simulator manager

.bazelrc

@@ -32,7 +32,13 @@ build:clang --host_cxxopt="-fvisibility=hidden"
 
 # iOS setup
 build:ios --ios_multi_cpus=sim_arm64
-build:ios --@rules_apple//apple/build_settings:ios_simulator_device="iPhone 16 Pro"
+# The simulator device is not pinned here: which device profiles exist depends on
+# the installed Xcode's iOS runtime, so any value checked in goes stale. Set it in
+# .bazelrc.user, e.g.
+#
+#     build:ios --@rules_apple//apple/build_settings:ios_simulator_device="iPhone 17"
+#
+# `xcrun simctl list devicetypes` lists what is available locally.
 
 # Platform-specific options for each supported platform.
 build:remote_linux_x64 --extra_execution_platforms=//platform/linux_x64

MODULE.bazel

@@ -129,6 +129,29 @@ use_repo(maven, "maven")
 bazel_dep(name = "rules_kotlin", version = "2.3.20")
 bazel_dep(name = "rules_perl", version = "1.1.1")
 bazel_dep(name = "rules_swift", version = "4.0.0-rc2")
+
+# Swift Package Manager deps, used by //tools/simulator_manager.
+bazel_dep(name = "rules_swift_package_manager", version = "1.22.0")
+
+swift_deps = use_extension(
+    "@rules_swift_package_manager//:extensions.bzl",
+    "swift_deps",
+)
+swift_deps.from_package(
+    declare_swift_deps_info = True,
+    resolved = "//tools/simulator_manager:Package.resolved",
+    swift = "//tools/simulator_manager:Package.swift",
+)
+use_repo(
+    swift_deps,
+    "swift_deps_info",
+    "swift_package",
+    "swiftpkg_shellout",
+    "swiftpkg_swift_argument_parser",
+    "swiftpkg_swift_nio",
+    "swiftpkg_swift_nio_extras",
+)
+
 bazel_dep(name = "rules_nodejs", version = "6.7.4")
 bazel_dep(name = "aspect_rules_ts", version = "3.8.10")
 bazel_dep(name = "aspect_bazel_lib", version = "2.22.5")

ios/BUILD

@@ -1,7 +1,34 @@
 load("@bazel_skylib//rules:build_test.bzl", "build_test")
 load("@rules_apple//apple:ios.bzl", "ios_application", "ios_ui_test", "ios_unit_test")
+load(
+    "@rules_apple//apple/testing/default_runner:ios_xctestrun_runner.bzl",
+    "ios_xctestrun_runner",
+)
 load("@rules_swift//swift:swift.bzl", "swift_library")
 
+# Leases each test its own simulator from //tools/simulator_manager, so
+# concurrent actions on one worker never share a device.
+ios_xctestrun_runner(
+    name = "ios_leased_simulator_runner",
+    clean_up_simulator_action = "//tools/simulator_manager:release_simulator",
+    create_simulator_action = "//tools/simulator_manager:lease_simulator",
+    random = False,
+    # reuse_simulator doubles as the exclusivity toggle for the lease: opting out
+    # of reuse means no other test shares this device.
+    reuse_simulator = False,
+)
+
+# Non-exclusive leases: concurrent tests may share one simulator. Sharing is
+# opportunistic, not bounded — a request only joins a device that is already
+# active, so tests dispatched simultaneously can still each get their own.
+ios_xctestrun_runner(
+    name = "ios_shared_simulator_runner",
+    clean_up_simulator_action = "//tools/simulator_manager:release_simulator",
+    create_simulator_action = "//tools/simulator_manager:lease_simulator",
+    random = False,
+    reuse_simulator = True,
+)
+
 swift_library(
     name = "HelloAppLibrary",
     srcs = ["HelloApp.swift"],
@@ -49,7 +76,48 @@ swift_library(
 ios_ui_test(
     name = "HelloAppUITest",
     minimum_os_version = "26.0",
-    runner = "@rules_apple//apple/testing/default_runner:ios_xctestrun_ordered_runner",
+    runner = ":ios_leased_simulator_runner",
     test_host = ":HelloApp",
     deps = [":HelloAppUITestLib"],
+    exec_properties = {
+        "sandboxAllowed": "False",
+    },
+)
+
+swift_library(
+    name = "HelloConcurrentTestLib",
+    testonly = True,
+    srcs = ["HelloConcurrentTest.swift"],
+    module_name = "HelloConcurrentTest",
 )
+
+# Exercises concurrent leasing: run with --runs_per_test to get several
+# simultaneous leases from one target. Unlike HelloAppUITest this has no test
+# host, so it takes rules_apple's `simctl spawn` path rather than `xcodebuild`.
+#
+#     bazel test --config=ios --runs_per_test=3 //ios:HelloConcurrentTest
+#
+# Each run's log reports its simulator UDID; no two concurrent runs should match.
+# Manual because it is sensitive to how much memory the host has — see the
+# simulator_manager README.
+ios_unit_test(
+    name = "HelloConcurrentTest",
+    minimum_os_version = "26.0",
+    runner = ":ios_leased_simulator_runner",
+    tags = ["manual"],
+    timeout = "long",
+    deps = [":HelloConcurrentTestLib"],
+)
+
+# Same source, but leasing non-exclusively: several concurrent runs should
+# collapse onto fewer devices. Measured on a 16 GiB worker, 8 runs shared 2
+# simulators.
+ios_unit_test(
+    name = "HelloSharedSimulatorTest",
+    minimum_os_version = "26.0",
+    runner = ":ios_shared_simulator_runner",
+    tags = ["manual"],
+    timeout = "long",
+    deps = [":HelloConcurrentTestLib"],
+)
+

ios/HelloConcurrentTest.swift

@@ -0,0 +1,17 @@
+import XCTest
+
+/// Checks that concurrent tests each lease their own simulator.
+///
+/// Unlike HelloAppConcurrentUITest this has no test host, so rules_apple runs it
+/// via `simctl spawn` rather than `xcodebuild`.
+///
+/// Several targets share this source; each logs its UDID, and no two should
+/// match. The sleep holds the lease long enough for the runs to actually overlap.
+class HelloConcurrentTest: XCTestCase {
+    func testHoldsItsOwnSimulator() {
+        let udid = ProcessInfo.processInfo.environment["SIMULATOR_UDID"] ?? "unknown"
+        print("CONCURRENT_UNIT_TEST_SIMULATOR_UDID=\(udid)")
+        XCTAssertEqual(1 + 1, 2)
+        Thread.sleep(forTimeInterval: 20)
+    }
+}

tools/simulator_manager/BUILD

@@ -0,0 +1,75 @@
+load("@rules_shell//shell:sh_binary.bzl", "sh_binary")
+load("@rules_swift//swift:swift.bzl", "swift_binary", "swift_library", "swift_test")
+
+# A daemon that hands out leases on iOS simulators. Ported from
+# https://github.com/bazelbuild/rules_apple/pull/2767, which builds it with an
+# internal `macos_swift_tool` macro; a swift_library plus swift_binary is
+# equivalent, and lets the tests link the library.
+swift_binary(
+    name = "simulator_manager",
+    srcs = glob(["Sources/*.swift"]),
+    tags = ["no-linux-ci"],
+    deps = [
+        "@swiftpkg_shellout//:ShellOut",
+        "@swiftpkg_swift_argument_parser//:ArgumentParser",
+        "@swiftpkg_swift_nio//:NIO",
+        "@swiftpkg_swift_nio//:NIOCore",
+        "@swiftpkg_swift_nio//:NIOHTTP1",
+        "@swiftpkg_swift_nio//:NIOPosix",
+        "@swiftpkg_swift_nio_extras//:NIOExtras",
+    ],
+)
+
+# Compiles the daemon's sources directly rather than depending on the binary
+# above: linking a swift_binary into a swift_test duplicates `_main`. Main.swift
+# is excluded for the same reason.
+swift_test(
+    name = "simulator_manager_test",
+    srcs = glob(["Tests/*.swift"]) + glob(
+        ["Sources/*.swift"],
+        exclude = ["Sources/Main.swift"],
+    ),
+    module_name = "simulator_manager",
+    tags = ["no-linux-ci"],
+    deps = [
+        "@swiftpkg_shellout//:ShellOut",
+        "@swiftpkg_swift_argument_parser//:ArgumentParser",
+        "@swiftpkg_swift_nio//:NIO",
+        "@swiftpkg_swift_nio//:NIOCore",
+        "@swiftpkg_swift_nio//:NIOHTTP1",
+        "@swiftpkg_swift_nio//:NIOPosix",
+        "@swiftpkg_swift_nio_extras//:NIOExtras",
+    ],
+)
+
+sh_binary(
+    name = "start",
+    srcs = ["start.sh"],
+    data = [
+        ":prepare_simulator",
+        ":simulator_manager",
+    ],
+    visibility = ["//visibility:public"],
+    deps = ["@bazel_tools//tools/bash/runfiles"],
+)
+
+sh_binary(
+    name = "prepare_simulator",
+    srcs = ["prepare_simulator.sh"],
+)
+
+# rules_apple's create/clean_up_simulator_action hooks. Using these instead of
+# patching the runner template (as PR 2767 does) keeps rules_apple unmodified.
+sh_binary(
+    name = "lease_simulator",
+    srcs = ["lease_simulator.sh"],
+    data = [":start"],
+    visibility = ["//visibility:public"],
+    deps = ["@bazel_tools//tools/bash/runfiles"],
+)
+
+sh_binary(
+    name = "release_simulator",
+    srcs = ["release_simulator.sh"],
+    visibility = ["//visibility:public"],
+)