watchman: rust: implement subscribe method

Summary:
adds `Client::subscribe` which is similar in usage to
`Client::query`, but returns a subscription handle that will
yield file changes as they occur.

An example of usage is in `examples/subscribe.rs`.

Note that the metadata fields are omitted in this version and commented out; to
implement those we need to have an alternative to `serde_json::Value` that can
handle the binary strings that watchman produces for some metadata; that will
be tackled in the next diff in this stack.

Reviewed By: jsgf

Differential Revision: D18598229

fbshipit-source-id: 6d89a8544d3571d820855c78c261265465fbe1cb

rust/watchman_client/examples/glob.rs

@@ -21,10 +21,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 }
 
 async fn run() -> Result<(), Box<dyn std::error::Error>> {
-    let mut client = Connector::new().connect().await?;
-
     let opt = Opt::from_args();
-
+    let client = Connector::new().connect().await?;
     let resolved = client
         .resolve_root(CanonicalPath::canonicalize(opt.path)?)
         .await?;

rust/watchman_client/examples/subscribe.rs

@@ -0,0 +1,40 @@
+//! This example shows how to initiate a subscription and print out
+//! file changes as they are reported
+use std::path::PathBuf;
+use structopt::StructOpt;
+use watchman_client::prelude::*;
+
+#[derive(Debug, StructOpt)]
+#[structopt(about = "Subscribe to watchman and stream file changes for a path")]
+struct Opt {
+    #[structopt(default_value = ".")]
+    path: PathBuf,
+}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    if let Err(err) = run().await {
+        // Print a prettier error than the default
+        eprintln!("{}", err);
+        std::process::exit(1);
+    }
+    Ok(())
+}
+
+async fn run() -> Result<(), Box<dyn std::error::Error>> {
+    let opt = Opt::from_args();
+    let client = Connector::new().connect().await?;
+    let resolved = client
+        .resolve_root(CanonicalPath::canonicalize(opt.path)?)
+        .await?;
+
+    let (mut sub, initial) = client
+        .subscribe::<NameOnly>(&resolved, SubscribeRequest::default())
+        .await?;
+
+    println!("{} {:#?}", sub.name(), initial);
+    loop {
+        let item = sub.next().await?;
+        println!("{:#?}", item);
+    }
+}