objectstore/packed: Check pack/idx checksums here.

We previously had helpers in format/pack/checksum that checks
.pack/.idx-related checksums with []byte-based APIs. But it only really
makes sense to use those []byte-based APIs on mmap's (otherwise it'd be
horribly inefficient).

Since the packed object-store only needs to check that the .pack and
.idx trailer match, we move the relevant part into objectstore/packed.
The rest are deleted for now; we'll definitely need a streaming version
for the pack verification (when ingesting packfiles from the network)
(though we might just make it a streaming API (writer? reader? not
decided yet) that *produces* a hash, then verify it in the caller; this
way we could reuse the function in the pack-producing routines). The
others might get the []byte-based APIs back, or perhaps they too get
streaming APIs.

Remember that "reading objects from a packed object store",
"creating/writing packfiles", and "ingesting an incoming pack (which
usually involves creating an .idx for it)", are all very different
tasks.

2 files changed, 31 insertions, 2 deletions

diff --git a/objectstore/packed/pack_idx_checksum.go b/objectstore/packed/pack_idx_checksum.go
new file mode 100644
index 00000000..2f55a469
--- /dev/null
+++ b/objectstore/packed/pack_idx_checksum.go
@@ -0,0 +1,30 @@
+package packed
+
+import (
+	"bytes"
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objectid"
+)
+
+// verifyMappedPackMatchesMappedIdx compares one mapped pack trailer hash with
+// the pack hash recorded in one mapped idx trailer.
+func verifyMappedPackMatchesMappedIdx(packData, idxData []byte, algo objectid.Algorithm) error {
+	hashSize := algo.Size()
+	if hashSize <= 0 {
+		return objectid.ErrInvalidAlgorithm
+	}
+	if len(packData) < hashSize {
+		return fmt.Errorf("objectstore/packed: pack too short for trailer hash")
+	}
+	if len(idxData) < hashSize*2 {
+		return fmt.Errorf("objectstore/packed: idx too short for trailer hashes")
+	}
+
+	packTrailerHash := packData[len(packData)-hashSize:]
+	idxPackHash := idxData[len(idxData)-hashSize*2 : len(idxData)-hashSize]
+	if !bytes.Equal(packTrailerHash, idxPackHash) {
+		return fmt.Errorf("objectstore/packed: pack hash does not match idx")
+	}
+	return nil
+}
diff --git a/objectstore/packed/store.go b/objectstore/packed/store.go
index bb6936ea..abd7175f 100644
--- a/objectstore/packed/store.go
+++ b/objectstore/packed/store.go
@@ -7,7 +7,6 @@ import (
 	"os"
 	"sync"
 
-	packchecksum "codeberg.org/lindenii/furgit/format/pack/checksum"
 	"codeberg.org/lindenii/furgit/objectid"
 	"codeberg.org/lindenii/furgit/objectstore"
 )
@@ -195,7 +194,7 @@ func (store *Store) verifyPackMatchesIndexes(pack *packFile) error {
 	if err != nil {
 		return err
 	}
-	if err := packchecksum.VerifyPackMatchesIdx(pack.data, index.data, store.algo); err != nil {
+	if err := verifyMappedPackMatchesMappedIdx(pack.data, index.data, store.algo); err != nil {
 		return fmt.Errorf("objectstore/packed: pack %q does not match idx %q: %w", pack.name, index.idxName, err)
 	}
 	return nil