feat(docker): tunable Solr heap via SOLR_HEAP (mendelu, 9.1)

[jr-rk]

Problem

On DSpace 9.x (customer/mendelu, 9.1) 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/jcu (9.3).
• 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

This PR makes the Solr JVM heap size configurable in the 9.x docker-compose-rest.yml stack by explicitly forwarding SOLR_HEAP through runuser into solr-foreground, with a default of 4g when unset. This aligns the 9.x Solr heap default with the existing 7.x fleet behavior referenced in #1309, while still allowing operators to override it via .env.

Changes:

• Update the Solr container startup command to run solr-foreground with SOLR_HEAP="${SOLR_HEAP:-4g}" passed via env under runuser.
• Establish an explicit default heap of 4g for 9.x when SOLR_HEAP is not provided.

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