feat(docker): tunable Solr heap via SOLR_HEAP (jcu, 9.3)

[jr-rk]

Problem

On DSpace 9.x (customer/jcu, 9.3) the Solr container has no heap knob. It launches via
runuser -u solr -- solr-foreground with no -m flag, so Solr falls back to the image default
heap (512m) and operators can't tune it without editing this tracked compose file. The whole
7.x customer fleet runs 4g (source PR #1309); 9.x had no equivalent.

Carved out of dataquest-dev/dspace-customers#746, which marked the 9.x branches N/A for the
-m 4g backport (no anchor to translate) and flagged a tunable heap as a separate feature.

Root cause

The 9.x Solr entrypoint never passed a heap argument, and solr-foreground runs under a runuser
user switch, so no SOLR_HEAP reached Solr's start scripts.

Change set

One line in docker/docker-compose-rest.yml, the Solr entrypoint's final command:

-      runuser -u solr -- solr-foreground
+      runuser -u solr -- env SOLR_HEAP="${SOLR_HEAP:-4g}" solr-foreground

• env SOLR_HEAP=… forwards the value explicitly through runuser into the solr user's
  environment, where solr-foreground honours it natively — no dependence on runuser env passing.
• Compose interpolates ${SOLR_HEAP:-4g} from the host/.env at parse time; quoted to survive
  word-splitting.
• Default decision: 4g when unset — aligns 9.x with the 7.x fleet and Make Solr heap tunable via SOLR_HEAP (default 4g, no behavior change) #1309. This is a
  deliberate behavior change on 9.x (image default 512m → 4g). Override with SOLR_HEAP=1g
  in .env. No other line changed.

Test evidence

docker compose config renders the interpolated entrypoint correctly:

# unset  -> default 4g
$ SOLR_HEAP= docker compose -f docker/docker-compose.yml -f docker/docker-compose-rest.yml config | grep solr-foreground
        runuser -u solr -- env SOLR_HEAP="4g" solr-foreground

# override
$ SOLR_HEAP=1g docker compose ... config | grep solr-foreground
        runuser -u solr -- env SOLR_HEAP="1g" solr-foreground

Risk & rollback

Low. Infra-only, single line. Default heap rises 512m → 4g on 9.x (intended; matches fleet). If a
host lacks 4g, set SOLR_HEAP lower in .env. Rollback = revert this commit.

Notes / assumptions

• Sibling PR opens the identical change on customer/mendelu (9.1).
• Out of scope: 7.x branches (UFAL/Shibboleth the user is not logged in after email with verification token #746), other services/ports, the Solr image/version.
• No standalone tracking issue created (per request this is delivered as PRs); source context is
  Make Solr heap tunable via SOLR_HEAP (default 4g, no behavior change) #1309 + dataquest-dev/dspace-customers#746.

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[Copilot]

Pull request overview

Adds a configurable JVM heap setting for the Solr container in the REST Docker Compose stack, allowing operators to tune Solr memory without modifying tracked compose files (defaulting to 4g when unset).

Changes:

• Updates the Solr entrypoint command to explicitly forward SOLR_HEAP through runuser via env.
• Introduces a default heap value of 4g when SOLR_HEAP is not provided (intentional behavior change vs image default).

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