Mount special filesystems in chroot runners

This can be used to mount special filesystems like '/proc' and '/sys' in
the input root of actions if 'chroot' is enabled. The filesystems are
required for many tools to work.

Solves: buildbarn#115

cmd/bb_runner/main.go

@@ -75,6 +75,13 @@ 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

pkg/proto/configuration/bb_runner/bb_runner.proto

@@ -131,4 +131,37 @@ 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_into_input_root`. 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.
+  //
+  // The mount point directories must exist in the input root.
+  //
+  // Typical choices are:
+  //
+  //   inputRootMounts: [
+  //     {
+  //       mountpoint: 'proc',
+  //       source: '/proc',
+  //       filesystemType: 'proc',
+  //     },
+  //     {
+  //       mountpoint: 'sys',
+  //       source: '/sys',
+  //       filesystemType: 'sysfs',
+  //     },
+  //   ],
+  repeated InputMountOptions input_root_mounts = 15;
+}
+
+message InputMountOptions {
+  // Mount a filesystem in the input root, a relative path.
+  string mountpoint = 1;
+  // Source filesystem from the runner's operating system.
+  string source = 2;
+  // Type of filesystem, see the mount(8) 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",
@@ -70,13 +72,15 @@ go_test(
         "apple_xcode_resolving_runner_test.go",
         "clean_runner_test.go",
         "local_runner_test.go",
+        "mounting_runner_test.go",
         "path_existence_checking_runner_test.go",
         "temporary_directory_symlinking_runner_test.go",
     ],
     deps = [
         ":runner",
         "//internal/mock",
         "//pkg/cleaner",
+        "//pkg/proto/configuration/bb_runner",
         "//pkg/proto/resourceusage",
         "//pkg/proto/runner",
         "@com_github_buildbarn_bb_storage//pkg/filesystem",