objectstore/packed: Separate idx candidate lookup vs actually opening it

1 files changed, 131 insertions, 0 deletions

diff --git a/objectstore/packed/idx_open.go b/objectstore/packed/idx_open.go
new file mode 100644
index 00000000..45f0f83d
--- /dev/null
+++ b/objectstore/packed/idx_open.go
@@ -0,0 +1,131 @@
+package packed
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+
+	"codeberg.org/lindenii/furgit/internal/intconv"
+	"codeberg.org/lindenii/furgit/objectid"
+)
+
+// idxFile stores one mapped and validated idx v2 file.
+type idxFile struct {
+	// idxName is the basename of this .idx file.
+	idxName string
+	// packName is the matching .pack basename.
+	packName string
+	// algo is the hash algorithm encoded by the index.
+	algo objectid.Algorithm
+
+	// file is the opened index file descriptor.
+	file *os.File
+	// data is the mapped index bytes.
+	data []byte
+
+	// fanout stores fanout table values.
+	fanout [256]uint32
+	// numObjects equals fanout[255].
+	numObjects int
+
+	// namesOffset starts the sorted object-id table.
+	namesOffset int
+	// offset32Offset starts the 32-bit offset table.
+	offset32Offset int
+	// offset64Offset starts the 64-bit offset table.
+	offset64Offset int
+	// offset64Count is the number of 64-bit offset entries.
+	offset64Count int
+}
+
+// candidateForPack returns one discovered candidate for a pack basename.
+func (store *Store) candidateForPack(packName string) (packCandidate, bool) {
+	store.stateMu.RLock()
+	candidate, ok := store.candidateByPack[packName]
+	store.stateMu.RUnlock()
+	return candidate, ok
+}
+
+// openIndex returns one opened and parsed index, caching it by pack basename.
+func (store *Store) openIndex(candidate packCandidate) (*idxFile, error) {
+	store.stateMu.RLock()
+	if index, ok := store.idxByPack[candidate.packName]; ok {
+		store.stateMu.RUnlock()
+		return index, nil
+	}
+	store.stateMu.RUnlock()
+
+	index, err := openIdxFile(store.root, candidate.idxName, candidate.packName, store.algo)
+	if err != nil {
+		return nil, err
+	}
+
+	store.stateMu.Lock()
+	if existing, ok := store.idxByPack[candidate.packName]; ok {
+		store.stateMu.Unlock()
+		_ = index.close()
+		return existing, nil
+	}
+	store.idxByPack[candidate.packName] = index
+	store.stateMu.Unlock()
+	return index, nil
+}
+
+// openIdxFile maps and validates one idx v2 file.
+func openIdxFile(root *os.Root, idxName, packName string, algo objectid.Algorithm) (*idxFile, error) {
+	file, err := root.Open(idxName)
+	if err != nil {
+		return nil, err
+	}
+	info, err := file.Stat()
+	if err != nil {
+		_ = file.Close()
+		return nil, err
+	}
+	size := info.Size()
+	if size < 0 || size > int64(int(^uint(0)>>1)) {
+		_ = file.Close()
+		return nil, fmt.Errorf("objectstore/packed: idx %q has unsupported size", idxName)
+	}
+	fd, err := intconv.UintptrToInt(file.Fd())
+	if err != nil {
+		_ = file.Close()
+		return nil, err
+	}
+	data, err := syscall.Mmap(fd, 0, int(size), syscall.PROT_READ, syscall.MAP_PRIVATE)
+	if err != nil {
+		_ = file.Close()
+		return nil, err
+	}
+
+	index := &idxFile{
+		idxName:  idxName,
+		packName: packName,
+		algo:     algo,
+		file:     file,
+		data:     data,
+	}
+	if err := index.parse(); err != nil {
+		_ = index.close()
+		return nil, err
+	}
+	return index, nil
+}
+
+// close unmaps and closes one idx handle.
+func (index *idxFile) close() error {
+	var closeErr error
+	if index.data != nil {
+		if err := syscall.Munmap(index.data); err != nil && closeErr == nil {
+			closeErr = err
+		}
+		index.data = nil
+	}
+	if index.file != nil {
+		if err := index.file.Close(); err != nil && closeErr == nil {
+			closeErr = err
+		}
+		index.file = nil
+	}
+	return closeErr
+}