HIGH: No disk space validation before OCI installation

[e-gineer]

Bug Description

The OCI installer (both DB and FDW) does not check available disk space before beginning installation. If the disk fills up during installation, the system can be left in a broken state with old files deleted but new files incomplete.

Severity: HIGH

Location

• pkg/ociinstaller/db.go:16-41 (InstallDB function)
• pkg/ociinstaller/fdw.go:18-45 (InstallFdw function)

Problem

The installation process requires significant disk space for:

1. Downloading OCI image layers to temp directory
2. Extracting/unzipping archives
3. Moving files to final destination

Currently, there is no validation that sufficient disk space is available before starting. If disk fills up during any of these steps:

• Temp files may be partially written
• Archives may be partially extracted
• File moves may be incomplete
• Old installation files may already be deleted (see CRITICAL: FDW binary removed before verifying new installation succeeds #4753)

Result: System left in broken state without working database or FDW.

Impact

• Severity: HIGH
• User Impact: Database or FDW installation corrupted, system may be unrecoverable
• Frequency: More common on systems with limited disk space or small partitions
• Affected Operations: All OCI installations (DB, FDW, potentially plugins)

Example Scenario

1. User has 500MB free disk space
2. New DB version is 400MB compressed, 1.2GB uncompressed
3. Installation begins:
   • Downloads 400MB to temp (100MB free remaining)
   • Attempts to extract 1.2GB (disk full error)
4. Old DB files may be deleted, new files incomplete
5. System cannot start database

Recommended Fix

Add disk space validation before installation:

func InstallDB(ctx context.Context, dblocation string) (string, error) {
    // Check available disk space BEFORE starting
    requiredSpace := estimateRequiredSpace(constants.PostgresImageRef)
    availableSpace, err := getAvailableDiskSpace(dblocation)
    if err != nil {
        return "", fmt.Errorf("could not check disk space: %w", err)
    }
    
    if availableSpace < requiredSpace {
        return "", fmt.Errorf(
            "insufficient disk space: need ~%s, have %s available", 
            humanize.Bytes(requiredSpace),
            humanize.Bytes(availableSpace),
        )
    }
    
    // Proceed with installation...
    tempDir := ociinstaller.NewTempDir(dblocation)
    // ...
}

Space estimate: ~2x archive size (for download + extraction, accounting for compression)

Benefits

1. Fail fast with clear error message before partial installation
2. Better UX - users know immediately if they need to free up space
3. System safety - prevents corrupted installations
4. Clear guidance - error message tells user exactly how much space needed

Related Tests

• pkg/ociinstaller/db_test.go::TestInstallDB_DiskSpaceExhaustion_BugDocumentation

References

• Task 7 completion report: .ai/milestones/wave-3-untested-packages/tasks/task-7-ociinstaller-COMPLETED.md
• Related to CRITICAL: FDW binary removed before verifying new installation succeeds #4753 (FDW binary removal bug)