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: NDavid Barr <david.barr@cordelta.com>
Signed-off-by: NJonathan 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);

vcs-svn/line_buffer.h

@@ -15,12 +15,17 @@ struct line_buffer {
 int buffer_init(struct line_buffer *buf, const char *filename);
 int buffer_fdinit(struct line_buffer *buf, int fd);
 int buffer_deinit(struct line_buffer *buf);
+void buffer_reset(struct line_buffer *buf);
+
+int buffer_tmpfile_init(struct line_buffer *buf);
+FILE *buffer_tmpfile_rewind(struct line_buffer *buf);	/* prepare to write. */
+long buffer_tmpfile_prepare_to_read(struct line_buffer *buf);
+
 char *buffer_read_line(struct line_buffer *buf);
 char *buffer_read_string(struct line_buffer *buf, uint32_t len);
 int buffer_read_char(struct line_buffer *buf);
 void buffer_read_binary(struct line_buffer *buf, struct strbuf *sb, uint32_t len);
 void buffer_copy_bytes(struct line_buffer *buf, uint32_t len);
 void buffer_skip_bytes(struct line_buffer *buf, uint32_t len);
-void buffer_reset(struct line_buffer *buf);
 
 #endif

vcs-svn/line_buffer.txt

@@ -24,6 +24,28 @@ The calling program:
 When finished, the caller can use `buffer_reset` to deallocate
 resources.
 
+Using temporary files
+---------------------
+
+Temporary files provide a place to store data that should not outlive
+the calling program.  A program
+
+ - initializes a `struct line_buffer` to LINE_BUFFER_INIT
+ - requests a temporary file with `buffer_tmpfile_init`
+ - acquires an output handle by calling `buffer_tmpfile_rewind`
+ - uses standard I/O functions like `fprintf` and `fwrite` to fill
+   the temporary file
+ - declares writing is over with `buffer_tmpfile_prepare_to_read`
+ - can re-read what was written with `buffer_read_line`,
+   `buffer_read_string`, and so on
+ - can reuse the temporary file by calling `buffer_tmpfile_rewind`
+   again
+ - removes the temporary file with `buffer_deinit`, perhaps to
+   reuse the line_buffer for some other file.
+
+When finished, the calling program can use `buffer_reset` to deallocate
+resources.
+
 Functions
 ---------