[snapshot] Avoid deriving the ID string each time

Store the (now immutable) ID string in a snapshot field and provide an
accessor to it, rather than deriving it each time it is needed. Adapt
client code accordingly.

src/platform/backends/qemu/qemu_snapshot.cpp

@@ -71,7 +71,7 @@ mp::QemuSnapshot::QemuSnapshot(const QString& filename, QemuVirtualMachine& vm,
 
 void mp::QemuSnapshot::capture_impl()
 {
-    const auto tag = derive_id();
+    const auto& tag = get_id();
 
     // Avoid creating more than one snapshot with the same tag (creation would succeed, but we'd then be unable to
     // identify the snapshot by tag)
@@ -86,7 +86,7 @@ void mp::QemuSnapshot::capture_impl()
 
 void mp::QemuSnapshot::erase_impl()
 {
-    const auto tag = derive_id();
+    const auto& tag = get_id();
     if (backend::instance_image_has_snapshot(image_path, tag))
         mp::backend::checked_exec_qemu_img(make_delete_spec(tag, image_path));
     else
@@ -107,6 +107,6 @@ void mp::QemuSnapshot::apply_impl()
     desc.mem_size = get_mem_size();
     desc.disk_space = get_disk_space();
 
-    mp::backend::checked_exec_qemu_img(make_restore_spec(derive_id(), image_path));
+    mp::backend::checked_exec_qemu_img(make_restore_spec(get_id(), image_path));
     rollback.dismiss();
 }

src/platform/backends/shared/base_snapshot.cpp

@@ -113,6 +113,7 @@ mp::BaseSnapshot::BaseSnapshot(const std::string& name,    // NOLINT(modernize-p
       comment{comment},
       parent{std::move(parent)},
       index{index},
+      id{snapshot_template.arg(index)},
       creation_timestamp{std::move(creation_timestamp)},
       num_cores{num_cores},
       mem_size{mem_size},
@@ -225,11 +226,6 @@ void mp::BaseSnapshot::persist() const
     MP_JSONUTILS.write_json(serialize(), snapshot_filepath);
 }
 
-QString mp::BaseSnapshot::derive_id() const
-{
-    return snapshot_template.arg(index);
-}
-
 auto mp::BaseSnapshot::erase_helper()
 {
     // Remove snapshot file

src/platform/backends/shared/base_snapshot.h

@@ -69,12 +69,12 @@ class BaseSnapshot : public Snapshot
     void apply() final;
 
 protected:
+    const QString& get_id() const noexcept;
+
     virtual void capture_impl() = 0;
     virtual void erase_impl() = 0;
     virtual void apply_impl() = 0;
 
-    QString derive_id() const;
-
 private:
     BaseSnapshot(const QJsonObject& json, VirtualMachine& vm);
     BaseSnapshot(const std::string& name,
@@ -103,6 +103,7 @@ class BaseSnapshot : public Snapshot
 
     // This class is non-copyable and having these const simplifies thread safety
     const int index;                                       // NOLINT(cppcoreguidelines-avoid-const-or-ref-data-members)
+    const QString id;                                      // NOLINT(cppcoreguidelines-avoid-const-or-ref-data-members)
     const QDateTime creation_timestamp;                    // NOLINT(cppcoreguidelines-avoid-const-or-ref-data-members)
     const int num_cores;                                   // NOLINT(cppcoreguidelines-avoid-const-or-ref-data-members)
     const MemorySize mem_size;                             // NOLINT(cppcoreguidelines-avoid-const-or-ref-data-members)
@@ -244,4 +245,9 @@ inline void multipass::BaseSnapshot::apply()
     // those cannot be affected by apply_impl (except by setters, which already persist)
 }
 
+inline const QString& multipass::BaseSnapshot::get_id() const noexcept
+{
+    return id;
+}
+
 #endif // MULTIPASS_BASE_SNAPSHOT_H