java/unsafe-double-checked-locking: missed when the field is re-read through a call that returns it #22968

Description of the issue

The query reports double-checked locking on the non-volatile field x in DclSeed.m(): the field is read into a local z, checked against null outside and again inside the synchronized block, and initialised there. DclVariant.m() is the same except the read inside the synchronized block is java.util.Collections.singletonList(x).get(0), which just returns the field's value. Nothing is reported for DclVariant.

Probably getAFieldRead in DoubleCheckedLocking.qll: a local counts as a field read when its assignment source is a field access or another such local, so a value coming back through a call isn't followed, and the inner z == null is no longer a null check of x. SpotBugs' DC_DOUBLECHECK missed the same pair (spotbugs/spotbugs#4212).

Query: java/unsafe-double-checked-locking (codeql/java-queries 1.1.10)

CodeQL CLI: 2.27.2

Code samples

seed/DclSeed.java:

class DclSeed {
    Object x;

    void m() {
        Object z = x;
        if (z == null) {
            synchronized (this) {                 // query fires here
                z = x;
                if (z == null) {
                    x = z = new Object();
                }
            }
        }
    }
}

variant/DclVariant.java:

class DclVariant {
    Object x;

    void m() {
        Object z = x;
        if (z == null) {
            synchronized (this) {                 // query silent
                z = java.util.Collections.singletonList(x).get(0);
                if (z == null) {
                    x = z = new Object();
                }
            }
        }
    }
}

How to reproduce

codeql database create db --language=java --build-mode=none \
    --source-root=<one directory> --overwrite
codeql database analyze db \
    codeql/java-queries:codeql-suites/java-security-and-quality.qls \
    --format=csv --output=out.csv
# one database per directory

Expected result

"Double-checked locking on the non-volatile field x is not thread-safe" on the synchronized block of DclVariant.m(), as on DclSeed.m().

See also #22967.

Output of the commands above:

### CodeQL 2.27.2  query: java/unsafe-double-checked-locking
### suite: java-security-and-quality.qls, --build-mode=none, per-file isolated databases
### extraction JDK: openjdk version "21.0.11" 2026-04-21 LTS

--- SEED -- expected: query fires (seed/)
"Double-checked locking is not thread-safe","A repeated check on a non-volatile field is not thread-safe, and

--- VARIANT -- expected: query fires, actual: not reported (the bug) (variant/)
    (query reported nothing)