core: introduce filesystem wrapper

core/src/trezor/sdcard.py

@@ -0,0 +1,46 @@
+from trezorio import FatFS, sdcard
+
+if False:
+    from typing import Any, Optional
+
+
+class FilesystemWrapper:
+    _INSTANCE = None  # type: Optional[FilesystemWrapper]
+
+    def __init__(self, mounted: bool = True) -> None:
+        self.fs = FatFS()
+        self.mounted = mounted
+        self.counter = 0
+
+    @classmethod
+    def get_instance(cls, mounted: bool = True) -> "FilesystemWrapper":
+        if cls._INSTANCE is None:
+            cls._INSTANCE = cls(mounted=mounted)
+        if cls._INSTANCE.mounted is not mounted:
+            raise RuntimeError  # cannot request mounted and non-mounted instance at the same time
+        return cls._INSTANCE
+
+    def __enter__(self) -> "FatFS":
+        if self.counter <= 0:
+            self.counter = 0
+            sdcard.power_on()
+            if self.mounted:
+                self.fs.mount()
+        self.counter += 1
+        return self.fs
+
+    def __exit__(self, exc_type: Any, exc_val: Any, tb: Any) -> None:
+        self.counter -= 1
+        if self.counter <= 0:
+            self.counter = 0
+            if self.mounted:
+                self.fs.unmount()
+            sdcard.power_off()
+            FilesystemWrapper._INSTANCE = None
+
+
+def get_filesystem(mounted: bool = True) -> FilesystemWrapper:
+    return FilesystemWrapper.get_instance(mounted=mounted)
+
+
+is_present = sdcard.is_present