Add some really cockeyed support for MySQL 5.7 users.

Sources/QueuesFluentDriver/FluentQueue.swift

@@ -1,12 +1,15 @@
 @preconcurrency import Queues
 @preconcurrency import SQLKit
+import NIOConcurrencyHelpers
 
 /// An implementation of `Queue` which stores job data and metadata in a Fluent database.
 public struct FluentQueue: Queue, Sendable {
     // See `Queue.context`.
     public let context: QueueContext
 
     let sqlDb: any SQLDatabase
+
+    let _sqlLockingClause: NIOLockedValueBox<(any SQLExpression)?> = .init(nil) // needs a lock for the queue to be `Sendable`
 
     // See `Queue.get(_:)`.
     public func get(_ id: JobIdentifier) -> EventLoopFuture<JobData> {
@@ -56,6 +59,27 @@ public struct FluentQueue: Queue, Sendable {
     // See `Queue.pop()`.
     public func pop() -> EventLoopFuture<JobIdentifier?> {
         self.sqlDb.eventLoop.makeFutureWithTask {
+            // Special case: For MySQL < 8.0, we can't use `SKIP LOCKED`. This is a really hackneyed solution,
+            // but we need to execute a database query to get the version information, `makeQueue(with:)`
+            // is purely synchronous, and `SQLDatabase.version` is not implemented in MySQLKit at the time
+            // of this writing.
+            if self._sqlLockingClause.withLockedValue({ $0 }) == nil {
+                switch self.sqlDb.dialect.name {
+                case "mysql":
+                    let version = try await self.sqlDb.select()
+                        .column(SQLFunction("version"), as: "version")
+                        .first(decodingColumn: "version", as: String.self) ?? "" // always returns one row
+                    // This is a really lazy check and it knows it; we know MySQLNIO doesn't support versions older than 5.x.
+                    if version.starts(with: "5.") || !(version.first?.isNumber ?? false) {
+                        self._sqlLockingClause.withLockedValue { $0 = SQLLockingClause.update }
+                    } else {
+                        fallthrough
+                    }
+                default:
+                    self._sqlLockingClause.withLockedValue { $0 = SQLLockingClauseWithSkipLocked.updateSkippingLocked }
+                }
+            }
+
             let select = self.sqlDb
                 .select()
                 .column("id")
@@ -65,7 +89,7 @@ public struct FluentQueue: Queue, Sendable {
                 .where(.dateValue(.function("coalesce", SQLColumn("delay_until"), SQLNow())), .lessThanOrEqual, .now())
                 .orderBy("delay_until")
                 .limit(1)
-                .lockingClause(SQLLockingClauseWithSkipLocked.updateSkippingLocked)
+                .lockingClause(self._sqlLockingClause.withLockedValue { $0! }) // we've always set it by the time we get here
 
             if self.sqlDb.dialect.supportsReturning {
                 return try await self.sqlDb.update(JobModel.schema)