Retain the existing timer backend for short scan deadlines

Cargo.lock

@@ -103,6 +103,24 @@ version = "1.0.96"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6b964d184e89d9b6b67dd2715bc8e74cf3107fb2b529990c90cf517326150bf4"
 
+[[package]]
+name = "async-io"
+version = "2.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "456b8a8feb6f42d237746d4b3e9a178494627745c3c56c6ea55d92ba50d026fc"
+dependencies = [
+ "autocfg",
+ "cfg-if",
+ "concurrent-queue",
+ "futures-io",
+ "futures-lite",
+ "parking",
+ "polling",
+ "rustix",
+ "slab",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "async-trait"
 version = "0.1.86"
@@ -288,6 +306,15 @@ version = "0.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ffb474a9c3219a8254ead020421ecf1b90427f29b55f6aae9a2471fa62c126ef"
 
+[[package]]
+name = "concurrent-queue"
+version = "2.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4ca0197aee26d1ae37445ee532fefce43251d24cc7c166799f4d46817f1d3973"
+dependencies = [
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "cpufeatures"
 version = "0.3.0"
@@ -521,6 +548,16 @@ version = "0.3.31"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9e5c1b78ca4aae1ac06c48a526a655760685149f0d465d21f37abfe57ce075c6"
 
+[[package]]
+name = "futures-lite"
+version = "2.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f78e10609fe0e0b3f4157ffab1876319b5b0db102a2c60dc4626306dc46b44ad"
+dependencies = [
+ "futures-core",
+ "pin-project-lite",
+]
+
 [[package]]
 name = "futures-macro"
 version = "0.3.31"
@@ -544,12 +581,6 @@ version = "0.3.31"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f90f7dce0722e95104fcb095585910c0977252f286e354b5e3bd38902cd99988"
 
-[[package]]
-name = "futures-timer"
-version = "3.0.4"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "af43fadb8a98512d547e37b4e92e0ced13e205c061b87b4623eff01d918d6968"
-
 [[package]]
 name = "futures-util"
 version = "0.3.31"
@@ -645,6 +676,12 @@ version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 
+[[package]]
+name = "hermit-abi"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e17592d60ebacc7d5e169f4663c5f84f9161cc90328abcfe8456f41e4dfcb284"
+
 [[package]]
 name = "hickory-proto"
 version = "0.24.4"
@@ -1120,6 +1157,12 @@ dependencies = [
  "syn 2.0.117",
 ]
 
+[[package]]
+name = "parking"
+version = "2.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f38d5652c16fde515bb1ecef450ab0f6a219d619a7274976324d5e377f7dceba"
+
 [[package]]
 name = "parking_lot"
 version = "0.12.3"
@@ -1189,6 +1232,20 @@ dependencies = [
  "plotters-backend",
 ]
 
+[[package]]
+name = "polling"
+version = "3.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d0e4f59085d47d8241c88ead0f274e8a0cb551f3625263c05eb8dd897c34218"
+dependencies = [
+ "cfg-if",
+ "concurrent-queue",
+ "hermit-abi",
+ "pin-project-lite",
+ "rustix",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "portable-atomic"
 version = "1.11.0"
@@ -1453,6 +1510,7 @@ dependencies = [
  "ansi_term",
  "anstream",
  "anyhow",
+ "async-io",
  "cidr-utils",
  "clap",
  "colored",
@@ -1461,7 +1519,6 @@ dependencies = [
  "dirs",
  "env_logger",
  "futures",
- "futures-timer",
  "gcd",
  "hickory-resolver",
  "itertools 0.15.0",

Cargo.toml

@@ -21,7 +21,7 @@ clap = { version = "4.6.1", features = ["derive", "wrap_help"] }
 colored = "3.1.1"
 tokio = { version = "1.53.1", features = ["rt", "net", "time"] }
 futures = "0.3"
-futures-timer = "3.0.4"
+async-io = "2.4.0"
 rlimit = "0.11.0"
 log = "0.4.32"
 env_logger = "0.11.10"

docs/tokio-migration.md

@@ -15,7 +15,8 @@ runtime. Script execution runs after the runtime has been dropped. Port ordering
 exclusions, retries, UDP payloads, closed-port reporting and CLI options retain
 their existing implementations.
 
-Timeouts and port intervals use the runtime-independent `futures-timer` crate.
+Timeouts and port intervals retain `async_io::Timer`, the timer underneath
+async-std, independently of the Tokio socket runtime.
 An expired timer maps to `std::io::ErrorKind::TimedOut`. Socket errors retain
 their original kind, so TCP refusal reporting and UDP timeout retries continue
 to distinguish replies from failures. Successful TCP streams are converted to
@@ -23,8 +24,8 @@ standard streams, shut down in both directions, and dropped before another
 attempt is polled.
 
 Using Tokio's timers for short waits added approximately one millisecond per
-interval in the first benchmark. `futures-timer` uses a shared helper thread and
-OS waits, avoiding that extra rounding while keeping the async runtime responsive.
+interval in the first benchmark. Retaining the existing timer avoids that extra
+rounding while keeping the async runtime responsive.
 Dropping a scan cancels its timers; runtime shutdown never has to wait for a
 sleeping blocking task. Immediately ready I/O operations avoid timer allocation,
 and the time spent polling a pending operation is charged to its original timeout.
@@ -71,10 +72,10 @@ owns its own runtime and performs blocking lookups.
 The direct Tokio dependency enables only `rt`, `net`, and `time`; `test-util` is
 enabled for deterministic tests with a paused clock. Tokio was already present
 through Hickory. The existing `futures` dependency continues to provide
-`FuturesUnordered`, `StreamExt` and future polling. The new `futures-timer` 3.0.4
-dependency has no transitive dependencies on native targets; its purpose is to
-preserve short timeout and interval performance. `async-std` is absent from the
-resolved dependency tree.
+`FuturesUnordered`, `StreamExt` and future polling. `async-io` becomes a direct
+dependency solely to retain the existing timer implementation and its short-wait
+performance; socket I/O uses Tokio. `async-std` and its executor are absent from
+the resolved dependency tree.
 
 Regression tests cover successful operations (including zero-byte responses),
 preservation of I/O errors, timeout cancellation, immediate results at a zero

src/scanner/mod.rs

@@ -165,7 +165,10 @@ impl Scanner {
         // Tokio's millisecond rounding adds about 1 ms to each short interval.
         // This cancellable delay keeps the requested spacing without blocking
         // the async runtime or accumulating that rounding on every port.
-        self.run_with_delay(futures_timer::Delay::new).await
+        self.run_with_delay(|interval| async move {
+            async_io::Timer::after(interval).await;
+        })
+        .await
     }
 
     /// Supplying the delay lets tests verify spacing with a virtual clock.

src/scanner/timeout.rs

@@ -20,7 +20,7 @@ pub(super) async fn io_timeout<T>(
 
     // Charge socket setup to the original deadline. The timer uses OS waits
     // without rounding each short timeout up to Tokio's next millisecond tick.
-    let delay = futures_timer::Delay::new(duration.saturating_sub(started.elapsed()));
+    let delay = async_io::Timer::after(duration.saturating_sub(started.elapsed()));
     match select(operation, delay).await {
         Either::Left((result, _)) => result,
         Either::Right(_) => Err(io::Error::new(io::ErrorKind::TimedOut, "future timed out")),