Add a new Xcode target and scheme for fuzz tests (#1364)

Created `Firestore_FuzzTests_iOS` Xcode target as a duplicate of the target
  `Firestore_Tests_iOS`.
    - Added `-fsanitize-coverage=trace-pc-guard` to the compiler flags to enable
      code coverage by libFuzzer fuzzing driver.
    - Modified the files to compile to the fuzz testing ones.
    - Added a Principal testing class.
* Modified the `Podfile` to include `LibFuzzer.podspec` as a dependency for
  the target `Firestore_FuzzTests_iOS`.
* Added a skeleton for fuzz testing. Currently does not test any code but
  the fuzzing works fine and calls an empty `LLVMFuzzerTestOneInput`
  with different input values.
    - This means that fuzzing does not finish or crash.

2 files changed, 88 insertions, 0 deletions

diff --git a/Firestore/Example/FuzzTests/FSTFuzzTestsPrincipal.mm b/Firestore/Example/FuzzTests/FSTFuzzTestsPrincipal.mm
new file mode 100644
index 0000000..63f6db0
--- /dev/null
+++ b/Firestore/Example/FuzzTests/FSTFuzzTestsPrincipal.mm
@@ -0,0 +1,64 @@
+/*
+ * Copyright 2018 Google
+ *
+ * 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 <Foundation/NSObject.h>
+
+#include "LibFuzzer/FuzzerDefs.h"
+
+namespace {
+
+// Contains the code to be fuzzed. Called by the fuzzing library with
+// different argument values for `data` and `size`.
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  // Code to be fuzz-tested here.
+  return 0;
+}
+
+// Simulates calling the main() function of libFuzzer (FuzzerMain.cpp).
+int RunFuzzTestingMain() {
+  // Arguments to libFuzzer main() function should be added to this array,
+  // e.g., dictionaries, corpus, number of runs, jobs, etc.
+  char *program_args[] = {
+      const_cast<char *>("RunFuzzTestingMain")  // First argument is program name.
+  };
+  char **argv = program_args;
+  int argc = sizeof(program_args) / sizeof(program_args[0]);
+
+  // Start fuzzing using libFuzzer's driver.
+  return fuzzer::FuzzerDriver(&argc, &argv, LLVMFuzzerTestOneInput);
+}
+
+}  // namespace
+
+/**
+ * This class is registered as the NSPrincipalClass in the
+ * Firestore_FuzzTests_iOS bundle's Info.plist. XCTest instantiates this class
+ * to perform one-time setup for the test bundle, as documented here:
+ *
+ *   https://developer.apple.com/documentation/xctest/xctestobservationcenter
+ */
+@interface FSTFuzzTestsPrincipal : NSObject
+@end
+
+@implementation FSTFuzzTestsPrincipal
+
+- (instancetype)init {
+  self = [super init];
+  RunFuzzTestingMain();
+  return self;
+}
+
+@end
diff --git a/Firestore/Example/FuzzTests/Firestore_FuzzTests_iOS-Info.plist b/Firestore/Example/FuzzTests/Firestore_FuzzTests_iOS-Info.plist
new file mode 100644
index 0000000..0d53e5f
--- /dev/null
+++ b/Firestore/Example/FuzzTests/Firestore_FuzzTests_iOS-Info.plist
@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>CFBundleDevelopmentRegion</key>
+	<string>en</string>
+	<key>CFBundleExecutable</key>
+	<string>${EXECUTABLE_NAME}</string>
+	<key>CFBundleIdentifier</key>
+	<string>$(PRODUCT_BUNDLE_IDENTIFIER)</string>
+	<key>CFBundleInfoDictionaryVersion</key>
+	<string>6.0</string>
+	<key>CFBundlePackageType</key>
+	<string>BNDL</string>
+	<key>CFBundleShortVersionString</key>
+	<string>1.0</string>
+	<key>CFBundleSignature</key>
+	<string>????</string>
+	<key>NSPrincipalClass</key>
+	<string>FSTFuzzTestsPrincipal</string>
+	<key>CFBundleVersion</key>
+	<string>1</string>
+</dict>
+</plist>