vcs-svn: teach line_buffer about temporary files

It can sometimes be useful to write information temporarily to file,
to read back later.  These functions allow a program to use the
line_buffer facilities when doing so.

It works like this:

 1. find a unique filename with buffer_tmpfile_init.
 2. rewind with buffer_tmpfile_rewind.  This returns a stdio
    handle for writing.
 3. when finished writing, declare so with
    buffer_tmpfile_prepare_to_read.  The return value indicates
    how many bytes were written.
 4. read whatever portion of the file is needed.
 5. if finished, remove the temporary file with buffer_deinit.
    otherwise, go back to step 2,

The svn support would use this to buffer the postimage from delta
application until the length is known and fast-import can receive
the resulting blob.

Based-on-patch-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

vcs-svn/line_buffer.c

@@ -25,6 +25,14 @@ int buffer_fdinit(struct line_buffer *buf, int fd)
 	return 0;
 }
 
+int buffer_tmpfile_init(struct line_buffer *buf)
+{
+	buf->infile = tmpfile();
+	if (!buf->infile)
+		return -1;
+	return 0;
+}
+
 int buffer_deinit(struct line_buffer *buf)
 {
 	int err;
@@ -35,6 +43,22 @@ int buffer_deinit(struct line_buffer *buf)
 	return err;
 }
 
+FILE *buffer_tmpfile_rewind(struct line_buffer *buf)
+{
+	rewind(buf->infile);
+	return buf->infile;
+}
+
+long buffer_tmpfile_prepare_to_read(struct line_buffer *buf)
+{
+	long pos = ftell(buf->infile);
+	if (pos < 0)
+		return error("ftell error: %s", strerror(errno));
+	if (fseek(buf->infile, 0, SEEK_SET))
+		return error("seek error: %s", strerror(errno));
+	return pos;
+}
+
 int buffer_read_char(struct line_buffer *buf)
 {
 	return fgetc(buf->infile);