host daemon (#2)

* feat: host daemon api scaffold

* fix: use sparse writes

* fix: unix socket length (<108 bytes)

internal/daemon/files_test.go

@@ -0,0 +1,94 @@
+package daemon
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+)
+
+func TestCloneFilePreservesSparseDiskUsage(t *testing.T) {
+	root := t.TempDir()
+	sourcePath := filepath.Join(root, "source.img")
+	targetPath := filepath.Join(root, "target.img")
+
+	sourceFile, err := os.OpenFile(sourcePath, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0o644)
+	if err != nil {
+		t.Fatalf("open source file: %v", err)
+	}
+	if _, err := sourceFile.Write([]byte("head")); err != nil {
+		sourceFile.Close()
+		t.Fatalf("write source prefix: %v", err)
+	}
+	if _, err := sourceFile.Seek(32<<20, io.SeekStart); err != nil {
+		sourceFile.Close()
+		t.Fatalf("seek source hole: %v", err)
+	}
+	if _, err := sourceFile.Write([]byte("tail")); err != nil {
+		sourceFile.Close()
+		t.Fatalf("write source suffix: %v", err)
+	}
+	if err := sourceFile.Close(); err != nil {
+		t.Fatalf("close source file: %v", err)
+	}
+
+	sourceInfo, err := os.Stat(sourcePath)
+	if err != nil {
+		t.Fatalf("stat source file: %v", err)
+	}
+	sourceUsage, err := allocatedBytes(sourcePath)
+	if err != nil {
+		t.Fatalf("allocated bytes for source: %v", err)
+	}
+	if sourceUsage >= sourceInfo.Size()/2 {
+		t.Skip("temp filesystem does not expose sparse allocation savings")
+	}
+
+	if err := cloneFile(sourcePath, targetPath); err != nil {
+		t.Fatalf("clone sparse file: %v", err)
+	}
+
+	targetInfo, err := os.Stat(targetPath)
+	if err != nil {
+		t.Fatalf("stat target file: %v", err)
+	}
+	if targetInfo.Size() != sourceInfo.Size() {
+		t.Fatalf("target size mismatch: got %d want %d", targetInfo.Size(), sourceInfo.Size())
+	}
+
+	targetUsage, err := allocatedBytes(targetPath)
+	if err != nil {
+		t.Fatalf("allocated bytes for target: %v", err)
+	}
+	if targetUsage >= targetInfo.Size()/2 {
+		t.Fatalf("target file is not sparse enough: allocated=%d size=%d", targetUsage, targetInfo.Size())
+	}
+
+	targetData, err := os.ReadFile(targetPath)
+	if err != nil {
+		t.Fatalf("read target file: %v", err)
+	}
+	if !bytes.Equal(targetData[:4], []byte("head")) {
+		t.Fatalf("target prefix mismatch: %q", string(targetData[:4]))
+	}
+	if !bytes.Equal(targetData[len(targetData)-4:], []byte("tail")) {
+		t.Fatalf("target suffix mismatch: %q", string(targetData[len(targetData)-4:]))
+	}
+	if !bytes.Equal(targetData[4:4+(1<<20)], make([]byte, 1<<20)) {
+		t.Fatal("target hole contents were not zeroed")
+	}
+}
+
+func allocatedBytes(path string) (int64, error) {
+	info, err := os.Stat(path)
+	if err != nil {
+		return 0, err
+	}
+	stat, ok := info.Sys().(*syscall.Stat_t)
+	if !ok {
+		return 0, syscall.EINVAL
+	}
+	return stat.Blocks * 512, nil
+}