tail: add WASI support by disabling notify and pid features (#11569)

WASI has no file watching (inotify/kqueue) or process management APIs.
Disable notify and libc on WASI, provide Observer stubs so tail
compiles, and add fallback values for BACKEND and polling_help.
The follow mode (-f) returns an error on WASI since it cannot work
without a file watching backend.

Cargo.toml

@@ -216,6 +216,7 @@ feat_wasm = [
   "shuf",
   "sleep",
   "sum",
+  "tail",
   "tee",
   "tr",
   "true",

src/uu/tail/Cargo.toml

@@ -21,13 +21,15 @@ path = "src/tail.rs"
 
 [dependencies]
 clap = { workspace = true }
-libc = { workspace = true }
 memchr = { workspace = true }
-notify = { workspace = true }
 uucore = { workspace = true, features = ["fs", "parser-size", "signals"] }
 same-file = { workspace = true }
 fluent = { workspace = true }
 
+[target.'cfg(not(target_os = "wasi"))'.dependencies]
+libc = { workspace = true }
+notify = { workspace = true }
+
 [target.'cfg(unix)'.dependencies]
 rustix = { workspace = true, features = ["fs"] }
 

src/uu/tail/src/args.rs

@@ -447,6 +447,8 @@ pub fn uu_app() -> Command {
     let polling_help = translate!("tail-help-polling-unix");
     #[cfg(target_os = "windows")]
     let polling_help = translate!("tail-help-polling-windows");
+    #[cfg(not(any(unix, target_os = "windows")))]
+    let polling_help = translate!("tail-help-polling-unix");
 
     Command::new(uucore::util_name())
         .version(uucore::crate_version!())

src/uu/tail/src/follow/mod.rs

@@ -4,6 +4,69 @@
 // file that was distributed with this source code.
 
 mod files;
+#[cfg(not(target_os = "wasi"))]
 mod watch;
 
+#[cfg(not(target_os = "wasi"))]
 pub use watch::{Observer, follow};
+
+// WASI: notify/inotify are unavailable, so `tail -f` cannot work.
+// Provide minimal stubs matching the real Observer API so tail compiles.
+#[cfg(target_os = "wasi")]
+mod wasi_stubs {
+    use crate::args::Settings;
+    use std::io::BufRead;
+    use std::path::Path;
+    use uucore::error::{UResult, USimpleError};
+
+    pub struct Observer {
+        pub use_polling: bool,
+        pub pid: super::super::platform::Pid,
+    }
+
+    impl Observer {
+        pub fn from(settings: &Settings) -> Self {
+            Self {
+                use_polling: false,
+                pid: settings.pid,
+            }
+        }
+
+        pub fn start(&mut self, _settings: &Settings) -> UResult<()> {
+            Ok(())
+        }
+
+        pub fn add_path(
+            &mut self,
+            _path: &Path,
+            _display_name: &str,
+            _reader: Option<Box<dyn BufRead>>,
+            _update_last: bool,
+        ) -> UResult<()> {
+            Ok(())
+        }
+
+        pub fn add_bad_path(
+            &mut self,
+            _path: &Path,
+            _display_name: &str,
+            _update_last: bool,
+        ) -> UResult<()> {
+            Ok(())
+        }
+
+        pub fn follow_name_retry(&self) -> bool {
+            false
+        }
+    }
+
+    pub fn follow(_observer: Observer, _settings: &Settings) -> UResult<()> {
+        Err(USimpleError::new(
+            1,
+            "follow mode is not supported on this platform",
+        ))
+    }
+}
+
+#[cfg(target_os = "wasi")]
+pub use wasi_stubs::{Observer, follow};

src/uu/tail/src/paths.rs

@@ -197,6 +197,10 @@ impl MetadataExtTail for Metadata {
             // }
             false
         }
+        #[cfg(not(any(unix, windows)))]
+        {
+            false
+        }
     }
 }
 

src/uu/tail/src/platform/mod.rs

@@ -14,6 +14,28 @@ pub use self::unix::{
 #[cfg(windows)]
 pub use self::windows::{Pid, ProcessChecker, supports_pid_checks};
 
+// WASI has no process management; provide stubs so tail compiles.
+#[cfg(target_os = "wasi")]
+pub type Pid = u64;
+
+#[cfg(target_os = "wasi")]
+pub struct ProcessChecker;
+
+#[cfg(target_os = "wasi")]
+impl ProcessChecker {
+    pub fn new(_pid: Pid) -> Self {
+        Self
+    }
+    pub fn is_dead(&mut self) -> bool {
+        true
+    }
+}
+
+#[cfg(target_os = "wasi")]
+pub fn supports_pid_checks(_pid: Pid) -> bool {
+    false
+}
+
 #[cfg(unix)]
 mod unix;
 

src/uu/tail/src/text.rs

@@ -18,3 +18,5 @@ pub const BACKEND: &str = "inotify";
 pub const BACKEND: &str = "kqueue";
 #[cfg(target_os = "windows")]
 pub const BACKEND: &str = "ReadDirectoryChanges";
+#[cfg(not(any(unix, target_os = "windows")))]
+pub const BACKEND: &str = "polling";