this, and an upcoming diff to do the same with the packidx, will allow
to avoid having to pass the algorithm around.  Subsequent diff will need
this.

diff /home/op/w/got
commit - 020f73dba5df9e319726af7526dbb3b478d52b76
path + /home/op/w/got
blob - 08acf40b3e797365f5cc41914e99e091f0f09fe7
file + lib/got_lib_pack.h
--- lib/got_lib_pack.h
+++ lib/got_lib_pack.h
@@ -24,6 +24,7 @@ struct got_pack_privsep_child {
 struct got_pack {
 	char *path_packfile;
 	int fd;
+	enum got_hash_algorithm algo;
 	uint8_t *map;
 	off_t filesize;
 	struct got_pack_privsep_child *privsep_child;
blob - b643ffc4d3b4304f1d21cd0eeb70e6170a63554e
file + lib/got_lib_privsep.h
--- lib/got_lib_privsep.h
+++ lib/got_lib_privsep.h
@@ -520,6 +520,7 @@ struct got_imsg_packidx {
 struct got_imsg_pack {
 	char path_packfile[PATH_MAX];
 	off_t filesize;
+	int algo;
 	/* Additionally, a file desciptor is passed via imsg. */
 } __attribute__((__packed__));
 
blob - 7e62208eb4b4a54c4cea9971137def3815fabedf
file + lib/privsep.c
--- lib/privsep.c
+++ lib/privsep.c
@@ -1975,6 +1975,7 @@ got_privsep_init_pack_child(struct imsgbuf *ibuf, stru
 	    sizeof(ipack.path_packfile)) >= sizeof(ipack.path_packfile))
 		return got_error(GOT_ERR_NO_SPACE);
 	ipack.filesize = pack->filesize;
+	ipack.algo = pack->algo;
 
 	fd = dup(pack->fd);
 	if (fd == -1)
blob - 271a0e86825dde1ffd9062349f1b0ff19c086697
file + lib/repository.c
--- lib/repository.c
+++ lib/repository.c
@@ -1609,6 +1609,7 @@ got_repo_cache_pack(struct got_pack **packp, struct go
 		goto done;
 	}
 	pack->filesize = sb.st_size;
+	pack->algo = repo->algo;
 
 	pack->privsep_child = NULL;
 
blob - 1a3634ab2fe9a44827da4a6597ff3582c37a42f3
file + libexec/got-read-pack/got-read-pack.c
--- libexec/got-read-pack/got-read-pack.c
+++ libexec/got-read-pack/got-read-pack.c
@@ -1880,6 +1880,7 @@ receive_pack(struct got_pack **packp, struct imsgbuf *
 	}
 	memcpy(&ipack, imsg.data, sizeof(ipack));
 
+	pack->algo = ipack.algo;
 	pack->filesize = ipack.filesize;
 	pack->fd = imsg_get_fd(&imsg);
 	if (pack->fd == -1) {