Problem

The Solr heap in docker/docker-compose-rest.yml is hardcoded to -m 4g. Combined with
-XX:+AlwaysPreTouch, the container commits the full 4 GB on start, which is wasteful on
shared dev hosts running several DSpace stacks. Operators cannot lower the heap without
editing a tracked file.

Backport of #1309 to customer/zcu-pub.
Tracking issue: dataquest-dev/dspace-customers#746

Root cause

The Solr launch command was written with a literal heap argument (-m 4g) instead of an
environment-driven one, so there is no runtime knob and AlwaysPreTouch forces the full
allocation up front.

Change set

Single-line change to the dspacesolr entrypoint in docker/docker-compose-rest.yml:

-      exec solr -p 898${INSTANCE} -f -m 4g
+      exec solr -p 898${INSTANCE} -f -m "${SOLR_HEAP:-4g}"

Translated, not cherry-picked (per the FE backport model). On this branch (7.6.1 family)
the Solr launch uses -p 898${INSTANCE}, not the source PR's -p 8983, so only the -m 4g
argument was changed; the port and every other line are untouched. The value is quoted
("${SOLR_HEAP:-4g}") to survive word-splitting, matching the source PR's follow-up fix.

Intentionally out of scope: the default heap value (stays 4g), other services, ports,
Solr image/version, and AlwaysPreTouch.

Test evidence

Static fingerprints:

$ grep -n 'm "${SOLR_HEAP:-4g}"' docker/docker-compose-rest.yml
164:      exec solr -p 898${INSTANCE} -f -m "${SOLR_HEAP:-4g}"

$ grep -n 'm 4g' docker/docker-compose-rest.yml
(no matches — hardcoded value gone)

$ git diff --stat
 docker/docker-compose-rest.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Compose render (Docker Compose interpolates ${SOLR_HEAP:-4g}, demonstrating both paths;
INSTANCE=0 set only to silence unset-var noise):

# Default path — SOLR_HEAP unset:
$ docker compose -f docker/docker-compose-rest.yml config | grep 'exec solr'
      exec solr -p 8980 -f -m "4g"

# Override path — SOLR_HEAP=1g:
$ SOLR_HEAP=1g docker compose -f docker/docker-compose-rest.yml config | grep 'exec solr'
      exec solr -p 8980 -f -m "1g"

# Valid render:
$ docker compose -f docker/docker-compose-rest.yml config --quiet ; echo $?
0

Default preserved (4g), override honored (1g), YAML/interpolation valid.

Risk & rollback

• Risk: Very low — single-line, default-preserving config change. Main failure mode would
  be an unquoted/typo'd interpolation; mitigated by the quoting and the compose-render check above.
• Backward compatible: Full. Unset SOLR_HEAP reproduces the prior 4 GB behavior exactly;
  existing .env/compose overrides that don't set SOLR_HEAP keep working.
• Rollback: Revert this one-line change (or the PR). No state, migration, or data implications.

Notes / assumptions

• Source PR: Make Solr heap tunable via SOLR_HEAP (default 4g, no behavior change) #1309 (commits 27253a3 env var, 0d652dc quoting).
• Sibling target customer/mendelu from the tracking issue is not included: that branch
  (DSpace 9.1) launches Solr via solr-foreground and has no hardcoded -m 4g line to
  translate, so the backport does not apply there. Reported separately on the tracking issue.

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