Rewrite KernelTimeout to support both absolute and relative timeouts

APIs that take KernelTimeout as a parameter can now query if an
absolute or relative timeout was requested. If the underlying API can
only use one type of timeout, the code will do a reasonable
conversion.

The goal is to eventually enable the possibility of using wait times
that are based on monotonic clocks that are safe against system clock
steps.

PiperOrigin-RevId: 508541507
Change-Id: Id08bf13515f3e1bfd78d88393cde98a6fd3ef72c

6 files changed, 546 insertions, 115 deletions

diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
index 52a563cd..3c0f6901 100644
--- a/CMake/AbseilDll.cmake
+++ b/CMake/AbseilDll.cmake
@@ -365,6 +365,7 @@ set(ABSL_INTERNAL_DLL_FILES
   "synchronization/internal/graphcycles.cc"
   "synchronization/internal/graphcycles.h"
   "synchronization/internal/kernel_timeout.h"
+  "synchronization/internal/kernel_timeout.cc"
   "synchronization/internal/per_thread_sem.cc"
   "synchronization/internal/per_thread_sem.h"
   "synchronization/internal/thread_pool.h"
diff --git a/absl/synchronization/BUILD.bazel b/absl/synchronization/BUILD.bazel
index ccaee796..9c89ac49 100644
--- a/absl/synchronization/BUILD.bazel
+++ b/absl/synchronization/BUILD.bazel
@@ -53,6 +53,7 @@ cc_library(
 
 cc_library(
     name = "kernel_timeout_internal",
+    srcs = ["internal/kernel_timeout.cc"],
     hdrs = ["internal/kernel_timeout.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
@@ -60,12 +61,25 @@ cc_library(
         "//absl/synchronization:__pkg__",
     ],
     deps = [
-        "//absl/base:core_headers",
+        "//absl/base:config",
         "//absl/base:raw_logging_internal",
         "//absl/time",
     ],
 )
 
+cc_test(
+    name = "kernel_timeout_internal_test",
+    srcs = ["internal/kernel_timeout_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":kernel_timeout_internal",
+        "//absl/base:config",
+        "//absl/time",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "synchronization",
     srcs = [
diff --git a/absl/synchronization/CMakeLists.txt b/absl/synchronization/CMakeLists.txt
index f64653bb..5eb9b566 100644
--- a/absl/synchronization/CMakeLists.txt
+++ b/absl/synchronization/CMakeLists.txt
@@ -39,14 +39,30 @@ absl_cc_library(
     kernel_timeout_internal
   HDRS
     "internal/kernel_timeout.h"
+  SRCS
+    "internal/kernel_timeout.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::core_headers
+    absl::config
     absl::raw_logging_internal
     absl::time
 )
 
+absl_cc_test(
+  NAME
+    kernel_timeout_internal_test
+  SRCS
+    "internal/kernel_timeout_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::kernel_timeout_internal
+    absl::config
+    absl::time
+    GTest::gmock_main
+)
+
 absl_cc_library(
   NAME
     synchronization
diff --git a/absl/synchronization/internal/kernel_timeout.cc b/absl/synchronization/internal/kernel_timeout.cc
new file mode 100644
index 00000000..4015bd0c
--- /dev/null
+++ b/absl/synchronization/internal/kernel_timeout.cc
@@ -0,0 +1,168 @@
+// Copyright 2023 The Abseil Authors
+//
+// 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
+//
+//     https://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.
+
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+#include <algorithm>
+#include <cstdint>
+#include <ctime>
+#include <limits>
+
+#include "absl/base/config.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace synchronization_internal {
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr uint64_t KernelTimeout::kNoTimeout;
+constexpr int64_t KernelTimeout::kMaxNanos;
+#endif
+
+KernelTimeout::KernelTimeout(absl::Time t) {
+  // `absl::InfiniteFuture()` is a common "no timeout" value and cheaper to
+  // compare than convert.
+  if (t == absl::InfiniteFuture()) {
+    rep_ = kNoTimeout;
+    return;
+  }
+
+  int64_t unix_nanos = absl::ToUnixNanos(t);
+
+  // A timeout that lands before the unix epoch is converted to 0.
+  // In theory implementations should expire these timeouts immediately.
+  if (unix_nanos < 0) {
+    unix_nanos = 0;
+  }
+
+  // Values greater than or equal to kMaxNanos are converted to infinite.
+  if (unix_nanos >= kMaxNanos) {
+    rep_ = kNoTimeout;
+    return;
+  }
+
+  rep_ = static_cast<uint64_t>(unix_nanos) << 1;
+}
+
+KernelTimeout::KernelTimeout(absl::Duration d) {
+  // `absl::InfiniteDuration()` is a common "no timeout" value and cheaper to
+  // compare than convert.
+  if (d == absl::InfiniteDuration()) {
+    rep_ = kNoTimeout;
+    return;
+  }
+
+  int64_t nanos = absl::ToInt64Nanoseconds(d);
+
+  // Negative durations are normalized to 0.
+  // In theory implementations should expire these timeouts immediately.
+  if (nanos < 0) {
+    nanos = 0;
+  }
+
+  // Values greater than or equal to kMaxNanos are converted to infinite.
+  if (nanos >= kMaxNanos) {
+    rep_ = kNoTimeout;
+    return;
+  }
+
+  rep_ = (static_cast<uint64_t>(nanos) << 1) | uint64_t{1};
+}
+
+int64_t KernelTimeout::MakeAbsNanos() const {
+  if (!has_timeout()) {
+    return kMaxNanos;
+  }
+
+  int64_t nanos = RawNanos();
+
+  if (is_relative_timeout()) {
+    int64_t now = ToUnixNanos(absl::Now());
+    if (nanos > kMaxNanos - now) {
+      // Overflow.
+      nanos = kMaxNanos;
+    } else {
+      nanos += now;
+    }
+  } else if (nanos == 0) {
+    // Some callers have assumed that 0 means no timeout, so instead we return a
+    // time of 1 nanosecond after the epoch.
+    nanos = 1;
+  }
+
+  return nanos;
+}
+
+struct timespec KernelTimeout::MakeAbsTimespec() const {
+  return absl::ToTimespec(absl::Nanoseconds(MakeAbsNanos()));
+}
+
+struct timespec KernelTimeout::MakeRelativeTimespec() const {
+  if (!has_timeout()) {
+    return absl::ToTimespec(absl::Nanoseconds(kMaxNanos));
+  }
+  if (is_relative_timeout()) {
+    return absl::ToTimespec(absl::Nanoseconds(RawNanos()));
+  }
+
+  int64_t nanos = RawNanos();
+  int64_t now = ToUnixNanos(absl::Now());
+  if (now > nanos) {
+    // Convert past values to 0 to be safe.
+    nanos = 0;
+  } else {
+    nanos -= now;
+  }
+  return absl::ToTimespec(absl::Nanoseconds(nanos));
+}
+
+KernelTimeout::DWord KernelTimeout::InMillisecondsFromNow() const {
+  constexpr DWord kInfinite = std::numeric_limits<DWord>::max();
+
+  if (!has_timeout()) {
+    return kInfinite;
+  }
+
+  const int64_t nanos = RawNanos();
+  constexpr uint64_t kNanosInMillis = uint64_t{1000000};
+
+  if (is_relative_timeout()) {
+    uint64_t ms = static_cast<uint64_t>(nanos) / kNanosInMillis;
+    if (ms > static_cast<uint64_t>(kInfinite)) {
+      ms = static_cast<uint64_t>(kInfinite);
+    }
+    return static_cast<DWord>(ms);
+  }
+
+  int64_t now = ToUnixNanos(absl::Now());
+  if (nanos >= now) {
+    // Round up so that now + ms_from_now >= nanos.
+    constexpr uint64_t kMaxValueNanos =
+        std::numeric_limits<int64_t>::max() - kNanosInMillis + 1;
+    uint64_t ms_from_now =
+        (std::min(kMaxValueNanos, static_cast<uint64_t>(nanos - now)) +
+         kNanosInMillis - 1) /
+        kNanosInMillis;
+    if (ms_from_now > kInfinite) {
+      return kInfinite;
+    }
+    return static_cast<DWord>(ms_from_now);
+  }
+  return DWord{0};
+}
+
+}  // namespace synchronization_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/synchronization/internal/kernel_timeout.h b/absl/synchronization/internal/kernel_timeout.h
index f5c2c0ef..1f4d82cd 100644
--- a/absl/synchronization/internal/kernel_timeout.h
+++ b/absl/synchronization/internal/kernel_timeout.h
@@ -11,26 +11,16 @@
 // 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.
-//
-
-// An optional absolute timeout, with nanosecond granularity,
-// compatible with absl::Time. Suitable for in-register
-// parameter-passing (e.g. syscalls.)
-// Constructible from a absl::Time (for a timeout to be respected) or {}
-// (for "no timeout".)
-// This is a private low-level API for use by a handful of low-level
-// components. Higher-level components should build APIs based on
-// absl::Time and absl::Duration.
 
 #ifndef ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
 #define ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
 
-#include <time.h>
-
 #include <algorithm>
 #include <cstdint>
+#include <ctime>
 #include <limits>
 
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
@@ -41,56 +31,59 @@ namespace synchronization_internal {
 
 class Waiter;
 
+// An optional timeout, with nanosecond granularity.
+//
+// This is a private low-level API for use by a handful of low-level
+// components. Higher-level components should build APIs based on
+// absl::Time and absl::Duration.
 class KernelTimeout {
  public:
-  // A timeout that should expire at <t>.  Any value, in the full
-  // InfinitePast() to InfiniteFuture() range, is valid here and will be
-  // respected.
-  explicit KernelTimeout(absl::Time t) : ns_(MakeNs(t)) {}
-  // No timeout.
-  KernelTimeout() : ns_(0) {}
+  // Construct an absolute timeout that should expire at `t`.
+  explicit KernelTimeout(absl::Time t);
+
+  // Construct a relative timeout that should expire after `d`.
+  explicit KernelTimeout(absl::Duration d);
+
+  // Infinite timeout.
+  constexpr KernelTimeout() : rep_(kNoTimeout) {}
+
+  // A more explicit factory for those who prefer it.
+  // Equivalent to `KernelTimeout()`.
+  static constexpr KernelTimeout Never() { return KernelTimeout(); }
 
-  // A more explicit factory for those who prefer it.  Equivalent to {}.
-  static KernelTimeout Never() { return {}; }
+  // Returns true if there is a timeout that will eventually expire.
+  // Returns false if the timeout is infinite.
+  bool has_timeout() const { return rep_ != kNoTimeout; }
 
-  // We explicitly do not support other custom formats: timespec, int64_t nanos.
-  // Unify on this and absl::Time, please.
+  // If `has_timeout()` is true, returns true if the timeout was provided as an
+  // `absl::Time`. The return value is undefined if `has_timeout()` is false
+  // because all indefinite timeouts are equivalent.
+  bool is_absolute_timeout() const { return (rep_ & 1) == 0; }
 
-  bool has_timeout() const { return ns_ != 0; }
+  // If `has_timeout()` is true, returns true if the timeout was provided as an
+  // `absl::Duration`. The return value is undefined if `has_timeout()` is false
+  // because all indefinite timeouts are equivalent.
+  bool is_relative_timeout() const { return (rep_ & 1) == 1; }
 
-  // Convert to parameter for sem_timedwait/futex/similar.  Only for approved
-  // users.  Do not call if !has_timeout.
+  // Convert to `struct timespec` for interfaces that expect an absolute
+  // timeout. If !has_timeout() or is_relative_timeout(), attempts to convert to
+  // a reasonable absolute timeout, but callers should to test has_timeout() and
+  // is_relative_timeout() and prefer to use a more appropriate interface.
   struct timespec MakeAbsTimespec() const;
 
-  // Convert to unix epoch nanos.  Do not call if !has_timeout.
+  // Convert to `struct timespec` for interfaces that expect a relative
+  // timeout. If !has_timeout() or is_absolute_timeout(), attempts to convert to
+  // a reasonable relative timeout, but callers should to test has_timeout() and
+  // is_absolute_timeout() and prefer to use a more appropriate interface.
+  struct timespec MakeRelativeTimespec() const;
+
+  // Convert to unix epoch nanos for interfaces that expect an absolute timeout
+  // in nanoseconds. If !has_timeout() or is_relative_timeout(), attempts to
+  // convert to a reasonable absolute timeout, but callers should to test
+  // has_timeout() and is_relative_timeout() and prefer to use a more
+  // appropriate interface.
   int64_t MakeAbsNanos() const;
 
- private:
-  // internal rep, not user visible: ns after unix epoch.
-  // zero = no timeout.
-  // Negative we treat as an unlikely (and certainly expired!) but valid
-  // timeout.
-  int64_t ns_;
-
-  static int64_t MakeNs(absl::Time t) {
-    // optimization--InfiniteFuture is common "no timeout" value
-    // and cheaper to compare than convert.
-    if (t == absl::InfiniteFuture()) return 0;
-    int64_t x = ToUnixNanos(t);
-
-    // A timeout that lands exactly on the epoch (x=0) needs to be respected,
-    // so we alter it unnoticably to 1.  Negative timeouts are in
-    // theory supported, but handled poorly by the kernel (long
-    // delays) so push them forward too; since all such times have
-    // already passed, it's indistinguishable.
-    if (x <= 0) x = 1;
-    // A time larger than what can be represented to the kernel is treated
-    // as no timeout.
-    if (x == (std::numeric_limits<int64_t>::max)()) x = 0;
-    return x;
-  }
-
-#ifdef _WIN32
   // Converts to milliseconds from now, or INFINITE when
   // !has_timeout(). For use by SleepConditionVariableSRW on
   // Windows. Callers should recognize that the return value is a
@@ -100,68 +93,29 @@ class KernelTimeout {
   // so we define our own DWORD and INFINITE instead of getting them from
   // <intsafe.h> and <WinBase.h>.
   typedef unsigned long DWord;  // NOLINT
-  DWord InMillisecondsFromNow() const {
-    constexpr DWord kInfinite = (std::numeric_limits<DWord>::max)();
-    if (!has_timeout()) {
-      return kInfinite;
-    }
-    // The use of absl::Now() to convert from absolute time to
-    // relative time means that absl::Now() cannot use anything that
-    // depends on KernelTimeout (for example, Mutex) on Windows.
-    int64_t now = ToUnixNanos(absl::Now());
-    if (ns_ >= now) {
-      // Round up so that Now() + ms_from_now >= ns_.
-      constexpr uint64_t max_nanos =
-          (std::numeric_limits<int64_t>::max)() - 999999u;
-      uint64_t ms_from_now =
-          ((std::min)(max_nanos, static_cast<uint64_t>(ns_ - now)) + 999999u) /
-          1000000u;
-      if (ms_from_now > kInfinite) {
-        return kInfinite;
-      }
-      return static_cast<DWord>(ms_from_now);
-    }
-    return 0;
-  }
-
-  friend class Waiter;
-#endif
-};
+  DWord InMillisecondsFromNow() const;
 
-inline struct timespec KernelTimeout::MakeAbsTimespec() const {
-  int64_t n = ns_;
-  static const int64_t kNanosPerSecond = 1000 * 1000 * 1000;
-  if (n == 0) {
-    ABSL_RAW_LOG(
-        ERROR, "Tried to create a timespec from a non-timeout; never do this.");
-    // But we'll try to continue sanely.  no-timeout ~= saturated timeout.
-    n = (std::numeric_limits<int64_t>::max)();
-  }
-
-  // Kernel APIs validate timespecs as being at or after the epoch,
-  // despite the kernel time type being signed.  However, no one can
-  // tell the difference between a timeout at or before the epoch (since
-  // all such timeouts have expired!)
-  if (n < 0) n = 0;
-
-  struct timespec abstime;
-  int64_t seconds = (std::min)(n / kNanosPerSecond,
-                               int64_t{(std::numeric_limits<time_t>::max)()});
-  abstime.tv_sec = static_cast<time_t>(seconds);
-  abstime.tv_nsec = static_cast<decltype(abstime.tv_nsec)>(n % kNanosPerSecond);
-  return abstime;
-}
-
-inline int64_t KernelTimeout::MakeAbsNanos() const {
-  if (ns_ == 0) {
-    ABSL_RAW_LOG(
-        ERROR, "Tried to create a timeout from a non-timeout; never do this.");
-    // But we'll try to continue sanely.  no-timeout ~= saturated timeout.
-    return (std::numeric_limits<int64_t>::max)();
-  }
-
-  return ns_;
-}
+ private:
+  // Internal representation.
+  //   - If the value is kNoTimeout, then the timeout is infinite, and
+  //     has_timeout() will return true.
+  //   - If the low bit is 0, then the high 63 bits is number of nanoseconds
+  //     after the unix epoch.
+  //   - If the low bit is 1, then the high 63 bits is a relative duration in
+  //     nanoseconds.
+  uint64_t rep_;
+
+  // Returns the number of nanoseconds stored in the internal representation.
+  // Together with is_absolute_timeout() and is_relative_timeout(), the return
+  // value is used to compute when the timeout should occur.
+  int64_t RawNanos() const { return static_cast<int64_t>(rep_ >> 1); }
+
+  // A value that represents no timeout (or an infinite timeout).
+  static constexpr uint64_t kNoTimeout = (std::numeric_limits<uint64_t>::max)();
+
+  // The maximum value that can be stored in the high 63 bits.
+  static constexpr int64_t kMaxNanos = (std::numeric_limits<int64_t>::max)();
+};
 
 }  // namespace synchronization_internal
 ABSL_NAMESPACE_END
diff --git a/absl/synchronization/internal/kernel_timeout_test.cc b/absl/synchronization/internal/kernel_timeout_test.cc
new file mode 100644
index 00000000..a89ae220
--- /dev/null
+++ b/absl/synchronization/internal/kernel_timeout_test.cc
@@ -0,0 +1,278 @@
+// Copyright 2023 The Abseil Authors
+//
+// 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
+//
+//     https://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.
+
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+#include <limits>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace {
+
+#if defined(ABSL_HAVE_ADDRESS_SANITIZER) || \
+    defined(ABSL_HAVE_MEMORY_SANITIZER) ||  \
+    defined(ABSL_HAVE_THREAD_SANITIZER) || defined(__ANDROID__)
+constexpr absl::Duration kTimingBound = absl::Milliseconds(5);
+#else
+constexpr absl::Duration kTimingBound = absl::Microseconds(250);
+#endif
+
+using absl::synchronization_internal::KernelTimeout;
+
+TEST(KernelTimeout, FiniteTimes) {
+  constexpr absl::Duration kDurationsToTest[] = {
+    absl::ZeroDuration(),
+    absl::Nanoseconds(1),
+    absl::Microseconds(1),
+    absl::Milliseconds(1),
+    absl::Seconds(1),
+    absl::Minutes(1),
+    absl::Hours(1),
+    absl::Hours(1000),
+    -absl::Nanoseconds(1),
+    -absl::Microseconds(1),
+    -absl::Milliseconds(1),
+    -absl::Seconds(1),
+    -absl::Minutes(1),
+    -absl::Hours(1),
+    -absl::Hours(1000),
+  };
+
+  for (auto duration : kDurationsToTest) {
+    const absl::Time now = absl::Now();
+    const absl::Time when = now + duration;
+    SCOPED_TRACE(duration);
+    KernelTimeout t(when);
+    EXPECT_TRUE(t.has_timeout());
+    EXPECT_TRUE(t.is_absolute_timeout());
+    EXPECT_FALSE(t.is_relative_timeout());
+    EXPECT_EQ(absl::TimeFromTimespec(t.MakeAbsTimespec()), when);
+    // MakeRelativeTimespec() doesn't quite round trip when using an absolute
+    // time, but it should get pretty close. Past times are converted to zero
+    // durations.
+    EXPECT_LE(
+        absl::AbsDuration(absl::DurationFromTimespec(t.MakeRelativeTimespec()) -
+                          std::max(duration, absl::ZeroDuration())),
+        kTimingBound);
+    EXPECT_EQ(absl::FromUnixNanos(t.MakeAbsNanos()), when);
+    EXPECT_LE(absl::AbsDuration(absl::Milliseconds(t.InMillisecondsFromNow()) -
+                                std::max(duration, absl::ZeroDuration())),
+              absl::Milliseconds(5));
+  }
+}
+
+TEST(KernelTimeout, InfiniteFuture) {
+  KernelTimeout t(absl::InfiniteFuture());
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+}
+
+TEST(KernelTimeout, DefaultConstructor) {
+  // The default constructor is equivalent to absl::InfiniteFuture().
+  KernelTimeout t;
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+}
+
+TEST(KernelTimeout, TimeMaxNanos) {
+  // Time >= kMaxNanos should behave as no timeout.
+  KernelTimeout t(absl::FromUnixNanos(std::numeric_limits<int64_t>::max()));
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+}
+
+TEST(KernelTimeout, Never) {
+  // KernelTimeout::Never() is equivalent to absl::InfiniteFuture().
+  KernelTimeout t = KernelTimeout::Never();
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+}
+
+TEST(KernelTimeout, InfinitePast) {
+  KernelTimeout t(absl::InfinitePast());
+  EXPECT_TRUE(t.has_timeout());
+  EXPECT_TRUE(t.is_absolute_timeout());
+  EXPECT_FALSE(t.is_relative_timeout());
+  EXPECT_LE(absl::TimeFromTimespec(t.MakeAbsTimespec()),
+            absl::FromUnixNanos(1));
+  EXPECT_EQ(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::ZeroDuration());
+  EXPECT_LE(absl::FromUnixNanos(t.MakeAbsNanos()), absl::FromUnixNanos(1));
+  EXPECT_EQ(t.InMillisecondsFromNow(), KernelTimeout::DWord{0});
+}
+
+TEST(KernelTimeout, FiniteDurations) {
+  constexpr absl::Duration kDurationsToTest[] = {
+    absl::ZeroDuration(),
+    absl::Nanoseconds(1),
+    absl::Microseconds(1),
+    absl::Milliseconds(1),
+    absl::Seconds(1),
+    absl::Minutes(1),
+    absl::Hours(1),
+    absl::Hours(1000),
+  };
+
+  for (auto duration : kDurationsToTest) {
+    SCOPED_TRACE(duration);
+    KernelTimeout t(duration);
+    EXPECT_TRUE(t.has_timeout());
+    EXPECT_FALSE(t.is_absolute_timeout());
+    EXPECT_TRUE(t.is_relative_timeout());
+    EXPECT_LE(absl::AbsDuration(absl::Now() + duration -
+                                absl::TimeFromTimespec(t.MakeAbsTimespec())),
+              absl::Milliseconds(5));
+    EXPECT_EQ(absl::DurationFromTimespec(t.MakeRelativeTimespec()), duration);
+    EXPECT_LE(absl::AbsDuration(absl::Now() + duration -
+                                absl::FromUnixNanos(t.MakeAbsNanos())),
+              absl::Milliseconds(5));
+    EXPECT_LE(absl::Milliseconds(t.InMillisecondsFromNow()) - duration,
+              absl::Milliseconds(5));
+  }
+}
+
+TEST(KernelTimeout, NegativeDurations) {
+  constexpr absl::Duration kDurationsToTest[] = {
+    -absl::ZeroDuration(),
+    -absl::Nanoseconds(1),
+    -absl::Microseconds(1),
+    -absl::Milliseconds(1),
+    -absl::Seconds(1),
+    -absl::Minutes(1),
+    -absl::Hours(1),
+    -absl::Hours(1000),
+    -absl::InfiniteDuration(),
+  };
+
+  for (auto duration : kDurationsToTest) {
+    // Negative durations should all be converted to zero durations or "now".
+    SCOPED_TRACE(duration);
+    KernelTimeout t(duration);
+    EXPECT_TRUE(t.has_timeout());
+    EXPECT_FALSE(t.is_absolute_timeout());
+    EXPECT_TRUE(t.is_relative_timeout());
+    EXPECT_LE(absl::AbsDuration(absl::Now() -
+                                absl::TimeFromTimespec(t.MakeAbsTimespec())),
+              absl::Milliseconds(5));
+    EXPECT_EQ(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+              absl::ZeroDuration());
+    EXPECT_LE(
+        absl::AbsDuration(absl::Now() - absl::FromUnixNanos(t.MakeAbsNanos())),
+        absl::Milliseconds(5));
+    EXPECT_EQ(t.InMillisecondsFromNow(), KernelTimeout::DWord{0});
+  }
+}
+
+TEST(KernelTimeout, InfiniteDuration) {
+  KernelTimeout t(absl::InfiniteDuration());
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+}
+
+TEST(KernelTimeout, DurationMaxNanos) {
+  // Duration >= kMaxNanos should behave as no timeout.
+  KernelTimeout t(absl::Nanoseconds(std::numeric_limits<int64_t>::max()));
+  EXPECT_FALSE(t.has_timeout());
+  // Callers are expected to check has_timeout() instead of using the methods
+  // below, but we do try to do something reasonable if they don't. We may not
+  // be able to round-trip back to absl::InfiniteDuration() or
+  // absl::InfiniteFuture(), but we should return a very large value.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_EQ(t.InMillisecondsFromNow(),
+            std::numeric_limits<KernelTimeout::DWord>::max());
+}
+
+TEST(KernelTimeout, OverflowNanos) {
+  // Test what happens when KernelTimeout is constructed with an absl::Duration
+  // that would overflow now_nanos + duration.
+  int64_t now_nanos = absl::ToUnixNanos(absl::Now());
+  int64_t limit = std::numeric_limits<int64_t>::max() - now_nanos;
+  absl::Duration duration = absl::Nanoseconds(limit) + absl::Seconds(1);
+  KernelTimeout t(duration);
+  EXPECT_TRUE(t.has_timeout());
+  // Timeouts should still be far in the future.
+  EXPECT_GT(absl::TimeFromTimespec(t.MakeAbsTimespec()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_GT(absl::DurationFromTimespec(t.MakeRelativeTimespec()),
+            absl::Hours(100000));
+  EXPECT_GT(absl::FromUnixNanos(t.MakeAbsNanos()),
+            absl::Now() + absl::Hours(100000));
+  EXPECT_LE(absl::Milliseconds(t.InMillisecondsFromNow()) - duration,
+            absl::Milliseconds(5));
+}
+
+}  // namespace