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CA-396751: write updated RRDD data before headers #5915

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Aug 13, 2024
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CA-396751: write updated RRDD data before headers #5915

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24 changes: 16 additions & 8 deletions ocaml/xcp-rrdd/scripts/rrdd/rrdd.py
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Original file line number Diff line number Diff line change
Expand Up @@ -283,7 +283,8 @@ def wait_until_next_reading(self, neg_shift=1):
self.lazy_complete_init()
next_reading = self.register()
wait_time = next_reading - neg_shift
if wait_time < 0: wait_time %= self.frequency_in_seconds
if wait_time < 0:
wait_time %= self.frequency_in_seconds
time.sleep(wait_time)
return
except socket.error:
Expand All @@ -310,20 +311,27 @@ def update(self):
metadata_json = json.dumps(metadata, sort_keys=True).encode('utf-8')
metadata_checksum = crc32(metadata_json) & 0xffffffff

self.dest.seek(0)
self.dest.write('DATASOURCES'.encode())
self.dest.write(pack(">LLLQ",
data_checksum,
metadata_checksum,
len(self.datasources),
timestamp))
# First write the updated data and metadata
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encoded_datasource_header = 'DATASOURCES'.encode()
# DATASOURCES + 20 for 32 + 32 + 32 + 64
self.dest.seek(len(encoded_datasource_header) + 20)
for val in data_values:
# This is already big endian encoded
self.dest.write(val)
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The reader can do its reading at any time? Is it reasonable to assume that this action of writing the the data/metadata is atomic?

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the reader should not read any more than the header unless and until the timestamp in the header changes from the last data read. Once that changes the data can be read.

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@lindig lindig Aug 7, 2024 •
edited
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Header and data are protected by checksums. So if you read garbage, the checksum should tell you. You can't avoid reading garbage in the general case because there is always a race between the data and its checksum being written as no locking is implemented.

The total amount of data can change, and in particular shrink. I believe the memory-mapped file is of constant size though.

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Ok, I think at least it is better to write the data before writing the header, that sounds like a safer appraoch than the other way round


self.dest.write(pack(">L", len(metadata_json)))
self.dest.write(metadata_json)
self.dest.flush()

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# Now write the updated header
self.dest.seek(0)
self.dest.write(encoded_datasource_header)
self.dest.write(pack(">LLLQ",
data_checksum,
metadata_checksum,
len(self.datasources),
timestamp))
self.dest.flush()
self.datasources = []
time.sleep(
0.003) # wait a bit to ensure wait_until_next_reading will block
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