[PATCH v4 4/7] bulk-checkin: implement `SOURCE_INCORE` mode for `bulk_checkin_source`

[Taylor Blau <me@ttaylorr.com>]

Continue to prepare for streaming an object's contents directly from
memory by teaching `bulk_checkin_source` how to perform reads and seeks
based on an address in memory.

Unlike file descriptors, which manage their own offset internally, we
have to keep track of how many bytes we've read out of the buffer, and
make sure we don't read past the end of the buffer.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 bulk-checkin.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/bulk-checkin.c b/bulk-checkin.c
index 28bc8d5ab4..60361b3e2e 100644
--- a/bulk-checkin.c
+++ b/bulk-checkin.c
@@ -141,11 +141,15 @@ static int already_written(struct bulk_checkin_packfile *state, struct object_id
 }
 
 struct bulk_checkin_source {
-	enum { SOURCE_FILE } type;
+	enum { SOURCE_FILE, SOURCE_INCORE } type;
 
 	/* SOURCE_FILE fields */
 	int fd;
 
+	/* SOURCE_INCORE fields */
+	const void *buf;
+	size_t read;
+
 	/* common fields */
 	size_t size;
 	const char *path;
@@ -157,6 +161,11 @@ static off_t bulk_checkin_source_seek_to(struct bulk_checkin_source *source,
 	switch (source->type) {
 	case SOURCE_FILE:
 		return lseek(source->fd, offset, SEEK_SET);
+	case SOURCE_INCORE:
+		if (!(0 <= offset && offset < source->size))
+			return (off_t)-1;
+		source->read = offset;
+		return source->read;
 	default:
 		BUG("unknown bulk-checkin source: %d", source->type);
 	}
@@ -168,6 +177,13 @@ static ssize_t bulk_checkin_source_read(struct bulk_checkin_source *source,
 	switch (source->type) {
 	case SOURCE_FILE:
 		return read_in_full(source->fd, buf, nr);
+	case SOURCE_INCORE:
+		assert(source->read <= source->size);
+		if (nr > source->size - source->read)
+			nr = source->size - source->read;
+		memcpy(buf, (unsigned char *)source->buf + source->read, nr);
+		source->read += nr;
+		return nr;
 	default:
 		BUG("unknown bulk-checkin source: %d", source->type);
 	}
-- 
2.42.0.405.g86fe3250c2