Clarify that we don't hop ELs in our callbacks (#526)

Author: Lukasa

Sources/NIOSSL/CustomPrivateKey.swift

@@ -156,7 +156,7 @@ extension SSLConnection {
         inputBytes.writeBytes(`in`)
 
         let result = customKey.sign(channel: channel, algorithm: wrappedAlgorithm, data: inputBytes)
-        result.whenComplete { signingResult in
+        result.hop(to: channel.eventLoop).assumeIsolated().whenComplete { signingResult in
             self.storeCustomPrivateKeyResult(signingResult, channel: channel)
         }
 
@@ -178,7 +178,7 @@ extension SSLConnection {
         inputBytes.writeBytes(`in`)
 
         let result = customKey.decrypt(channel: channel, data: inputBytes)
-        result.whenComplete { decryptionResult in
+        result.hop(to: channel.eventLoop).assumeIsolated().whenComplete { decryptionResult in
             self.storeCustomPrivateKeyResult(decryptionResult, channel: channel)
         }
 
@@ -207,7 +207,7 @@ extension SSLConnection {
         // When we complete here we need to set our result state, and then ask to respin the handshake.
         // If we can't respin the handshake because we've dropped the parent handler, that's fine, no harm no foul.
         // For that reason, we tolerate both the verify manager and the parent handler being nil.
-        channel.eventLoop.execute {
+        channel.eventLoop.assumeIsolated().execute {
             precondition(self.customPrivateKeyResult == nil)
             self.customPrivateKeyResult = result
             self.parentHandler?.resumeHandshake()

Sources/NIOSSL/SSLCallbacks.swift

@@ -333,10 +333,10 @@ extension CustomContextManager {
             let promise = eventLoop.makePromise(of: NIOSSLContextConfigurationOverride.self)
             self.callback(values, promise)
 
-            promise.futureResult.whenComplete { result in
+            promise.futureResult.assumeIsolated().whenComplete { result in
                 // Ensure we execute any completion on the next event loop tick
                 // This ensures that we suspend before calling resume
-                eventLoop.execute {
+                eventLoop.assumeIsolated().execute {
                     connection.parentContext.customContextManager?.state = .complete(result)
                     connection.parentHandler?.resumeHandshake()
                 }
@@ -445,11 +445,11 @@ extension CustomVerifyManager {
 
         // We need to attach our "do the thing" callback. This will always invoke the "ask me again" API, and it will do so in a separate
         // event loop tick to avoid awkward re-entrancy with this method.
-        promise.futureResult.whenComplete { result in
+        promise.futureResult.assumeIsolated().whenComplete { result in
             // When we complete here we need to set our result state, and then ask to respin certificate verification.
             // If we can't respin verification because we've dropped the parent handler, that's fine, no harm no foul.
             // For that reason, we tolerate both the verify manager and the parent handler being nil.
-            eventLoop.execute {
+            eventLoop.assumeIsolated().execute {
                 // Note that we don't close over self here: that's to deal with the fact that this is a struct, and we don't want to
                 // escape the mutable ownership of self.
                 precondition(