_rd_compressed_file: read signals in 1048576-sample chunks.

A bug in libsndfile [1] means that if we try to read a chunk of more
than 16777216 (2**24) total samples (across all channels) from a FLAC
file, and the number of channels is not a power of two, then
sf_read_short will return zero.

Work around this bug by allocating an array ourselves and reading a
small block of samples at a time (a FLAC file has at most 8 channels,
at most 7 if not a power of two, and 7 * 1048576 < 16777216).

[1] libsndfile/libsndfile#431

Author: Benjamin Moody

wfdb/io/_signal.py

@@ -1875,7 +1875,23 @@ def _rd_compressed_file(
             start_samp = start_frame * samps_per_frame[0]
             end_samp = end_frame * samps_per_frame[0]
             sf.seek(start_samp + sample_offset)
-            sig_data = sf.read(end_samp - start_samp, dtype=read_dtype)
+
+            # We could do this:
+            #  sig_data = sf.read(end_samp - start_samp, dtype=read_dtype)
+            # However, sf.read fails for huge blocks (over 2**24 total
+            # samples) due to a bug in libsndfile:
+            # https://github.com/libsndfile/libsndfile/issues/431
+            # So read the data in chunks instead.
+            n_samp = end_samp - start_samp
+            sig_data = np.empty((n_samp, n_sig), dtype=read_dtype)
+            CHUNK_SIZE = 1024 * 1024
+            for chunk_start in range(0, n_samp, CHUNK_SIZE):
+                chunk_end = chunk_start + CHUNK_SIZE
+                chunk_data = sf.read(out=sig_data[chunk_start:chunk_end])
+                samples_read = chunk_data.shape[0]
+                if samples_read != CHUNK_SIZE:
+                    sig_data = sig_data[: chunk_start + samples_read]
+                    break
 
             # If we read an 8-bit stream as int16 or a 24-bit stream as
             # int32, soundfile shifts each sample left by 8 bits.  We