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"""Atomic append of gzipped data.
The point is - if several gzip streams are concatenated,
they are read back as one whole stream.
"""
import gzip
import io
__all__ = ('gzip_append',)
def gzip_append(filename: str, data: bytes, level: int = 6) -> None:
"""Append a block of data to file with safety checks."""
# compress data
buf = io.BytesIO()
with gzip.GzipFile(fileobj=buf, compresslevel=level, mode="w") as g:
g.write(data)
zdata = buf.getvalue()
# append, safely
with open(filename, "ab+", 0) as f:
f.seek(0, 2)
pos = f.tell()
try:
f.write(zdata)
except Exception as ex:
# rollback on error
f.seek(pos, 0)
f.truncate()
raise ex
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