5 files changed, 33 insertions, 10 deletions

diff --git a/archival/libarchive/Kbuild.src b/archival/libarchive/Kbuild.src
index 70c7fae5b..1af3cf61d 100644
--- a/archival/libarchive/Kbuild.src
+++ b/archival/libarchive/Kbuild.src
@@ -39,6 +39,7 @@ DPKG_FILES:= \
         get_header_tar_bz2.o \
         get_header_tar_lzma.o \
         get_header_tar_xz.o \
+        get_header_tar_zstd.o \
 
 INSERT
 
diff --git a/archival/libarchive/decompress_unzstd.c b/archival/libarchive/decompress_unzstd.c
index 08930a3e5..c55ab8575 100644
--- a/archival/libarchive/decompress_unzstd.c
+++ b/archival/libarchive/decompress_unzstd.c
@@ -4,16 +4,9 @@
 IF_DESKTOP(long long) int FAST_FUNC
 unpack_zstd_stream(transformer_state_t *xstate)
 {
-        char *argv[] = {(char *)"unzstd", NULL};
+        // FIXME Provide an internal implementation of zstd decompression.
+        char *argv[] = {(char *)"unzstd", (char *)"-cf", (char *)"-", NULL};
 
-        /* Exec external unzstd. */
-#if BB_MMU
-        if (xstate->signature_skipped) {
-                /* If called from fork_transformer_with_no_sig() signature_skipped
-                 * is just a flag, not the actual offset.  But we know it's 4. */
-                xlseek(xstate->src_fd, -4, SEEK_CUR);
-        }
-#endif
         xmove_fd(xstate->src_fd, STDIN_FILENO);
         xmove_fd(xstate->dst_fd, STDOUT_FILENO);
         BB_EXECVP_or_die(argv);
diff --git a/archival/libarchive/filter_accept_list_reassign.c b/archival/libarchive/filter_accept_list_reassign.c
index 826c5c29d..a78186a6e 100644
--- a/archival/libarchive/filter_accept_list_reassign.c
+++ b/archival/libarchive/filter_accept_list_reassign.c
@@ -55,6 +55,12 @@ char FAST_FUNC filter_accept_list_reassign(archive_handle_t *archive_handle)
                         archive_handle->dpkg__action_data_subarchive = get_header_tar_xz;
                         return EXIT_SUCCESS;
                 }
+                if (ENABLE_FEATURE_SEAMLESS_ZSTD
+                 && strcmp(name_ptr, "zst") == 0
+                ) {
+                        archive_handle->dpkg__action_data_subarchive = get_header_tar_zstd;
+                        return EXIT_SUCCESS;
+                }
         }
         return EXIT_FAILURE;
 }
diff --git a/archival/libarchive/get_header_tar_zstd.c b/archival/libarchive/get_header_tar_zstd.c
new file mode 100644
index 000000000..6f1b4b977
--- /dev/null
+++ b/archival/libarchive/get_header_tar_zstd.c
@@ -0,0 +1,20 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Licensed under GPLv2 or later, see file LICENSE in this source tree.
+ */
+#include "libbb.h"
+#include "bb_archive.h"
+
+char FAST_FUNC get_header_tar_zstd(archive_handle_t *archive_handle)
+{
+        /* Can't lseek over pipes */
+        archive_handle->seek = seek_by_read;
+
+        fork_transformer_with_sig(archive_handle->src_fd, unpack_zstd_stream, "unzstd");
+        archive_handle->offset = 0;
+        while (get_header_tar(archive_handle) == EXIT_SUCCESS)
+                continue;
+
+        /* Can only do one file at a time */
+        return EXIT_FAILURE;
+}
diff --git a/archival/libarchive/open_transformer.c b/archival/libarchive/open_transformer.c
index 9f41c7b4f..375e7b249 100644
--- a/archival/libarchive/open_transformer.c
+++ b/archival/libarchive/open_transformer.c
@@ -211,11 +211,14 @@ static transformer_state_t *setup_transformer_on_fd(int fd, int die_if_not_compr
         }
 
         if (ENABLE_FEATURE_SEAMLESS_ZSTD && xstate->magic.b16[0] == ZSTD_MAGIC1) {
-                xstate->signature_skipped = 4;
                 xread(fd, &xstate->magic.b16[1], 2);
                 if (xstate->magic.b16[1] == ZSTD_MAGIC2) {
                         xstate->xformer = unpack_zstd_stream;
                         USE_FOR_NOMMU(xstate->xformer_prog = "unzstd";)
+                        // FIXME We execute an external decompressor even on MMU.
+                        // Force a seek back to the signature.
+                        xstate->signature_skipped = 0;
+                        xlseek(xstate->src_fd, -4, SEEK_CUR);
                         goto found_magic;
                 }
         }