fix: use utf8mb4_unicode_520_ci for hashtags to fix BMP-outside collation (PR #6098)

MySQL/MariaDB's utf8mb4_unicode_ci collation treats all characters outside
the Basic Multilingual Plane as equal, conflating distinct same-length
hashtags (e.g. Shavian vs cuneiform) on the unique name/slug indexes.

Migrate the hashtags name/slug columns to utf8mb4_unicode_520_ci, which
differentiates supplementary-plane characters. Improvements over the
original PR:
- Use an explicit ALTER ... MODIFY, since Laravel's fluent ->change() emits
  no collation change on MySQL/MariaDB and silently no-ops.
- Match both 'mysql' and 'mariadb' drivers (Laravel 11+ reports MariaDB as a
  distinct driver, so a mysql-only check would skip the fix on MariaDB).
- Provide an accurate, reversible down() and preserve NOT NULL + unique keys.

Add feature tests covering the no-op path on non-MySQL drivers and, on
MySQL/MariaDB, that distinct BMP-outside hashtags coexist while same-slug
and case-insensitive dedup still work.

database/migrations/2025_07_31_164635_change_hashtags_collation.php

@@ -0,0 +1,76 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+
+return new class extends Migration
+{
+    /**
+     * The collation that correctly differentiates characters outside the
+     * Basic Multilingual Plane (codepoints >= 0x10000). The historic default
+     * of utf8mb4_unicode_ci treats all such characters as equal, which causes
+     * distinct hashtags (e.g. Shavian vs cuneiform of the same length) to
+     * collide on the unique name/slug indexes.
+     */
+    private const TARGET_COLLATION = 'utf8mb4_unicode_520_ci';
+
+    /**
+     * The collation to restore on rollback (the previous project default).
+     */
+    private const PREVIOUS_COLLATION = 'utf8mb4_unicode_ci';
+
+    /**
+     * Column definitions to keep intact while altering the collation. Both are
+     * VARCHAR(255) NOT NULL with unique indexes; MODIFY preserves the index.
+     */
+    private const COLUMNS = ['name', 'slug'];
+
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        $this->setCollation(self::TARGET_COLLATION);
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        $this->setCollation(self::PREVIOUS_COLLATION);
+    }
+
+    /**
+     * Apply the given collation to the hashtags name and slug columns.
+     *
+     * Laravel's fluent ->change() does not reliably emit a collation-only
+     * change on MySQL/MariaDB, so issue an explicit MODIFY per column.
+     */
+    private function setCollation(string $collation): void
+    {
+        if (! $this->isMysql()) {
+            return;
+        }
+
+        foreach (self::COLUMNS as $column) {
+            DB::statement(
+                'ALTER TABLE `hashtags` MODIFY `'.$column.'` '.
+                'VARCHAR(255) CHARACTER SET utf8mb4 COLLATE '.$collation.' NOT NULL'
+            );
+        }
+    }
+
+    /**
+     * This migration only applies to MySQL/MariaDB. Postgres compares
+     * hashtags with ILIKE (no collation quirk) and other drivers (e.g. the
+     * sqlite test database) do not support these collations.
+     *
+     * Note: Laravel 11+ reports MariaDB as the distinct "mariadb" driver, so
+     * both must be matched here.
+     */
+    private function isMysql(): bool
+    {
+        return in_array(DB::connection()->getDriverName(), ['mysql', 'mariadb'], true);
+    }
+};

tests/Feature/HashtagCollationTest.php

@@ -0,0 +1,108 @@
+<?php
+
+use App\Models\Hashtag;
+use Illuminate\Foundation\Testing\LazilyRefreshDatabase;
+use Illuminate\Support\Facades\Artisan;
+use Illuminate\Support\Facades\DB;
+
+uses(LazilyRefreshDatabase::class);
+
+/*
+|--------------------------------------------------------------------------
+| Hashtag collation fix (PR #6098)
+|--------------------------------------------------------------------------
+|
+| Verifies that the hashtags collation migration runs safely on all drivers
+| and, on MySQL/MariaDB, prevents conflation of distinct hashtags that use
+| characters outside the Basic Multilingual Plane (codepoints >= 0x10000).
+|
+*/
+
+it('migration runs without error regardless of database driver', function () {
+    // The test suite uses sqlite by default. The migration detects the driver
+    // and no-ops gracefully rather than attempting unsupported ALTER syntax.
+    Artisan::call('migrate', [
+        '--path' => 'database/migrations/2025_07_31_164635_change_hashtags_collation.php',
+        '--force' => true,
+    ]);
+
+    // If we reach here without exception the no-op path worked.
+    expect(true)->toBeTrue();
+});
+
+it('migration rollback runs without error regardless of database driver', function () {
+    Artisan::call('migrate', [
+        '--path' => 'database/migrations/2025_07_31_164635_change_hashtags_collation.php',
+        '--force' => true,
+    ]);
+
+    Artisan::call('migrate:rollback', [
+        '--path' => 'database/migrations/2025_07_31_164635_change_hashtags_collation.php',
+        '--force' => true,
+    ]);
+
+    expect(true)->toBeTrue();
+});
+
+it('distinct BMP-outside hashtags do not collide on the unique index', function () {
+    // This test is only meaningful on MySQL/MariaDB where the collation fix matters.
+    if (! in_array(DB::connection()->getDriverName(), ['mysql', 'mariadb'], true)) {
+        $this->markTestSkipped('Collation behavior is MySQL/MariaDB-specific.');
+    }
+
+    // Ensure the migration has been applied.
+    Artisan::call('migrate', [
+        '--path' => 'database/migrations/2025_07_31_164635_change_hashtags_collation.php',
+        '--force' => true,
+    ]);
+
+    // Two distinct 5-character hashtags using characters outside the BMP.
+    // Under the old utf8mb4_unicode_ci collation these were considered equal.
+    $shavian = '𐑖𐑱𐑝𐑾𐑯';     // Shavian script
+    $cuneiform = '𒆳𒆍𒀭𒊏𒆠';   // Cuneiform script
+
+    $tag1 = Hashtag::create(['name' => $shavian, 'slug' => $shavian]);
+    $tag2 = Hashtag::create(['name' => $cuneiform, 'slug' => $cuneiform]);
+
+    // Both must coexist as separate rows with distinct IDs.
+    expect($tag1->id)->not->toBe($tag2->id);
+    expect(Hashtag::where('slug', $shavian)->first()->id)->toBe($tag1->id);
+    expect(Hashtag::where('slug', $cuneiform)->first()->id)->toBe($tag2->id);
+});
+
+it('same-script hashtags with the same slug still correctly deduplicate', function () {
+    // Sanity check: two identical hashtags should NOT create duplicates.
+    if (! in_array(DB::connection()->getDriverName(), ['mysql', 'mariadb'], true)) {
+        $this->markTestSkipped('Collation behavior is MySQL/MariaDB-specific.');
+    }
+
+    Artisan::call('migrate', [
+        '--path' => 'database/migrations/2025_07_31_164635_change_hashtags_collation.php',
+        '--force' => true,
+    ]);
+
+    $tag = Hashtag::firstOrCreate(['slug' => 'hello'], ['name' => 'hello']);
+    $same = Hashtag::firstOrCreate(['slug' => 'hello'], ['name' => 'hello']);
+
+    expect($tag->id)->toBe($same->id);
+    expect(Hashtag::where('slug', 'hello')->count())->toBe(1);
+});
+
+it('case-insensitivity is preserved after collation change', function () {
+    // utf8mb4_unicode_520_ci is still case-insensitive, so #Hello == #hello.
+    if (! in_array(DB::connection()->getDriverName(), ['mysql', 'mariadb'], true)) {
+        $this->markTestSkipped('Collation behavior is MySQL/MariaDB-specific.');
+    }
+
+    Artisan::call('migrate', [
+        '--path' => 'database/migrations/2025_07_31_164635_change_hashtags_collation.php',
+        '--force' => true,
+    ]);
+
+    Hashtag::create(['name' => 'Pixelfed', 'slug' => 'pixelfed']);
+
+    // Case-insensitive lookup should find it with different casing.
+    $found = Hashtag::where('slug', 'PIXELFED')->first();
+    expect($found)->not->toBeNull();
+    expect($found->slug)->toBe('pixelfed');
+});