Do not panic if hg command is not available

Reviewed By: tyao1

Differential Revision: D32066385

fbshipit-source-id: 3ff1acca165a9e77f1f306dde132882f417e3469

compiler/crates/graphql-watchman/src/subscription.rs

@@ -39,7 +39,7 @@ pub struct WatchmanFileSourceResult {
 pub struct WatchmanFileSourceSubscription {
     resolved_root: ResolvedRoot,
     subscription: WatchmanSubscription<WatchmanFile>,
-    base_revision: String,
+    base_revision: Option<String>,
 }
 
 impl WatchmanFileSourceSubscription {
@@ -50,7 +50,7 @@ impl WatchmanFileSourceSubscription {
         Self {
             resolved_root,
             subscription,
-            base_revision: get_base_revision(None),
+            base_revision: get_base_hg_revision(None),
         }
     }
 
@@ -88,7 +88,7 @@ impl WatchmanFileSourceSubscription {
                     } else {
                         None
                     };
-                    let current_base_revision = get_base_revision(current_commit);
+                    let current_base_revision = get_base_hg_revision(current_commit);
                     if current_base_revision != self.base_revision {
                         self.base_revision = current_base_revision;
                         return Ok(WatchmanFileSourceSubscriptionNextChange::SourceControlUpdate);
@@ -113,7 +113,7 @@ impl WatchmanFileSourceSubscription {
 /// `master` and current commit hash or `.`
 ///
 /// TODO: Make this dynamic on the default branch name.
-fn get_base_revision(commit_hash: Option<String>) -> String {
+fn get_base_hg_revision(commit_hash: Option<String>) -> Option<String> {
     let output = Command::new("hg")
         .arg("log".to_string())
         .arg("-r".to_string())
@@ -123,14 +123,11 @@ fn get_base_revision(commit_hash: Option<String>) -> String {
         ))
         .arg("-T={node}")
         .output()
-        .expect("Expect `hg` command getting base revision.");
+        .ok()?;
 
     if output.stdout.is_empty() {
-        panic!(
-            "Failed to get base revision hash:\n {:?}",
-            String::from_utf8_lossy(&output.stderr)
-        );
+        return None;
     }
 
-    String::from_utf8_lossy(&output.stdout).to_string()
+    Some(String::from_utf8_lossy(&output.stdout).to_string())
 }