Add methods for privHelper client to interact with fam

Summary:
## This Diff
Defined two methods: startFam and stopFam for privHelper client. These methods will be called directly by serviceHandler upon receiving the Thrift requests.

## Context
File Access Monitor(FAM) is a tool to monitor file system access, aiming to find out EdenFS crawlers. The tool will be implemented after the toolchain is ready. Before that, we are going to use [this third-party binary](https://github.com/objective-see/FileMonitor).
This diff stack focuses on integrating the binary into Eden package and spawn a process from privHelper because of the required privileged permission.

Reviewed By: jdelliot

Differential Revision: D69340304

fbshipit-source-id: 815423a1d38ea9af6bcd74d481e60f6299a5baf8

eden/fs/privhelper/PrivHelper.h

@@ -187,6 +187,25 @@ class PrivHelper {
    */
   FOLLY_NODISCARD virtual folly::Future<pid_t> getServerPid() = 0;
 
+  /**
+   * Start File Access Monitor(FAM).
+   *
+   * @param paths A list of paths to be monitored by FAM.
+   * @param outputPath The path to the output file.
+   * @return pid of the started FAM process
+   */
+  FOLLY_NODISCARD virtual folly::Future<pid_t> startFam(
+      const std::vector<std::string>& paths,
+      const std::string& tmpOutputPath,
+      const std::string& specifiedOutputPath,
+      const bool shouldUpload) = 0;
+
+  /**
+   * Stop File Access Monitor(FAM).
+   */
+  FOLLY_NODISCARD virtual folly::Future<StopFileAccessMonitorResponse>
+  stopFam() = 0;
+
   /**
    * setLogFileBlocking() is a wrapper around setLogFile() that blocks until
    * the call has completed.

eden/fs/privhelper/PrivHelperImpl.cpp

@@ -126,6 +126,12 @@ class PrivHelperClientImpl : public PrivHelper,
       std::chrono::nanoseconds duration) override;
   Future<folly::Unit> setUseEdenFs(bool useEdenFs) override;
   Future<pid_t> getServerPid() override;
+  Future<pid_t> startFam(
+      const std::vector<std::string>& paths,
+      const std::string& tmpOutputPath,
+      const std::string& specifiedOutputPath,
+      const bool shouldUpload) override;
+  Future<StopFileAccessMonitorResponse> stopFam() override;
   int stop() override;
   int getRawClientFd() const override {
     auto state = state_.rlock();
@@ -588,6 +594,37 @@ Future<pid_t> PrivHelperClientImpl::getServerPid() {
       });
 }
 
+Future<pid_t> PrivHelperClientImpl::startFam(
+    const std::vector<std::string>& paths,
+    const std::string& tmpOutputPath,
+    const std::string& specifiedOutputPath,
+    const bool shouldUpload) {
+  auto xid = getNextXid();
+  auto request = PrivHelperConn::serializeStartFamRequest(
+      xid, paths, tmpOutputPath, specifiedOutputPath, shouldUpload);
+
+  return sendAndRecv(xid, std::move(request))
+      .thenValue([](UnixSocket::Message&& response) {
+        return PrivHelperConn::parseStartFamResponse(response);
+      });
+}
+
+Future<StopFileAccessMonitorResponse> PrivHelperClientImpl::stopFam() {
+  auto xid = getNextXid();
+  auto request = PrivHelperConn::serializeStopFamRequest(xid);
+
+  return sendAndRecv(xid, std::move(request))
+      .thenValue([&](UnixSocket::Message&& response) {
+        StopFileAccessMonitorResponse stopResponse{};
+        PrivHelperConn::parseStopFamResponse(
+            response,
+            stopResponse.tmpOutputPath,
+            stopResponse.specifiedOutputPath,
+            stopResponse.shouldUpload);
+        return stopResponse;
+      });
+}
+
 int PrivHelperClientImpl::stop() {
   const auto result = cleanup();
   if (result.hasError()) {
@@ -880,6 +917,18 @@ class StubPrivHelper final : public PrivHelper {
     return -1;
   }
 
+  folly::Future<pid_t> startFam(
+      const std::vector<std::string>& paths,
+      const std::string& tmpOutputPath,
+      const std::string& specifiedOutputPath,
+      const bool shouldUpload) override {
+    NOT_IMPLEMENTED();
+  }
+
+  folly::Future<StopFileAccessMonitorResponse> stopFam() override {
+    NOT_IMPLEMENTED();
+  }
+
   int stop() override {
     return 0;
   }

eden/fs/testharness/FakePrivHelper.cpp

@@ -177,4 +177,18 @@ folly::Future<pid_t> FakePrivHelper::getServerPid() {
   return -1;
 }
 
+folly::Future<pid_t> FakePrivHelper::startFam(
+    const std::vector<std::string>& /* paths */,
+    const std::string& /* tmpOutputPath */,
+    const std::string& /* specifiedOutputPath */,
+    const bool /* shouldUpload */) {
+  return makeFuture<pid_t>(
+      runtime_error("FakePrivHelper::startFam() not implemented"));
+}
+
+folly::Future<StopFileAccessMonitorResponse> FakePrivHelper::stopFam() {
+  return makeFuture<StopFileAccessMonitorResponse>(
+      runtime_error("FakePrivHelper::stopFam() not implemented"));
+}
+
 } // namespace facebook::eden

eden/fs/testharness/FakePrivHelper.h

@@ -74,6 +74,12 @@ class FakePrivHelper final : public PrivHelper {
       std::chrono::nanoseconds duration) override;
   folly::Future<folly::Unit> setUseEdenFs(bool useEdenFs) override;
   folly::Future<pid_t> getServerPid() override;
+  folly::Future<pid_t> startFam(
+      const std::vector<std::string>& paths,
+      const std::string& tmpOutputPath,
+      const std::string& specifiedOutputPath,
+      const bool shouldUpload) override;
+  folly::Future<StopFileAccessMonitorResponse> stopFam() override;
   int stop() override;
   int getRawClientFd() const override {
     return -1;