++++++++++
 lib/scratch_buffer_grow_preserve.c  |  67 ++++++++++++++++++
 lib/scratch_buffer_set_array_size.c |  64 +++++++++++++++++
 modules/scratch_buffer              |  32 +++++++++
 6 files changed, 362 insertions(+)
 create mode 100644 lib/scratch_buffer.h
 create mode 100644 lib/scratch_buffer_grow.c
 create mode 100644 lib/scratch_buffer_grow_preserve.c
 create mode 100644 lib/scratch_buffer_set_array_size.c
 create mode 100644 modules/scratch_buffer

diff --git a/ChangeLog b/ChangeLog
index af34bedbb..078d5243c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,13 @@
 2017-09-01  Paul Eggert  <address@hidden>
 
+       scratch_buffer: new module
+       * lib/scratch_buffer.h, lib/scratch_buffer_grow.c:
+       * lib/scratch_buffer_grow_preserve.c:
+       * lib/scratch_buffer_set_array_size.c:
+       New files, copied from glibc with very minor changes that can be
+       copied back.
+       * modules/scratch_buffer: New file.
+
        libc-config: new module
        * MODULES.html.sh: Add libc-config.
        * lib/cdefs.h: New file, copied from the GNU C Library with very
diff --git a/lib/scratch_buffer.h b/lib/scratch_buffer.h
new file mode 100644
index 000000000..bb04662eb
--- /dev/null
+++ b/lib/scratch_buffer.h
@@ -0,0 +1,135 @@
+/* Variable-sized buffer with on-stack default allocation.
+   Copyright (C) 2015-2017 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/>.  */
+
+#ifndef _SCRATCH_BUFFER_H
+#define _SCRATCH_BUFFER_H
+
+/* Scratch buffers with a default stack allocation and fallback to
+   heap allocation.  It is expected that this function is used in this
+   way:
+
+     struct scratch_buffer tmpbuf;
+     scratch_buffer_init (&tmpbuf);
+
+     while (!function_that_uses_buffer (tmpbuf.data, tmpbuf.length))
+       if (!scratch_buffer_grow (&tmpbuf))
+        return -1;
+
+     scratch_buffer_free (&tmpbuf);
+     return 0;
+
+   The allocation functions (scratch_buffer_grow,
+   scratch_buffer_grow_preserve, scratch_buffer_set_array_size) make
+   sure that the heap allocation, if any, is freed, so that the code
+   above does not have a memory leak.  The buffer still remains in a
+   state that can be deallocated using scratch_buffer_free, so a loop
+   like this is valid as well:
+
+     struct scratch_buffer tmpbuf;
+     scratch_buffer_init (&tmpbuf);
+
+     while (!function_that_uses_buffer (tmpbuf.data, tmpbuf.length))
+       if (!scratch_buffer_grow (&tmpbuf))
+        break;
+
+     scratch_buffer_free (&tmpbuf);
+
+   scratch_buffer_grow and scratch_buffer_grow_preserve are guaranteed
+   to grow the buffer by at least 512 bytes.  This means that when
+   using the scratch buffer as a backing store for a non-character
+   array whose element size, in bytes, is 512 or smaller, the scratch
+   buffer only has to grow once to make room for at least one more
+   element.
+*/
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdlib.h>
+
+/* Scratch buffer.  Must be initialized with scratch_buffer_init
+   before its use.  */
+struct scratch_buffer {
+  void *data;    /* Pointer to the beginning of the scratch area.  */
+  size_t length; /* Allocated space at the data pointer, in bytes.  */
+  max_align_t __space[(1023 + sizeof (max_align_t)) / sizeof (max_align_t)];
+};
+
+/* Initializes *BUFFER so that BUFFER->data points to BUFFER->__space
+   and BUFFER->length reflects the available space.  */
+static inline void
+scratch_buffer_init (struct scratch_buffer *buffer)
+{
+  buffer->data = buffer->__space;
+  buffer->length = sizeof (buffer->__space);
+}
+
+/* Deallocates *BUFFER (if it was heap-allocated).  */
+static inline void
+scratch_buffer_free (struct scratch_buffer *buffer)
+{
+  if (buffer->data != buffer->__space)
+    free (buffer->data);
+}
+
+/* Grow *BUFFER by some arbitrary amount.  The buffer contents is NOT
+   preserved.  Return true on success, false on allocation failure (in
+   which case the old buffer is freed).  On success, the new buffer is
+   larger than the previous size.  On failure, *BUFFER is deallocated,
+   but remains in a free-able state, and errno is set.  */
+bool __libc_scratch_buffer_grow (struct scratch_buffer *buffer);
+libc_hidden_proto (__libc_scratch_buffer_grow)
+
+/* Alias for __libc_scratch_buffer_grow.  */
+static __always_inline bool
+scratch_buffer_grow (struct scratch_buffer *buffer)
+{
+  return __glibc_likely (__libc_scratch_buffer_grow (buffer));
+}
+
+/* Like __libc_scratch_buffer_grow, but preserve the old buffer
+   contents on success, as a prefix of the new buffer.  */
+bool __libc_scratch_buffer_grow_preserve (struct scratch_buffer *buffer);
+libc_hidden_proto (__libc_scratch_buffer_grow_preserve)
+
+/* Alias for __libc_scratch_buffer_grow_preserve.  */
+static __always_inline bool
+scratch_buffer_grow_preserve (struct scratch_buffer *buffer)
+{
+  return __glibc_likely (__libc_scratch_buffer_grow_preserve (buffer));
+}
+
+/* Grow *BUFFER so that it can store at least NELEM elements of SIZE
+   bytes.  The buffer contents are NOT preserved.  Both NELEM and SIZE
+   can be zero.  Return true on success, false on allocation failure
+   (in which case the old buffer is freed, but *BUFFER remains in a
+   free-able state, and errno is set).  It is unspecified whether this
+   function can reduce the array size.  */
+bool __libc_scratch_buffer_set_array_size (struct scratch_buffer *buffer,
+                                          size_t nelem, size_t size);
+libc_hidden_proto (__libc_scratch_buffer_set_array_size)
+
+/* Alias for __libc_scratch_set_array_size.  */
+static __always_inline bool
+scratch_buffer_set_array_size (struct scratch_buffer *buffer,
+                              size_t nelem, size_t size)
+{
+  return __glibc_likely (__libc_scratch_buffer_set_array_size
+                        (buffer, nelem, size));
+}
+
+#endif /* _SCRATCH_BUFFER_H */
diff --git a/lib/scratch_buffer_grow.c b/lib/scratch_buffer_grow.c
new file mode 100644
index 000000000..d2df02865
--- /dev/null
+++ b/lib/scratch_buffer_grow.c
@@ -0,0 +1,56 @@
+/* Variable-sized buffer with on-stack default allocation.
+   Copyright (C) 2015-2017 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/>.  */
+
+#ifndef _LIBC
+# include <libc-config.h>
+#endif
+
+#include <scratch_buffer.h>
+#include <errno.h>
+
+bool
+__libc_scratch_buffer_grow (struct scratch_buffer *buffer)
+{
+  void *new_ptr;
+  size_t new_length = buffer->length * 2;
+
+  /* Discard old buffer.  */
+  scratch_buffer_free (buffer);
+
+  /* Check for overflow.  */
+  if (__glibc_likely (new_length >= buffer->length))
+    new_ptr = malloc (new_length);
+  else
+    {
+      __set_errno (ENOMEM);
+      new_ptr = NULL;
+    }
+
+  if (__glibc_unlikely (new_ptr == NULL))
+    {
+      /* Buffer must remain valid to free.  */
+      scratch_buffer_init (buffer);
+      return false;
+    }
+
+  /* Install new heap-based buffer.  */
+  buffer->data = new_ptr;
+  buffer->length = new_length;
+  return true;
+}
+libc_hidden_def (__libc_scratch_buffer_grow)
diff --git a/lib/scratch_buffer_grow_preserve.c 
b/lib/scratch_buffer_grow_preserve.c
new file mode 100644
index 000000000..926861531
--- /dev/null
+++ b/lib/scratch_buffer_grow_preserve.c
@@ -0,0 +1,67 @@
+/* Variable-sized buffer with on-stack default allocation.
+   Copyright (C) 2015-2017 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/>.  */
+
+#ifndef _LIBC
+# include <libc-config.h>
+#endif
+
+#include <scratch_buffer.h>
+#include <errno.h>
+#include <string.h>
+
+bool
+__libc_scratch_buffer_grow_preserve (struct scratch_buffer *buffer)
+{
+  size_t new_length = 2 * buffer->length;
+  void *new_ptr;
+
+  if (buffer->data == buffer->__space)
+    {
+      /* Move buffer to the heap.  No overflow is possible because
+        buffer->length describes a small buffer on the stack.  */
+      new_ptr = malloc (new_length);
+      if (new_ptr == NULL)
+       return false;
+      memcpy (new_ptr, buffer->__space, buffer->length);
+    }
+  else
+    {
+      /* Buffer was already on the heap.  Check for overflow.  */
+      if (__glibc_likely (new_length >= buffer->length))
+       new_ptr = realloc (buffer->data, new_length);
+      else
+       {
+         __set_errno (ENOMEM);
+         new_ptr = NULL;
+       }
+
+      if (__glibc_unlikely (new_ptr == NULL))
+       {
+         /* Deallocate, but buffer must remain valid to free.  */
+         free (buffer->data);
+         scratch_buffer_init (buffer);
+         return false;
+       }
+    }
+
+  /* Install new heap-based buffer.  */
+  buffer->data = new_ptr;
+  buffer->length = new_length;
+  return true;
+}
+libc_hidden_def (__libc_scratch_buffer_grow_preserve)
diff --git a/lib/scratch_buffer_set_array_size.c 
b/lib/scratch_buffer_set_array_size.c
new file mode 100644
index 000000000..6fcc11534
--- /dev/null
+++ b/lib/scratch_buffer_set_array_size.c
@@ -0,0 +1,64 @@
+/* Variable-sized buffer with on-stack default allocation.
+   Copyright (C) 2015-2017 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/>.  */
+
+#ifndef _LIBC
+# include <libc-config.h>
+#endif
+
+#include <scratch_buffer.h>
+#include <errno.h>
+#include <limits.h>
+
+bool
+__libc_scratch_buffer_set_array_size (struct scratch_buffer *buffer,
+                                     size_t nelem, size_t size)
+{
+  size_t new_length = nelem * size;
+
+  /* Avoid overflow check if both values are small. */
+  if ((nelem | size) >> (sizeof (size_t) * CHAR_BIT / 2) != 0
+      && nelem != 0 && size != new_length / nelem)
+    {
+      /* Overflow.  Discard the old buffer, but it must remain valid
+        to free.  */
+      scratch_buffer_free (buffer);
+      scratch_buffer_init (buffer);
+      __set_errno (ENOMEM);
+      return false;
+    }
+
+  if (new_length <= buffer->length)
+    return true;
+
+  /* Discard old buffer.  */
+  scratch_buffer_free (buffer);
+
+  char *new_ptr = malloc (new_length);
+  if (new_ptr == NULL)
+    {
+      /* Buffer must remain valid to free.  */
+      scratch_buffer_init (buffer);
+      return false;
+    }
+
+  /* Install new heap-based buffer.  */
+  buffer->data = new_ptr;
+  buffer->length = new_length;
+  return true;
+}
+libc_hidden_def (__libc_scratch_buffer_set_array_size)
diff --git a/modules/scratch_buffer b/modules/scratch_buffer
new file mode 100644
index 000000000..7502d707d
--- /dev/null
+++ b/modules/scratch_buffer
@@ -0,0 +1,32 @@
+Description:
+Variable-sized buffer with on-stack default allocation.
+
+Files:
+lib/scratch_buffer.h
+lib/scratch_buffer_grow.c
+lib/scratch_buffer_grow_preserve.c
+lib/scratch_buffer_set_array_size.c
+
+Depends-on:
+libc-config
+stdbool
+stddef
+
+configure.ac:
+AC_CHECK_FUNCS_ONCE([__libc_scratch_buffer_grow])
+if test "$ac_cv_func___libc_scratch_buffer_grow" != yes; then
+  AC_LIBOBJ([scratch_buffer_grow])
+  AC_LIBOBJ([scratch_buffer_grow_preserve])
+  AC_LIBOBJ([scratch_buffer_set_array_size])
+fi
+
+Makefile.am:
+
+Include:
+<scratch_buffer.h>
+
+License:
+LGPLv2+
+
+Maintainer:
+all, glibc
-- 
2.13.5