C. Scott Andreas created CASSANDRA-21505:
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Summary: PaxosV1MultiNodeTableWalkTest Failure - CAS on static
column w/empty bytes
Key: CASSANDRA-21505
URL: https://issues.apache.org/jira/browse/CASSANDRA-21505
Project: Apache Cassandra
Issue Type: Bug
Components: Feature/Lightweight Transactions
Reporter: C. Scott Andreas
{{PaxosV1MultiNodeTableWalkTest}} fails deterministically at a {{UPDATE ... IF
<double static column> = <empty bytes>}} CAS. The Harry model predicts the CAS
does not apply (expecting a failed-CAS result of {{{}[applied], s0, s1{}}}),
while Paxos v1 applies it (returning {{[applied]}} only). The mismatch surfaces
as an assertion in the model when it validates the returned rows against the
expected columns.
The disagreement is in how {{s1 = <empty bytes>}} is evaluated when {{s1}} is a
{{double}} static column whose current value is empty bytes. This bug covers
determining whether Paxos v1's evaluation or the Harry model's expectation is
correct; and updating either the implementation or test.
*Environment*
* Apache Cassandra trunk at {{1332e21}}
* {{CasMultiNodeTableWalkBase}} uses {{ReadRepairStrategy.NONE}} and
{{paxos_variant = v1}} (this is a pure Paxos v1 path; no read repair).
* Reproduced locally with JDK 11 (in-JVM dtest). Observed in CI on
{{{}jvm21-dtests{}}}.
*Reproducing*
{{org.apache.cassandra.distributed.test.cql3.PaxosV1MultiNodeTableWalkTest}}
(3-node cluster), pinned to the failing seed via the harness hook:
// in
PaxosV1MultiNodeTableWalkTest.preCheck(...):builder.withSeed(-570398361159823834L);
then:
{{ant test-jvm-dtest-some
-Dtest.name=org.apache.cassandra.distributed.test.cql3.PaxosV1MultiNodeTableWalkTest
-Dtest.methods=test}}
It fails at step 387 (of 400) deterministically ({{{}Seed =
-570398361159823834, Examples = 10{}}}).
*Failure Observed*
Schema:
{{CREATE TABLE ks1.tbl (
pk0 timeuuid, ck0 inet,
s0 boolean static, s1 double static,
v0 uuid, v1 frozen<set<frozen<tuple<float>>>>,
PRIMARY KEY (pk0, ck0)
)}}
Shrunk operation history:
{{INSERT INTO ks1.tbl (pk0, ck0, s0, s1, v0, v1)
VALUES (00000000-0000-1200-b500-000000000000, '3484:...:762b', false, <empty
bytes>, ...); -- s1 set to empty bytes
...}}
{{UPDATE ks1.tbl SET s1 = <double> WHERE pk0 =
00000000-0000-1200-b500-000000000000
IF s0 = false AND s1 = <empty bytes>;}}
Model annotation for the CAS: "would apply false" (the model expects the
condition to be false, so the CAS should not apply).
Assertion (Harry model validating the returned rows):
{{java.lang.AssertionError: actual rows do not match the schema [[applied], s0,
s1];
found [java.nio.HeapByteBuffer[pos=0 lim=1 cap=1]]
at org.apache.cassandra.harry.model.ASTSingleTableModel
(ASTSingleTableModel.java:1474)}}
The model expected a failed-CAS result with three columns
({{{}[applied]=false{}}}, plus the current {{s0}} and {{{}s1{}}}), but the
actual result had a single 1-byte value — the shape of a CAS that applied
({{{}[applied]=true{}}}).
*Analysis*
* The row's {{s1}} (a {{double}} static column) was written as empty bytes.
The CAS condition compares {{{}s1 = <empty bytes>{}}}. Since the stored value
equals the compared value, Paxos v1 evaluates the condition as true and applies
the update (result {{{}[applied]{}}}, one column).
* The Harry model evaluated the same condition as false (annotating "would
apply false") and therefore expected the failed-CAS result shape {{{}[applied],
s0, s1{}}}.
* So the two disagree on the outcome of comparing a fixed-length numeric
column to an empty-bytes value. Empty bytes for a fixed-length type
({{{}double{}}} is 8 bytes) is a degenerate value; the disagreement is in
empty-value comparison semantics.
Two possibilities:
# Paxos v1 is correct (empty == empty, CAS applies) and the Harry model's
condition evaluation for empty bytes on a fixed-length column is wrong (model
bug in {{{}test/harry{}}}).
# The Harry model is correct and Paxos v1 mis-evaluates the condition for an
empty-bytes fixed-length column (product bug in the CAS condition path).
Determining which requires comparing Cassandra's CQL semantics for {{= <empty
bytes>}} on a {{{}double{}}}column against the model's evaluation, and checking
whether the row's {{s1}} is treated as empty vs null.
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