Add support for reading from TAR archives to flatdata-py (heremaps#182)

Signed-off-by: Christian Ocker <christian.ocker@here.com>

flatdata-py/README.md

@@ -4,6 +4,12 @@
 
 Python 3 implementation of [flatdata](https://github.com/heremaps/flatdata).
 
+## Running the tests
+
+```sh
+python3 -m nose
+```
+
 ## Basic usage
 
 Once you have [created a flatdata schema file](../README.md#creating-a-schema), you can generate a Python module to read your existing `flatdata` archive:

flatdata-py/flatdata/lib/tar_archive_resource_storage.py

@@ -0,0 +1,101 @@
+'''
+ Copyright (c) 2021 HERE Europe B.V.
+ See the LICENSE file in the root of this project for license details.
+'''
+
+import tarfile
+
+from .errors import CorruptResourceError
+from .errors import MissingResourceError
+from .file_resource_storage import FileResourceStorage
+
+
+class TarArchiveResourceStorage:
+    """
+    Resource storage based on a memory-mapped TAR archive.
+    """
+
+    def __init__(self, tar_map, file_entries, dir_entries, sub_path):
+        self.tar_map = tar_map
+        self.file_entries = file_entries
+        self.dir_entries = dir_entries
+        self.sub_path = sub_path
+
+    @classmethod
+    def create(cls, tar_path, sub_path=""):
+        tar_map = FileResourceStorage.memory_map(tar_path)
+        file_entries = dict()
+        dir_entries = set()
+        with tarfile.open(tar_path, "r:") as tar:
+            for file in tar:
+                name = file.name
+                if name.startswith("./"):
+                    name = name[2:]
+                if file.type == tarfile.GNUTYPE_SPARSE:
+                    raise CorruptResourceError("Sparse files are not supported")
+                if file.isreg():
+                    file_entries[name] = (file.offset_data, file.size)
+                if file.isdir():
+                    dir_entries.add(name)
+
+        return cls(tar_map, file_entries, dir_entries, sub_path)
+
+    def get(self, key, is_optional=False):
+        path = self._path(key)
+        if path in self.file_entries:
+            (offset, length) = self.file_entries[path]
+            return MemoryMapSection(self.tar_map, offset, length)
+
+        if path in self.dir_entries:
+            return TarArchiveResourceStorage(self.tar_map, self.file_entries, self.dir_entries, path)
+
+        if not is_optional:
+            raise MissingResourceError(key)
+        else:
+            return None
+
+    def _path(self, key):
+        if not self.sub_path:
+            return key
+        else:
+            return self.sub_path + '/' + key
+
+
+class MemoryMapSection:
+    """
+    Represent a slice of a memory mapped file.
+    Keeps track of its position, as to emulate pointing to a dedicated file.
+    """
+
+    def __init__(self, inner, offset, length):
+        self.inner = inner
+        self.offset = offset
+        self.length = length;
+        self.pos = 0
+
+    def __len__(self):
+        return self.size()
+
+    def __getitem__(self, key):
+        if isinstance(key, slice):
+            start = key.start if key.start is not None else 0
+            start = self.offset + min(start, self.length)
+            stop = key.stop if key.stop is not None else self.length
+            stop = self.offset + min(stop, self.length)
+            return self.inner[slice(start, stop, key.step)]
+        else:
+            if key < self.length:
+                return self.inner.__getitem__(self.offset + key)
+            else:
+                raise IndexError('index out of range')
+
+    def read(self, n=None):
+        if n is None:
+            n = self.length - self.pos
+        self.inner.seek(self.offset + self.pos)
+        data = self.inner.read(min(n, self.length - self.pos))
+        self.pos += len(data)
+        return data
+
+    def size(self):
+        return min(self.length, self.inner.size() - self.offset)

flatdata-py/tests/test_tar_resource_storage.py

@@ -0,0 +1,40 @@
+from common import *
+from flatdata.generator.engine import Engine
+from flatdata.lib.tar_archive_resource_storage import TarArchiveResourceStorage
+
+import tarfile
+import tempfile
+import os
+from nose.tools import eq_, assert_is_instance
+
+
+def check_signed_struct(s):
+    eq_(-0x1, s.a)
+    eq_(0x01234567, s.b)
+    eq_(-0x28, s.c)
+    eq_(0, s.d)
+
+
+def test_tar_resource_storage():
+    module = Engine(INSTANCE_TEST_SCHEMA).render_python_module()
+    valid_data = {
+        "Archive.archive": ARCHIVE_SIGNATURE_PAYLOAD,
+        "Archive.archive.schema": module.backward_compatibility_Archive.schema().encode(),
+        "resource": RESOURCE_PAYLOAD,
+        "resource.schema": module.backward_compatibility_Archive.resource_schema('resource').encode()
+    }
+
+    with tempfile.TemporaryDirectory() as tmpdir:
+        archive_path = os.path.join(tmpdir, "archive.tar")
+        cwd = os.getcwd()
+        os.chdir(tmpdir)
+        tar = tarfile.open(archive_path, "w")
+        for key, value in valid_data.items():
+            with open(os.path.join(tmpdir, key), "wb") as file:
+                file.write(value)
+            tar.add(key)
+        tar.close()
+        os.chdir(cwd)
+
+        archive = module.backward_compatibility_Archive(TarArchiveResourceStorage.create(archive_path))
+        check_signed_struct(archive.resource)