Adapt the SEVIRI native format reader in Satpy to support remote reading

AUTHORS.md

@@ -43,6 +43,7 @@ The following people have made contributions to this project:
 - [Jactry Zeng](https://github.com/jactry)
 - [Johannes Johansson (JohannesSMHI)](https://github.com/JohannesSMHI)
 - [Sauli Joro (sjoro)](https://github.com/sjoro)
+- [Pouria Khalaj](https://github.com/pkhalaj)
 - [Janne Kotro (jkotro)](https://github.com/jkotro)
 - [Ralph Kuehn (ralphk11)](https://github.com/ralphk11)
 - [Panu Lahtinen (pnuu)](https://github.com/pnuu)
@@ -94,5 +95,4 @@ The following people have made contributions to this project:
 - [Will Sharpe (wjsharpe)](https://github.com/wjsharpe)
 - [Sara Hörnquist (shornqui)](https://github.com/shornqui)
 - [Antonio Valentino](https://github.com/avalentino)
-- [Pouria Khalaj](https://github.com/pkhalaj)
 - [Clément (ludwigvonkoopa)](https://github.com/ludwigVonKoopa)

satpy/etc/readers/seviri_l1b_native.yaml

@@ -5,7 +5,7 @@ reader:
   description: >
     Reader for EUMETSAT MSG SEVIRI Level 1b native format files.
   status: Nominal
-  supports_fsspec: false
+  supports_fsspec: true
   sensors: [seviri]
   default_channels: [HRV, IR_016, IR_039, IR_087, IR_097, IR_108, IR_120, IR_134, VIS006, VIS008, WV_062, WV_073]
   reader: !!python/name:satpy.readers.yaml_reader.GEOFlippableFileYAMLReader

satpy/readers/seviri_l1b_native.py

@@ -138,7 +138,7 @@
     get_native_header,
     native_trailer,
 )
-from satpy.readers.utils import reduce_mda
+from satpy.readers.utils import fromfile, generic_open, reduce_mda
 from satpy.utils import get_legacy_chunk_size
 
 logger = logging.getLogger("native_msg")
@@ -193,10 +193,27 @@ def __init__(self, filename, filename_info, filetype_info,
         # Available channels are known only after the header has been read
         self.header_type = get_native_header(has_archive_header(self.filename))
         self._read_header()
-        self.dask_array = da.from_array(self._get_memmap(), chunks=(CHUNK_SIZE,))
+        self._make_dask_array_with_map_blocks()
         self._read_trailer()
         self.image_boundaries = ImageBoundaries(self.header, self.trailer, self.mda)
 
+    def _make_dask_array_with_map_blocks(self):
+        """Makes the dask array using the ``da.map_blocks()`` functionality."""
+        dtype = self._get_data_dtype()
+        chunks = da.core.normalize_chunks(
+            "auto",
+            shape=(self.mda["number_of_lines"],),
+            dtype=dtype)
+        self.dask_array = da.map_blocks(
+            _get_array,
+            dtype=dtype,
+            chunks=chunks,
+            meta=np.zeros(1, dtype=dtype),
+            # The following will be passed as keyword arguments to the `_get_array()` function.
+            filename=self.filename,
+            hdr_size=self.header_type.itemsize
+        )
+
     @property
     def _repeat_cycle_duration(self):
         """Get repeat cycle duration from the trailer."""
@@ -266,25 +283,17 @@ def get_lrec(cols):
         # each pixel is 10-bits -> one line of data has 25% more bytes
         # than the number of columns suggest (10/8 = 1.25)
         visir_rec = get_lrec(int(self.mda["number_of_columns"] * 1.25))
-        number_of_visir_channels = len(
-            [s for s in self.mda["channel_list"] if not s == "HRV"])
-        drec = [("visir", (visir_rec, number_of_visir_channels))]
+        drec = [("visir", (visir_rec, self._number_of_visir_channels()))]
 
         if self.mda["available_channels"]["HRV"]:
             hrv_rec = get_lrec(int(self.mda["hrv_number_of_columns"] * 1.25))
             drec.append(("hrv", (hrv_rec, 3)))
 
         return np.dtype(drec)
 
-    def _get_memmap(self):
-        """Get the memory map for the SEVIRI data."""
-        with open(self.filename) as fp:
-            data_dtype = self._get_data_dtype()
-            hdr_size = self.header_type.itemsize
-
-            return np.memmap(fp, dtype=data_dtype,
-                             shape=(self.mda["number_of_lines"],),
-                             offset=hdr_size, mode="r")
+    def _number_of_visir_channels(self):
+        """Returns the number of visir channels, i.e. all channels excluding ``HRV``."""
+        return len([s for s in self.mda["channel_list"] if not s == "HRV"])
 
     def _read_header(self):
         """Read the header info."""
@@ -387,9 +396,7 @@ def _read_trailer(self):
         data_size = (self._get_data_dtype().itemsize *
                      self.mda["number_of_lines"])
 
-        with open(self.filename) as fp:
-            fp.seek(hdr_size + data_size)
-            data = np.fromfile(fp, dtype=native_trailer, count=1)
+        data = fromfile(self.filename, dtype=native_trailer, count=1, offset=hdr_size + data_size)
 
         self.trailer.update(recarray2dict(data))
 
@@ -888,12 +895,23 @@ def get_available_channels(header):
 
 def has_archive_header(filename):
     """Check whether the file includes an ASCII archive header."""
-    with open(filename, mode="rb") as istream:
+    with generic_open(filename, mode="rb") as istream:
         return istream.read(36) == ASCII_STARTSWITH
 
 
 def read_header(filename):
     """Read SEVIRI L1.5 native header."""
     dtype = get_native_header(has_archive_header(filename))
-    hdr = np.fromfile(filename, dtype=dtype, count=1)
+    hdr = fromfile(filename, dtype=dtype, count=1)
     return recarray2dict(hdr)
+
+
+def _get_array(filename=None, hdr_size=None, block_info=None):
+    """Get the numpy array for the SEVIRI data."""
+    output_block_info = block_info[None]
+    data_dtype = output_block_info["dtype"]
+    return fromfile(
+        filename,
+        dtype=data_dtype,
+        offset=hdr_size + output_block_info["array-location"][0][0] * data_dtype.itemsize,
+        count=output_block_info["chunk-shape"][0])

satpy/readers/utils.py

@@ -98,8 +98,8 @@ def get_geostationary_angle_extent(geos_area):
     h = float(h) / 1000 + req
 
     # compute some constants
-    aeq = 1 - req**2 / (h ** 2)
-    ap_ = 1 - rp**2 / (h ** 2)
+    aeq = 1 - req ** 2 / (h ** 2)
+    ap_ = 1 - rp ** 2 / (h ** 2)
 
     # generate points around the north hemisphere in satellite projection
     # make it a bit smaller so that we stay inside the valid area
@@ -142,15 +142,15 @@ def _lonlat_from_geos_angle(x, y, geos_area):
     b__ = (a / float(b)) ** 2
 
     sd = np.sqrt((h__ * np.cos(x) * np.cos(y)) ** 2 -
-                 (np.cos(y)**2 + b__ * np.sin(y)**2) *
-                 (h__**2 - (float(a) / 1000)**2))
+                 (np.cos(y) ** 2 + b__ * np.sin(y) ** 2) *
+                 (h__ ** 2 - (float(a) / 1000) ** 2))
     # sd = 0
 
-    sn = (h__ * np.cos(x) * np.cos(y) - sd) / (np.cos(y)**2 + b__ * np.sin(y)**2)
+    sn = (h__ * np.cos(x) * np.cos(y) - sd) / (np.cos(y) ** 2 + b__ * np.sin(y) ** 2)
     s1 = h__ - sn * np.cos(x) * np.cos(y)
     s2 = sn * np.sin(x) * np.cos(y)
     s3 = -sn * np.sin(y)
-    sxy = np.sqrt(s1**2 + s2**2)
+    sxy = np.sqrt(s1 ** 2 + s2 ** 2)
 
     lons = np.rad2deg(np.arctan2(s2, s1)) + lon_0
     lats = np.rad2deg(-np.arctan2(b__ * s3, sxy))
@@ -256,7 +256,7 @@ def _unzip_with_pbzip(filename, tmpfilepath, fdn):
     if n_thr:
         runner = [pbzip,
                   "-dc",
-                  "-p"+str(n_thr),
+                  "-p" + str(n_thr),
                   filename]
     else:
         runner = [pbzip,
@@ -361,6 +361,27 @@ def generic_open(filename, *args, **kwargs):
     fp.close()
 
 
+def fromfile(filename, dtype, count=1, offset=0):
+    """Reads the numpy array from a (remote or local) file using a buffer.
+
+    Note:
+        This function relies on the :func:`generic_open` context manager to read a file remotely.
+
+    Args:
+        filename: Either the name of the file to read or a :class:`satpy.readers.FSFile` object.
+        dtype: The data type of the numpy array
+        count (Optional, default ``1``): Number of items to read
+        offset (Optional, default ``0``): Starting point for reading the buffer from
+
+    Returns:
+        The content of the filename as a numpy array with the given data type.
+    """
+    with generic_open(filename, mode="rb") as istream:
+        istream.seek(offset)
+        content = np.frombuffer(istream.read(dtype.itemsize * count), dtype=dtype, count=count)
+    return content
+
+
 def bbox(img):
     """Find the bounding box around nonzero elements in the given array.
 
@@ -395,7 +416,7 @@ def get_earth_radius(lon, lat, a, b):
     latlong = pyproj.CRS.from_dict({"proj": "latlong", "a": a, "b": b, "units": "m"})
     transformer = pyproj.Transformer.from_crs(latlong, geocent)
     x, y, z = transformer.transform(lon, lat, 0.0)
-    return np.sqrt(x**2 + y**2 + z**2)
+    return np.sqrt(x ** 2 + y ** 2 + z ** 2)
 
 
 def reduce_mda(mda, max_size=100):