sha1_file: don't malloc the whole compressed result when writing out objects

There is no real advantage to malloc the whole output buffer and
deflate the data in a single pass when writing loose objects. That is
like only 1% faster while using more memory, especially with large
files where memory usage is far more. It is best to deflate and write
the data out in small chunks reusing the same memory instead.

For example, using 'git add' on a few large files averaging 40 MB ...

Before:
21.45user 1.10system 0:22.57elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+828040outputs (0major+142640minor)pagefaults 0swaps

After:
21.50user 1.25system 0:22.76elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+828040outputs (0major+104408minor)pagefaults 0swaps

While the runtime stayed relatively the same, the number of minor page
faults went down significantly.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -2281,8 +2281,7 @@ static int write_loose_object(const unsigned char *sha1, char *hdr, int hdrlen,
 			      void *buf, unsigned long len, time_t mtime)
 {
 	int fd, ret;
-	size_t size;
-	unsigned char *compressed;
+	unsigned char compressed[4096];
 	z_stream stream;
 	char *filename;
 	static char tmpfile[PATH_MAX];
@@ -2301,12 +2300,8 @@ static int write_loose_object(const unsigned char *sha1, char *hdr, int hdrlen,
 	/* Set it up */
 	memset(&stream, 0, sizeof(stream));
 	deflateInit(&stream, zlib_compression_level);
-	size = 8 + deflateBound(&stream, len+hdrlen);
-	compressed = xmalloc(size);
-
-	/* Compress it */
 	stream.next_out = compressed;
-	stream.avail_out = size;
+	stream.avail_out = sizeof(compressed);
 
 	/* First header.. */
 	stream.next_in = (unsigned char *)hdr;
@@ -2317,20 +2312,21 @@ static int write_loose_object(const unsigned char *sha1, char *hdr, int hdrlen,
 	/* Then the data itself.. */
 	stream.next_in = buf;
 	stream.avail_in = len;
-	ret = deflate(&stream, Z_FINISH);
+	do {
+		ret = deflate(&stream, Z_FINISH);
+		if (write_buffer(fd, compressed, stream.next_out - compressed) < 0)
+			die("unable to write sha1 file");
+		stream.next_out = compressed;
+		stream.avail_out = sizeof(compressed);
+	} while (ret == Z_OK);
+
 	if (ret != Z_STREAM_END)
 		die("unable to deflate new object %s (%d)", sha1_to_hex(sha1), ret);
-
 	ret = deflateEnd(&stream);
 	if (ret != Z_OK)
 		die("deflateEnd on object %s failed (%d)", sha1_to_hex(sha1), ret);
 
-	size = stream.total_out;
-
-	if (write_buffer(fd, compressed, size) < 0)
-		die("unable to write sha1 file");
 	close_sha1_file(fd);
-	free(compressed);
 
 	if (mtime) {
 		struct utimbuf utb;