Allow runners to mount special filesystem in the input root

This introduces a new runner decorator that can create mounts in the
input root. This is useful for 'chroot' runners that have userland tools
that require '/proc' or '/sys', which are created by the operating
system. This is how to configure the runner:

  inputRootMounts: [{
    mountpoint: 'proc',
    source: '/proc',
    filesystemType: 'proc',
  }, {
    mountpoint: 'sys',
    source: '/sys',
    filesystemType: 'sysfs',
  }],

.bazelrc

@@ -1,2 +1,3 @@
+# Required for @com_google_absl and remoteapis that use it.
 build --host_cxxopt=-std=c++17
 run --workspace_status_command="bash tools/workspace-status.sh"

cmd/bb_runner/main.go

@@ -68,6 +68,13 @@ func main() {
 			buildDirectoryPath,
 			commandCreator,
 			configuration.SetTmpdirEnvironmentVariable)
+		for _, mountinfo := range configuration.InputRootMounts {
+			r = runner.NewMountingRunner(
+				r,
+				buildDirectory,
+				mountinfo,
+			)
+		}
 
 		// Let bb_runner replace temporary directories with symbolic
 		// links pointing to the temporary directory set up by

cmd/bb_scheduler/main.go

@@ -108,7 +108,7 @@ func main() {
 		}
 		killOperationsAuthorizer, err := authorizerFactory.NewAuthorizerFromConfiguration(configuration.KillOperationsAuthorizer)
 		if err != nil {
-			return util.StatusWrap(err, "Failed to create kill operaitons authorizer")
+			return util.StatusWrap(err, "Failed to create kill operations authorizer")
 		}
 
 		platformQueueWithNoWorkersTimeout := configuration.PlatformQueueWithNoWorkersTimeout

pkg/filesystem/lazy_directory.go

@@ -245,3 +245,21 @@ func (d *lazyDirectory) Apply(arg interface{}) error {
 	defer underlying.Close()
 	return underlying.Apply(arg)
 }
+
+func (d *lazyDirectory) Mount(mountpoint path.Component, source string, fstype string) error {
+	underlying, err := d.openUnderlying()
+	if err != nil {
+		return err
+	}
+	defer underlying.Close()
+	return underlying.Mount(mountpoint, source, fstype)
+}
+
+func (d *lazyDirectory) Unmount(mountpoint path.Component) error {
+	underlying, err := d.openUnderlying()
+	if err != nil {
+		return err
+	}
+	defer underlying.Close()
+	return underlying.Unmount(mountpoint)
+}

pkg/proto/configuration/bb_runner/bb_runner.proto

@@ -131,4 +131,23 @@ message ApplicationConfiguration {
   // https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/exec/local/XcodeLocalEnvProvider.java
   // https://www.smileykeith.com/2021/03/08/locking-xcode-in-bazel/
   map<string, string> apple_xcode_developer_directories = 14;
+
+  // Mount special filesystems in the input root.
+  // This is useful when running with `chroot`.
+  // some tools require access to special filesystems
+  // that are created when the operating system boots.
+  // An input root with a full userland implementation may need these.
+  // Typical choices are:
+  //    {"proc", "/proc", "proc"}
+  //    {"sys", "/sys", "sysfs"}
+  repeated mount_info input_root_mounts = 15;
+}
+
+message mount_info {
+    // Mount a filesystem in the input root, a relative path.
+    string mountpoint = 1;
+    // Source filesystem from the runner's operating system, absolute path.
+    string source = 2;
+    // Type of filesystem, see `mount`'s man page.
+    string filesystem_type = 3;
 }

pkg/runner/BUILD.bazel

@@ -11,6 +11,7 @@ go_library(
         "local_runner_rss_kibibytes.go",
         "local_runner_unix.go",
         "local_runner_windows.go",
+        "mounting_runner.go",
         "path_existence_checking_runner.go",
         "temporary_directory_installing_runner.go",
         "temporary_directory_symlinking_runner.go",
@@ -19,6 +20,7 @@ go_library(
     visibility = ["//visibility:public"],
     deps = [
         "//pkg/cleaner",
+        "//pkg/proto/configuration/bb_runner",
         "//pkg/proto/runner",
         "//pkg/proto/tmp_installer",
         "@com_github_buildbarn_bb_storage//pkg/filesystem",