script: Create a CrossProcessInstant to enable serializable monotonic time (#33282)

Up until now, Servo was using a very old version of time to get a
cross-process monotonic timestamp (using `time::precise_time_ns()`).
This change replaces the usage of old time with a new serializable
monotonic time called `CrossProcessInstant` and uses it where `u64`
timestamps were stored before. The standard library doesn't provide this
functionality because it isn't something you can do reliably on all
platforms. The idea is that we do our best and then fall back
gracefully.

This is a big change, because Servo was using `u64` timestamps all over
the place some as raw values taken from `time::precise_time_ns()` and
some as relative offsets from the "navigation start," which is a concept
similar to DOM's `timeOrigin` (but not exactly the same). It's very
difficult to fix this situation without fixing it everywhere as the
`Instant` concept is supposed to be opaque. The good thing is that this
change clears up all ambiguity when passing times as a `time::Duration`
is unit agnostic and a `CrossProcessInstant` represents an absolute
moment in time.

The `time` version of `Duration` is used because it can both be negative
and is also serializable.

Good things:
 - No need too pass around `time` and `time_precise` any longer.
   `CrossProcessInstant` is also precise and monotonic.
 - The distinction between a time that is unset or at `0` (at some kind
   of timer epoch) is now gone.

There still a lot of work to do to clean up timing, but this is the
first step. In general, I've tried to preserve existing behavior, even
when not spec compliant, as much as possible. I plan to submit followup
PRs fixing some of the issues I've noticed.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/shared/net/tests/lib.rs

@@ -2,16 +2,20 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use base::cross_process_instant::CrossProcessInstant;
 use net_traits::{ResourceAttribute, ResourceFetchTiming, ResourceTimeValue, ResourceTimingType};
 
 #[test]
 fn test_set_start_time_to_fetch_start_if_nonzero_tao() {
     let mut resource_timing: ResourceFetchTiming =
         ResourceFetchTiming::new(ResourceTimingType::Resource);
-    resource_timing.fetch_start = 1;
-    assert_eq!(resource_timing.start_time, 0, "`start_time` should be zero");
+    resource_timing.fetch_start = Some(CrossProcessInstant::now());
     assert!(
-        resource_timing.fetch_start > 0,
+        resource_timing.start_time.is_none(),
+        "`start_time` should be zero"
+    );
+    assert!(
+        resource_timing.fetch_start.is_some(),
         "`fetch_start` should have a positive value"
     );
 
@@ -27,13 +31,13 @@ fn test_set_start_time_to_fetch_start_if_nonzero_tao() {
 fn test_set_start_time_to_fetch_start_if_zero_tao() {
     let mut resource_timing: ResourceFetchTiming =
         ResourceFetchTiming::new(ResourceTimingType::Resource);
-    resource_timing.start_time = 1;
+    resource_timing.start_time = Some(CrossProcessInstant::now());
     assert!(
-        resource_timing.start_time > 0,
+        resource_timing.start_time.is_some(),
         "`start_time` should have a positive value"
     );
-    assert_eq!(
-        resource_timing.fetch_start, 0,
+    assert!(
+        resource_timing.fetch_start.is_none(),
         "`fetch_start` should be zero"
     );
 
@@ -50,10 +54,13 @@ fn test_set_start_time_to_fetch_start_if_nonzero_no_tao() {
     let mut resource_timing: ResourceFetchTiming =
         ResourceFetchTiming::new(ResourceTimingType::Resource);
     resource_timing.mark_timing_check_failed();
-    resource_timing.fetch_start = 1;
-    assert_eq!(resource_timing.start_time, 0, "`start_time` should be zero");
+    resource_timing.fetch_start = Some(CrossProcessInstant::now());
     assert!(
-        resource_timing.fetch_start > 0,
+        resource_timing.start_time.is_none(),
+        "`start_time` should be zero"
+    );
+    assert!(
+        !resource_timing.fetch_start.is_none(),
         "`fetch_start` should have a positive value"
     );
 
@@ -70,13 +77,13 @@ fn test_set_start_time_to_fetch_start_if_zero_no_tao() {
     let mut resource_timing: ResourceFetchTiming =
         ResourceFetchTiming::new(ResourceTimingType::Resource);
     resource_timing.mark_timing_check_failed();
-    resource_timing.start_time = 1;
+    resource_timing.start_time = Some(CrossProcessInstant::now());
     assert!(
-        resource_timing.start_time > 0,
+        resource_timing.start_time.is_some(),
         "`start_time` should have a positive value"
     );
-    assert_eq!(
-        resource_timing.fetch_start, 0,
+    assert!(
+        resource_timing.fetch_start.is_none(),
         "`fetch_start` should be zero"
     );
 
@@ -92,10 +99,13 @@ fn test_set_start_time_to_fetch_start_if_zero_no_tao() {
 fn test_set_start_time_to_redirect_start_if_nonzero_tao() {
     let mut resource_timing: ResourceFetchTiming =
         ResourceFetchTiming::new(ResourceTimingType::Resource);
-    resource_timing.redirect_start = 1;
-    assert_eq!(resource_timing.start_time, 0, "`start_time` should be zero");
+    resource_timing.redirect_start = Some(CrossProcessInstant::now());
     assert!(
-        resource_timing.redirect_start > 0,
+        resource_timing.start_time.is_none(),
+        "`start_time` should be zero"
+    );
+    assert!(
+        resource_timing.redirect_start.is_some(),
         "`redirect_start` should have a positive value"
     );
 
@@ -113,13 +123,13 @@ fn test_set_start_time_to_redirect_start_if_nonzero_tao() {
 fn test_not_set_start_time_to_redirect_start_if_zero_tao() {
     let mut resource_timing: ResourceFetchTiming =
         ResourceFetchTiming::new(ResourceTimingType::Resource);
-    resource_timing.start_time = 1;
+    resource_timing.start_time = Some(CrossProcessInstant::now());
     assert!(
-        resource_timing.start_time > 0,
+        resource_timing.start_time.is_some(),
         "`start_time` should have a positive value"
     );
-    assert_eq!(
-        resource_timing.redirect_start, 0,
+    assert!(
+        resource_timing.redirect_start.is_none(),
         "`redirect_start` should be zero"
     );
 
@@ -139,10 +149,13 @@ fn test_not_set_start_time_to_redirect_start_if_nonzero_no_tao() {
         ResourceFetchTiming::new(ResourceTimingType::Resource);
     resource_timing.mark_timing_check_failed();
     // Note: properly-behaved redirect_start should never be nonzero once TAO check has failed
-    resource_timing.redirect_start = 1;
-    assert_eq!(resource_timing.start_time, 0, "`start_time` should be zero");
+    resource_timing.redirect_start = Some(CrossProcessInstant::now());
     assert!(
-        resource_timing.redirect_start > 0,
+        resource_timing.start_time.is_none(),
+        "`start_time` should be zero"
+    );
+    assert!(
+        resource_timing.redirect_start.is_some(),
         "`redirect_start` should have a positive value"
     );
 
@@ -161,13 +174,13 @@ fn test_not_set_start_time_to_redirect_start_if_zero_no_tao() {
     let mut resource_timing: ResourceFetchTiming =
         ResourceFetchTiming::new(ResourceTimingType::Resource);
     resource_timing.mark_timing_check_failed();
-    resource_timing.start_time = 1;
+    resource_timing.start_time = Some(CrossProcessInstant::now());
     assert!(
-        resource_timing.start_time > 0,
+        resource_timing.start_time.is_some(),
         "`start_time` should have a positive value"
     );
-    assert_eq!(
-        resource_timing.redirect_start, 0,
+    assert!(
+        resource_timing.redirect_start.is_none(),
         "`redirect_start` should be zero"
     );
 
@@ -185,28 +198,37 @@ fn test_not_set_start_time_to_redirect_start_if_zero_no_tao() {
 fn test_set_start_time() {
     let mut resource_timing: ResourceFetchTiming =
         ResourceFetchTiming::new(ResourceTimingType::Resource);
-    assert_eq!(resource_timing.start_time, 0, "`start_time` should be zero");
+    assert!(
+        resource_timing.start_time.is_none(),
+        "`start_time` should be zero"
+    );
 
     // verify setting `start_time` to current time succeeds
     resource_timing.set_attribute(ResourceAttribute::StartTime(ResourceTimeValue::Now));
-    assert!(resource_timing.start_time > 0, "failed to set `start_time`");
+    assert!(
+        resource_timing.start_time.is_some(),
+        "failed to set `start_time`"
+    );
 }
 #[test]
 fn test_reset_start_time() {
     let mut resource_timing: ResourceFetchTiming =
         ResourceFetchTiming::new(ResourceTimingType::Resource);
-    assert_eq!(resource_timing.start_time, 0, "`start_time` should be zero");
-
-    resource_timing.start_time = 1;
     assert!(
-        resource_timing.start_time > 0,
+        resource_timing.start_time.is_none(),
+        "`start_time` should be zero"
+    );
+
+    resource_timing.start_time = Some(CrossProcessInstant::now());
+    assert!(
+        resource_timing.start_time.is_some(),
         "`start_time` should have a positive value"
     );
 
     // verify resetting `start_time` (to zero) succeeds
     resource_timing.set_attribute(ResourceAttribute::StartTime(ResourceTimeValue::Zero));
-    assert_eq!(
-        resource_timing.start_time, 0,
+    assert!(
+        resource_timing.start_time.is_none(),
         "failed to reset `start_time`"
     );
 }