Switch Buffers to memoryviews

When this was written in the code, Python's Buffer Protocol support was
inconsistent across Python versions (specifically on Python 2.7). Since
Python 2.7 reached EOL and it was dropped from Numcodecs, the Python
Buffer Protocol support has become more consistent.

At this stage the `memoryview` object, which Cython also supports, does
all the same things that `Buffer` would do for us. Plus it is builtin to
the Python standard library. It behaves similarly in a lot of ways.

Given this, switch the code over to `memoryview`s internally and drop
`Buffer`.

Author: jakirkham

numcodecs/blosc.pyx

@@ -8,12 +8,11 @@ import multiprocessing
 import os
 
 
-from cpython.buffer cimport PyBUF_ANY_CONTIGUOUS, PyBUF_WRITEABLE
+from cpython.buffer cimport PyBuffer_IsContiguous
 from cpython.bytes cimport PyBytes_FromStringAndSize, PyBytes_AS_STRING
+from cpython.memoryview cimport PyMemoryView_GET_BUFFER
 
 
-from .compat_ext cimport Buffer
-from .compat_ext import Buffer
 from .compat import ensure_contiguous_ndarray
 from .abc import Codec
 
@@ -146,34 +145,36 @@ def cbuffer_sizes(source):
 
     """
     cdef:
-        Buffer buffer
+        memoryview source_mv
+        const Py_buffer* source_pb
         size_t nbytes, cbytes, blocksize
 
-    # obtain buffer
-    buffer = Buffer(source, PyBUF_ANY_CONTIGUOUS)
+    # obtain source memoryview
+    source_mv = memoryview(source)
+    source_pb = PyMemoryView_GET_BUFFER(source_mv)
+    if not PyBuffer_IsContiguous(source_pb, b'A'):
+        raise BufferError("`source` must contain contiguous memory")
 
     # determine buffer size
-    blosc_cbuffer_sizes(buffer.ptr, &nbytes, &cbytes, &blocksize)
-
-    # release buffers
-    buffer.release()
+    blosc_cbuffer_sizes(source_pb.buf, &nbytes, &cbytes, &blocksize)
 
     return nbytes, cbytes, blocksize
 
 
 def cbuffer_complib(source):
     """Return the name of the compression library used to compress `source`."""
     cdef:
-        Buffer buffer
+        memoryview source_mv
+        const Py_buffer* source_pb
 
-    # obtain buffer
-    buffer = Buffer(source, PyBUF_ANY_CONTIGUOUS)
+    # obtain source memoryview
+    source_mv = memoryview(source)
+    source_pb = PyMemoryView_GET_BUFFER(source_mv)
+    if not PyBuffer_IsContiguous(source_pb, b'A'):
+        raise BufferError("`source` must contain contiguous memory")
 
     # determine buffer size
-    complib = blosc_cbuffer_complib(buffer.ptr)
-
-    # release buffers
-    buffer.release()
+    complib = blosc_cbuffer_complib(source_pb.buf)
 
     complib = complib.decode('ascii')
 
@@ -193,18 +194,19 @@ def cbuffer_metainfo(source):
 
     """
     cdef:
-        Buffer buffer
+        memoryview source_mv
+        const Py_buffer* source_pb
         size_t typesize
         int flags
 
-    # obtain buffer
-    buffer = Buffer(source, PyBUF_ANY_CONTIGUOUS)
+    # obtain source memoryview
+    source_mv = memoryview(source)
+    source_pb = PyMemoryView_GET_BUFFER(source_mv)
+    if not PyBuffer_IsContiguous(source_pb, b'A'):
+        raise BufferError("`source` must contain contiguous memory")
 
     # determine buffer size
-    blosc_cbuffer_metainfo(buffer.ptr, &typesize, &flags)
-
-    # release buffers
-    buffer.release()
+    blosc_cbuffer_metainfo(source_pb.buf, &typesize, &flags)
 
     # decompose flags
     if flags & BLOSC_DOSHUFFLE:
@@ -252,9 +254,10 @@ def compress(source, char* cname, int clevel, int shuffle=SHUFFLE,
     """
 
     cdef:
-        char *source_ptr
-        char *dest_ptr
-        Buffer source_buffer
+        memoryview source_mv
+        const Py_buffer* source_pb
+        const char* source_ptr
+        char* dest_ptr
         size_t nbytes, itemsize
         int cbytes
         bytes dest
@@ -264,11 +267,16 @@ def compress(source, char* cname, int clevel, int shuffle=SHUFFLE,
     if cname_str not in list_compressors():
         err_bad_cname(cname_str)
 
-    # setup source buffer
-    source_buffer = Buffer(source, PyBUF_ANY_CONTIGUOUS)
-    source_ptr = source_buffer.ptr
-    nbytes = source_buffer.nbytes
-    itemsize = source_buffer.itemsize
+    # obtain source memoryview
+    source_mv = memoryview(source)
+    source_pb = PyMemoryView_GET_BUFFER(source_mv)
+    if not PyBuffer_IsContiguous(source_pb, b'A'):
+        raise BufferError("`source` must contain contiguous memory")
+
+    # extract metadata
+    source_ptr = <const char*>source_pb.buf
+    nbytes = source_pb.len
+    itemsize = source_pb.itemsize
 
     # determine shuffle
     if shuffle == AUTOSHUFFLE:
@@ -280,46 +288,40 @@ def compress(source, char* cname, int clevel, int shuffle=SHUFFLE,
         raise ValueError('invalid shuffle argument; expected -1, 0, 1 or 2, found %r' %
                          shuffle)
 
-    try:
-
-        # setup destination
-        dest = PyBytes_FromStringAndSize(NULL, nbytes + BLOSC_MAX_OVERHEAD)
-        dest_ptr = PyBytes_AS_STRING(dest)
-
-        # perform compression
-        if _get_use_threads():
-            # allow blosc to use threads internally
+    # setup destination
+    dest = PyBytes_FromStringAndSize(NULL, nbytes + BLOSC_MAX_OVERHEAD)
+    dest_ptr = PyBytes_AS_STRING(dest)
 
-            # N.B., we are using blosc's global context, and so we need to use a lock
-            # to ensure no-one else can modify the global context while we're setting it
-            # up and using it.
-            with get_mutex():
+    # perform compression
+    if _get_use_threads():
+        # allow blosc to use threads internally
 
-                # set compressor
-                compressor_set = blosc_set_compressor(cname)
-                if compressor_set < 0:
-                    # shouldn't happen if we checked against list of compressors
-                    # already, but just in case
-                    err_bad_cname(cname_str)
+        # N.B., we are using blosc's global context, and so we need to use a lock
+        # to ensure no-one else can modify the global context while we're setting it
+        # up and using it.
+        with get_mutex():
 
-                # set blocksize
-                blosc_set_blocksize(blocksize)
+            # set compressor
+            compressor_set = blosc_set_compressor(cname)
+            if compressor_set < 0:
+                # shouldn't happen if we checked against list of compressors
+                # already, but just in case
+                err_bad_cname(cname_str)
 
-                # perform compression
-                with nogil:
-                    cbytes = blosc_compress(clevel, shuffle, itemsize, nbytes, source_ptr,
-                                            dest_ptr, nbytes + BLOSC_MAX_OVERHEAD)
+            # set blocksize
+            blosc_set_blocksize(blocksize)
 
-        else:
+            # perform compression
             with nogil:
-                cbytes = blosc_compress_ctx(clevel, shuffle, itemsize, nbytes, source_ptr,
-                                            dest_ptr, nbytes + BLOSC_MAX_OVERHEAD,
-                                            cname, blocksize, 1)
+                cbytes = blosc_compress(clevel, shuffle, itemsize, nbytes, source_ptr,
+                                        dest_ptr, nbytes + BLOSC_MAX_OVERHEAD)
 
-    finally:
+    else:
+        with nogil:
+            cbytes = blosc_compress_ctx(clevel, shuffle, itemsize, nbytes, source_ptr,
+                                        dest_ptr, nbytes + BLOSC_MAX_OVERHEAD,
+                                        cname, blocksize, 1)
 
-        # release buffers
-        source_buffer.release()
 
     # check compression was successful
     if cbytes <= 0:
@@ -350,15 +352,22 @@ def decompress(source, dest=None):
     """
     cdef:
         int ret
-        char *source_ptr
-        char *dest_ptr
-        Buffer source_buffer
-        Buffer dest_buffer = None
+        memoryview source_mv
+        const Py_buffer* source_pb
+        const char* source_ptr
+        memoryview dest_mv
+        Py_buffer* dest_pb
+        char* dest_ptr
         size_t nbytes, cbytes, blocksize
 
-    # setup source buffer
-    source_buffer = Buffer(source, PyBUF_ANY_CONTIGUOUS)
-    source_ptr = source_buffer.ptr
+    # obtain source memoryview
+    source_mv = memoryview(source)
+    source_pb = PyMemoryView_GET_BUFFER(source_mv)
+    if not PyBuffer_IsContiguous(source_pb, b'A'):
+        raise BufferError("`source` must contain contiguous memory")
+
+    # get source pointer
+    source_ptr = <const char*>source_pb.buf
 
     # determine buffer size
     blosc_cbuffer_sizes(source_ptr, &nbytes, &cbytes, &blocksize)
@@ -367,36 +376,28 @@ def decompress(source, dest=None):
     if dest is None:
         # allocate memory
         dest = PyBytes_FromStringAndSize(NULL, nbytes)
-        dest_ptr = PyBytes_AS_STRING(dest)
-        dest_nbytes = nbytes
     else:
-        arr = ensure_contiguous_ndarray(dest)
-        dest_buffer = Buffer(arr, PyBUF_ANY_CONTIGUOUS | PyBUF_WRITEABLE)
-        dest_ptr = dest_buffer.ptr
-        dest_nbytes = dest_buffer.nbytes
-
-    try:
-
-        # guard condition
-        if dest_nbytes < nbytes:
-            raise ValueError('destination buffer too small; expected at least %s, '
-                             'got %s' % (nbytes, dest_nbytes))
-
-        # perform decompression
-        if _get_use_threads():
-            # allow blosc to use threads internally
-            with nogil:
-                ret = blosc_decompress(source_ptr, dest_ptr, nbytes)
-        else:
-            with nogil:
-                ret = blosc_decompress_ctx(source_ptr, dest_ptr, nbytes, 1)
-
-    finally:
-
-        # release buffers
-        source_buffer.release()
-        if dest_buffer is not None:
-            dest_buffer.release()
+        dest = ensure_contiguous_ndarray(dest)
+
+    # obtain dest memoryview
+    dest_mv = memoryview(dest)
+    dest_pb = PyMemoryView_GET_BUFFER(dest_mv)
+    dest_ptr = <char*>dest_pb.buf
+    dest_nbytes = dest_pb.len
+
+    # guard condition
+    if dest_nbytes < nbytes:
+        raise ValueError('destination buffer too small; expected at least %s, '
+                         'got %s' % (nbytes, dest_nbytes))
+
+    # perform decompression
+    if _get_use_threads():
+        # allow blosc to use threads internally
+        with nogil:
+            ret = blosc_decompress(source_ptr, dest_ptr, nbytes)
+    else:
+        with nogil:
+            ret = blosc_decompress_ctx(source_ptr, dest_ptr, nbytes, 1)
 
     # handle errors
     if ret <= 0:
@@ -433,14 +434,22 @@ def decompress_partial(source, start, nitems, dest=None):
         int encoding_size
         int nitems_bytes
         int start_bytes
-        char *source_ptr
-        char *dest_ptr
-        Buffer source_buffer
-        Buffer dest_buffer = None
-
-    # setup source buffer
-    source_buffer = Buffer(source, PyBUF_ANY_CONTIGUOUS)
-    source_ptr = source_buffer.ptr
+        const char* source_ptr
+        memoryview source_mv
+        const Py_buffer* source_pb
+        memoryview dest_mv
+        Py_buffer* dest_pb
+        char* dest_ptr
+        size_t dest_nbytes
+
+    # obtain source memoryview
+    source_mv = memoryview(source)
+    source_pb = PyMemoryView_GET_BUFFER(source_mv)
+    if not PyBuffer_IsContiguous(source_pb, b"A"):
+        raise BufferError("`source` must contain contiguous memory")
+
+    # setup source pointer
+    source_ptr = <const char*>source_pb.buf
 
     # get encoding size from source buffer header
     encoding_size = source[3]
@@ -451,26 +460,22 @@ def decompress_partial(source, start, nitems, dest=None):
 
     # setup destination buffer
     if dest is None:
+        # allocate memory
         dest = PyBytes_FromStringAndSize(NULL, nitems_bytes)
-        dest_ptr = PyBytes_AS_STRING(dest)
-        dest_nbytes = nitems_bytes
     else:
-        arr = ensure_contiguous_ndarray(dest)
-        dest_buffer = Buffer(arr, PyBUF_ANY_CONTIGUOUS | PyBUF_WRITEABLE)
-        dest_ptr = dest_buffer.ptr
-        dest_nbytes = dest_buffer.nbytes
+        dest = ensure_contiguous_ndarray(dest)
+
+    # obtain dest memoryview
+    dest_mv = memoryview(dest)
+    dest_pb = PyMemoryView_GET_BUFFER(dest_mv)
+    dest_ptr = <char*>dest_pb.buf
+    dest_nbytes = dest_pb.len
 
     # try decompression
-    try:
-        if dest_nbytes < nitems_bytes:
-            raise ValueError('destination buffer too small; expected at least %s, '
-                             'got %s' % (nitems_bytes, dest_nbytes))
-        ret = blosc_getitem(source_ptr, start, nitems, dest_ptr)
-
-    finally:
-        source_buffer.release()
-        if dest_buffer is not None:
-            dest_buffer.release()
+    if dest_nbytes < nitems_bytes:
+        raise ValueError('destination buffer too small; expected at least %s, '
+                         'got %s' % (nitems_bytes, dest_nbytes))
+    ret = blosc_getitem(source_ptr, start, nitems, dest_ptr)
 
     # ret refers to the number of bytes returned from blosc_getitem.
     if ret <= 0: