Use CLOCK_BOOTTIME on Fuchsia (#848)

Fuchsia added kernel support for a monotonic clock that keeps running
while the system is suspended. They technically have not yet exposed it
through their libc, but one maintainer said that they will do so soon.

Cargo.toml

@@ -84,7 +84,7 @@ tiny-skia = { version = "0.11.1", default-features = false, features = [
 tiny-skia-path = { version = "0.11.1", default-features = false, optional = true }
 
 # SVG Rendering
-foldhash = { version = "0.1.3", optional = true }
+foldhash = { version = "0.1.3", default-features = false, optional = true }
 
 # Networking
 splits-io-api = { version = "0.4.0", optional = true }
@@ -117,7 +117,7 @@ windows-sys = { version = "0.59.0", features = [
     "Win32_Graphics_Gdi",
 ], optional = true }
 
-[target.'cfg(any(target_os = "linux", target_os = "l4re", target_os = "android", target_os = "macos", target_os = "ios"))'.dependencies]
+[target.'cfg(any(target_os = "linux", target_os = "l4re", target_os = "android", target_os = "fuchsia", target_os = "macos", target_os = "ios"))'.dependencies]
 # We need libc for our own implementation of Instant
 libc = { version = "0.2.101", optional = true }
 

src/platform/normal/mod.rs

@@ -19,8 +19,8 @@ cfg_if::cfg_if! {
     // correctly implemented in Linux and due to backwards compatibility
     // concerns they were never able to fix it properly. Thus `CLOCK_MONOTONIC`
     // means uptime on Linux whereas on other Unixes it means real time (the BSD
-    // family). So the solution is to use this on all operating systems that are
-    // based on the Linux kernel.
+    // family). As a solution the Linux kernel provides `CLOCK_BOOTTIME` to
+    // measure the real time.
     //
     // # macOS and iOS
     //
@@ -36,26 +36,19 @@ cfg_if::cfg_if! {
     //
     // # Fuchsia
     //
-    // Fuchsia is based on the new Zircon kernel. It has two functions for
-    // querying the time:
+    // Fuchsia is based on the new Zircon kernel. It has `zx_clock_get_boot` to
+    // query the real time  and `zx_clock_get_monotonic` to query the uptime.
     //
-    // zx_clock_get:
-    // https://fuchsia.dev/fuchsia-src/reference/syscalls/clock_get
-    // zx_clock_get_monotonic:
-    // https://fuchsia.dev/fuchsia-src/reference/syscalls/clock_get_monotonic
+    // https://fuchsia.dev/reference/syscalls/clock_get_boot
     //
-    // `zx_clock_get_monotonic` specifically calls out that it `does not adjust
-    // during sleep` which seems to mean that it doesn't count the time the OS
-    // is suspended. This is further evidenced by their libc implementation not
-    // treating `CLOCK_BOOTTIME` differently and a bug ticket being linked
-    // there:
-    // https://cs.opensource.google/fuchsia/fuchsia/+/main:zircon/third_party/ulib/musl/src/time/clock_gettime.c;l=40;drc=35e7a15cb21e16f0705560e5812b7a045d42c8a5
+    // They are supposed to be available through `CLOCK_BOOTTIME` and
+    // `CLOCK_MONOTONIC` respectively, just like on Linux.
     //
     // # WASI
     //
     // https://github.com/WebAssembly/WASI/blob/5ab83a68d4eb4f218a898ed03b963b7393caaedc/phases/snapshot/docs.md#variant-cases
     //
-    // WASI seems to underspecify its `monotonic` a bit, but says that it `is
+    // WASI seems to under specify its `monotonic` a bit, but says that it `is
     // defined as a clock measuring real time`, making it sound like a compliant
     // implementation should measure the time the OS is suspended as well.
     //
@@ -98,10 +91,14 @@ cfg_if::cfg_if! {
     // the Linux kernel and instead has its own implementation in JavaScript
     // where it actually errors out on `CLOCK_BOOTTIME`:
     // https://github.com/emscripten-core/emscripten/blob/9bdb310b89472a0f4d64f36e4a79273d8dc7fa98/system/lib/libc/emscripten_time.c#L50-L57
+    //
+    // And we add Fuchsia to this list as it's based on the Zircon kernel which
+    // uses `CLOCK_BOOTTIME` in the same way as Linux.
     if #[cfg(any(
         target_os = "linux",
         target_os = "l4re",
         target_os = "android",
+        target_os = "fuchsia",
     ))] {
         use core::{mem::MaybeUninit, ops::Sub};