Add fuzzing failure cases (#1190)

Motivation:

grpc-swift is now fuzzed by oss-fuzz; we should keep track of any
failure cases to check we don't regress.

Modifications:

- Add test inputs to FuzzTesting/FailCases
- Add unit tests which run the input

Result:

Easier to catch regressions.

FuzzTesting/FailCases/clusterfuzz-testcase-minimized-grpc-swift-fuzz-debug-4645975625957376

@@ -0,0 +1,4 @@
+POST * HTTP/1.1
+
+POST * HTTP/1.1
+

FuzzTesting/FailCases/clusterfuzz-testcase-minimized-grpc-swift-fuzz-release-5413100925878272

@@ -0,0 +1,4 @@
+PUT * HTTP/1.1
+
+PUT * HTTP/1.1
+

FuzzTesting/README.md

@@ -24,3 +24,8 @@ xcrun --toolchain swift \
     -Xswiftc -sanitize=fuzzer,address \
     -Xswiftc -parse-as-library
 ```
+
+## Failures
+
+The `FailCases` directory contains fuzzing test input which previously caused
+failures in gRPC.

Tests/GRPCTests/ServerFuzzingRegressionTests.swift

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2021, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import EchoImplementation
+import struct Foundation.Data
+import struct Foundation.URL
+import GRPC
+import NIO
+import XCTest
+
+final class ServerFuzzingRegressionTests: GRPCTestCase {
+  private static let failCasesURL = URL(fileURLWithPath: #file)
+    .deletingLastPathComponent() // ServerFuzzingRegressionTests.swift
+    .deletingLastPathComponent() // GRPCTests
+    .deletingLastPathComponent() // Tests
+    .appendingPathComponent("FuzzTesting")
+    .appendingPathComponent("FailCases")
+
+  private func runTest(withInput buffer: ByteBuffer) {
+    let channel = EmbeddedChannel()
+    defer {
+      _ = try? channel.finish()
+    }
+
+    let configuration = Server.Configuration(
+      target: .unixDomainSocket("/ignored"),
+      eventLoopGroup: channel.eventLoop,
+      serviceProviders: [EchoProvider()]
+    )
+
+    XCTAssertNoThrow(try channel._configureForServerFuzzing(configuration: configuration))
+    // We're okay with errors. Crashes are bad though.
+    _ = try? channel.writeInbound(buffer)
+    channel.embeddedEventLoop.run()
+  }
+
+  private func runTest(withInputNamed name: String) throws {
+    let url = ServerFuzzingRegressionTests.failCasesURL.appendingPathComponent(name)
+    let data = try Data(contentsOf: url)
+    let buffer = ByteBuffer(data: data)
+    self.runTest(withInput: buffer)
+  }
+
+  func testFuzzCase_debug_4645975625957376() {
+    let name = "clusterfuzz-testcase-minimized-grpc-swift-fuzz-debug-4645975625957376"
+    XCTAssertNoThrow(try self.runTest(withInputNamed: name))
+  }
+
+  func testFuzzCase_release_5413100925878272() {
+    let name = "clusterfuzz-testcase-minimized-grpc-swift-fuzz-release-5413100925878272"
+    XCTAssertNoThrow(try self.runTest(withInputNamed: name))
+  }
+}