git-pack-objects: write the pack files with a SHA1 csum

We want to be able to check their integrity later, and putting the
sha1-sum of the contents at the end is a good thing.  The writing
routines are generic, so we could try to re-use them for the index file,
instead of having the same logic duplicated.

Update unpack-objects to know about the extra 20 bytes at the end
of the index.

csum-file.c

@@ -0,0 +1,120 @@
+/*
+ * csum-file.c
+ *
+ * Copyright (C) 2005 Linus Torvalds
+ *
+ * Simple file write infrastructure for writing SHA1-summed
+ * files. Useful when you write a file that you want to be
+ * able to verify hasn't been messed with afterwards.
+ */
+#include "cache.h"
+#include "csum-file.h"
+
+static int sha1flush(struct sha1file *f, unsigned int count)
+{
+	void *buf = f->buffer;
+
+	for (;;) {
+		int ret = write(f->fd, buf, count);
+		if (ret > 0) {
+			buf += ret;
+			count -= ret;
+			if (count)
+				continue;
+			return 0;
+		}
+		if (!ret)
+			die("sha1 file write error. Out of diskspace");
+		if (errno == EAGAIN || errno == EINTR)
+			continue;
+		die("sha1 file write error (%s)", strerror(errno));
+	}
+}
+
+int sha1close(struct sha1file *f)
+{
+	unsigned offset = f->offset;
+	if (offset) {
+		SHA1_Update(&f->ctx, f->buffer, offset);
+		sha1flush(f, offset);
+	}
+	SHA1_Final(f->buffer, &f->ctx);
+	sha1flush(f, 20);
+	return 0;
+}
+
+int sha1write(struct sha1file *f, void *buf, unsigned int count)
+{
+	while (count) {
+		unsigned offset = f->offset;
+		unsigned left = sizeof(f->buffer) - offset;
+		unsigned nr = count > left ? left : count;
+
+		memcpy(f->buffer + offset, buf, nr);
+		count -= nr;
+		offset += nr;
+		left -= nr;
+		if (!left) {
+			SHA1_Update(&f->ctx, f->buffer, offset);
+			sha1flush(f, offset);
+			offset = 0;
+		}
+		f->offset = offset;
+	}
+	return 0;
+}
+
+struct sha1file *sha1create(const char *fmt, ...)
+{
+	static char filename[PATH_MAX];
+	struct sha1file *f;
+	unsigned len;
+	va_list arg;
+	int fd;
+
+	va_start(arg, fmt);
+	len = vsnprintf(filename, PATH_MAX, fmt, arg);
+	va_end(arg);
+
+	if (len >= PATH_MAX)
+		die("you wascally wabbit, you");
+	fd = open(filename, O_CREAT | O_EXCL | O_WRONLY, 0644);
+	if (fd < 0)
+		die("unable to open %s (%s)", filename, strerror(errno));
+	f = xmalloc(sizeof(*f));
+	f->fd = fd;
+	f->error = 0;
+	f->offset = 0;
+	SHA1_Init(&f->ctx);
+	return f;
+}
+
+int sha1write_compressed(struct sha1file *f, void *in, unsigned int size)
+{
+	z_stream stream;
+	unsigned long maxsize;
+	void *out;
+
+	memset(&stream, 0, sizeof(stream));
+	deflateInit(&stream, Z_DEFAULT_COMPRESSION);
+	maxsize = deflateBound(&stream, size);
+	out = xmalloc(maxsize);
+
+	/* Compress it */
+	stream.next_in = in;
+	stream.avail_in = size;
+
+	stream.next_out = out;
+	stream.avail_out = maxsize;
+
+	while (deflate(&stream, Z_FINISH) == Z_OK)
+		/* nothing */;
+	deflateEnd(&stream);
+
+	size = stream.total_out;
+	sha1write(f, out, size);
+	free(out);
+	return size;
+}
+
+