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diff --git a/toolkit/components/glean/api/src/private/timespan.rs b/toolkit/components/glean/api/src/private/timespan.rs
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+++ b/toolkit/components/glean/api/src/private/timespan.rs
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+// This Source Code Form is subject to the terms of the Mozilla Public
+// 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 inherent::inherent;
+
+use super::{CommonMetricData, MetricId, TimeUnit};
+use std::convert::TryInto;
+use std::time::Duration;
+
+use glean::traits::Timespan;
+
+use crate::ipc::need_ipc;
+
+/// A timespan metric.
+///
+/// Timespans are used to make a measurement of how much time is spent in a particular task.
+pub enum TimespanMetric {
+    Parent(glean::private::TimespanMetric, TimeUnit),
+    Child,
+}
+
+impl TimespanMetric {
+    /// Create a new timespan metric.
+    pub fn new(_id: MetricId, meta: CommonMetricData, time_unit: TimeUnit) -> Self {
+        if need_ipc() {
+            TimespanMetric::Child
+        } else {
+            TimespanMetric::Parent(
+                glean::private::TimespanMetric::new(meta, time_unit),
+                time_unit,
+            )
+        }
+    }
+
+    /// Only to be called from the MLA FFI.
+    /// If you don't know what that is, don't call this.
+    pub fn set_raw_unitless(&self, duration: u64) {
+        match self {
+            TimespanMetric::Parent(p, time_unit) => {
+                p.set_raw(Duration::from_nanos(time_unit.as_nanos(duration)));
+            }
+            TimespanMetric::Child => {
+                log::error!(
+                    "Unable to set_raw_unitless on timespan in non-main process. This operation will be ignored."
+                );
+                // If we're in automation we can panic so the instrumentor knows they've gone wrong.
+                // This is a deliberate violation of Glean's "metric APIs must not throw" design.
+                assert!(!crate::ipc::is_in_automation(), "Attempted to set_raw_unitless on timespan metric in non-main process, which is forbidden. This panics in automation.");
+                // TODO: Record an error. bug 1704504.
+            }
+        }
+    }
+}
+
+#[inherent]
+impl Timespan for TimespanMetric {
+    pub fn start(&self) {
+        match self {
+            TimespanMetric::Parent(p, _) => p.start(),
+            TimespanMetric::Child => {
+                log::error!("Unable to start timespan metric in non-main process. This operation will be ignored.");
+                // If we're in automation we can panic so the instrumentor knows they've gone wrong.
+                // This is a deliberate violation of Glean's "metric APIs must not throw" design.assert!(!crate::ipc::is_in_automation());
+                assert!(!crate::ipc::is_in_automation(), "Attempted to start timespan metric in non-main process, which is forbidden. This panics in automation.");
+                // TODO: Record an error. bug 1704504.
+            }
+        }
+    }
+
+    pub fn stop(&self) {
+        match self {
+            TimespanMetric::Parent(p, _) => p.stop(),
+            TimespanMetric::Child => {
+                log::error!("Unable to stop timespan metric in non-main process. This operation will be ignored.");
+                // If we're in automation we can panic so the instrumentor knows they've gone wrong.
+                // This is a deliberate violation of Glean's "metric APIs must not throw" design.assert!(!crate::ipc::is_in_automation());
+                assert!(!crate::ipc::is_in_automation(), "Attempted to stop timespan metric in non-main process, which is forbidden. This panics in automation.");
+                // TODO: Record an error. bug 1704504.
+            }
+        }
+    }
+
+    pub fn cancel(&self) {
+        match self {
+            TimespanMetric::Parent(p, _) => p.cancel(),
+            TimespanMetric::Child => {
+                log::error!("Unable to cancel timespan metric in non-main process. This operation will be ignored.");
+                // If we're in automation we can panic so the instrumentor knows they've gone wrong.
+                // This is a deliberate violation of Glean's "metric APIs must not throw" design.assert!(!crate::ipc::is_in_automation());
+                assert!(!crate::ipc::is_in_automation(), "Attempted to cancel timespan metric in non-main process, which is forbidden. This panics in automation.");
+                // TODO: Record an error. bug 1704504.
+            }
+        }
+    }
+
+    pub fn set_raw(&self, elapsed: Duration) {
+        let elapsed = elapsed.as_nanos().try_into().unwrap_or(i64::MAX);
+        match self {
+            TimespanMetric::Parent(p, _) => p.set_raw_nanos(elapsed),
+            TimespanMetric::Child => {
+                log::error!("Unable to set_raw on timespan in non-main process. This operation will be ignored.");
+                // If we're in automation we can panic so the instrumentor knows they've gone wrong.
+                // This is a deliberate violation of Glean's "metric APIs must not throw" design.assert!(!crate::ipc::is_in_automation());
+                assert!(!crate::ipc::is_in_automation(), "Attempted to set_raw on timespan metric in non-main process, which is forbidden. This panics in automation.");
+                // TODO: Record an error. bug 1704504.
+            }
+        }
+    }
+
+    pub fn test_get_value<'a, S: Into<Option<&'a str>>>(&self, ping_name: S) -> Option<u64> {
+        let ping_name = ping_name.into().map(|s| s.to_string());
+        match self {
+            // Conversion is ok here:
+            // Timespans are really tricky to set to excessive values with the pleasant APIs.
+            TimespanMetric::Parent(p, _) => p.test_get_value(ping_name).map(|i| i as u64),
+            TimespanMetric::Child => {
+                panic!("Cannot get test value for in non-main process!");
+            }
+        }
+    }
+
+    pub fn test_get_num_recorded_errors(&self, error: glean::ErrorType) -> i32 {
+        match self {
+            TimespanMetric::Parent(p, _) => p.test_get_num_recorded_errors(error),
+            TimespanMetric::Child => {
+                panic!("Cannot get the number of recorded errors for timespan metric in non-main process!");
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::{common_test::*, ipc, metrics};
+
+    #[test]
+    fn smoke_test_timespan() {
+        let _lock = lock_test();
+
+        let metric = TimespanMetric::new(
+            0.into(),
+            CommonMetricData {
+                name: "timespan_metric".into(),
+                category: "telemetry".into(),
+                send_in_pings: vec!["store1".into()],
+                disabled: false,
+                ..Default::default()
+            },
+            TimeUnit::Nanosecond,
+        );
+
+        metric.start();
+        // Stopping right away might not give us data, if the underlying clock source is not precise
+        // enough.
+        // So let's cancel and make sure nothing blows up.
+        metric.cancel();
+
+        assert_eq!(None, metric.test_get_value("store1"));
+    }
+
+    #[test]
+    fn timespan_ipc() {
+        let _lock = lock_test();
+        let _raii = ipc::test_set_need_ipc(true);
+
+        let child_metric = &metrics::test_only::can_we_time_it;
+
+        // Instrumentation calls do not panic.
+        child_metric.start();
+        // Stopping right away might not give us data,
+        // if the underlying clock source is not precise enough.
+        // So let's cancel and make sure nothing blows up.
+        child_metric.cancel();
+
+        // (They also shouldn't do anything,
+        // but that's not something we can inspect in this test)
+    }
+}