Allow to disable clocks in WasiCtx (#6007)

Takes the approach described in #6004, but also creates a wrapper for the monotonic time that encapsulates the `creation_time` field as well, since they logically belong and are always used together.

This makes it easier to configure `WasiCtx` with custom clocks as well as disable them for security or determinism reasons.

Closes #6004.

crates/wasi-common/src/clocks.rs

@@ -1,3 +1,4 @@
+use crate::{Error, ErrorExt};
 use cap_std::time::{Duration, Instant, SystemTime};
 
 pub enum SystemTimeSpec {
@@ -15,8 +16,52 @@ pub trait WasiMonotonicClock: Send + Sync {
     fn now(&self, precision: Duration) -> Instant;
 }
 
-pub struct WasiClocks {
-    pub system: Box<dyn WasiSystemClock>,
-    pub monotonic: Box<dyn WasiMonotonicClock>,
+pub struct WasiMonotonicOffsetClock {
     pub creation_time: cap_std::time::Instant,
+    pub abs_clock: Box<dyn WasiMonotonicClock>,
+}
+
+impl WasiMonotonicOffsetClock {
+    pub fn new(clock: impl 'static + WasiMonotonicClock) -> Self {
+        Self {
+            creation_time: clock.now(clock.resolution()),
+            abs_clock: Box::new(clock),
+        }
+    }
+}
+
+pub struct WasiClocks {
+    pub system: Option<Box<dyn WasiSystemClock>>,
+    pub monotonic: Option<WasiMonotonicOffsetClock>,
+}
+
+impl WasiClocks {
+    pub fn new() -> Self {
+        Self {
+            system: None,
+            monotonic: None,
+        }
+    }
+
+    pub fn with_system(mut self, clock: impl 'static + WasiSystemClock) -> Self {
+        self.system = Some(Box::new(clock));
+        self
+    }
+
+    pub fn with_monotonic(mut self, clock: impl 'static + WasiMonotonicClock) -> Self {
+        self.monotonic = Some(WasiMonotonicOffsetClock::new(clock));
+        self
+    }
+
+    pub fn system(&self) -> Result<&dyn WasiSystemClock, Error> {
+        self.system
+            .as_deref()
+            .ok_or_else(|| Error::badf().context("system clock is not supported"))
+    }
+
+    pub fn monotonic(&self) -> Result<&WasiMonotonicOffsetClock, Error> {
+        self.monotonic
+            .as_ref()
+            .ok_or_else(|| Error::badf().context("monotonic clock is not supported"))
+    }
 }