blackbox: do not inherit session_id if pid has changed

Summary:
D17429691 made blackbox reuse session_id unconditionally. That has an
undesirable side effect that chg processes are all logged as a same session id.
Fix that by detecting pid change and avoid reusing session_id in that case.

Reviewed By: singhsrb

Differential Revision: D17532555

fbshipit-source-id: cf11bb66f7d7242429b90ab5e5ea85ca307f92c3

lib/blackbox/src/blackbox.rs

@@ -175,6 +175,11 @@ impl Blackbox {
         }
     }
 
+    /// Get the pid stored in session_id.
+    pub(crate) fn session_pid(&self) -> u32 {
+        (self.session_id & 0xffffff) as u32
+    }
+
     pub fn session_id(&self) -> u64 {
         self.session_id
     }

lib/blackbox/src/singleton.rs

@@ -36,7 +36,11 @@ pub fn init(mut blackbox: Blackbox) {
             }
         }
     }
-    blackbox.session_id = old_blackbox.session_id;
+
+    // Perserve session_id if pid hasn't been changed.
+    if blackbox.session_pid() == old_blackbox.session_pid() {
+        blackbox.session_id = old_blackbox.session_id;
+    }
 
     *singleton.deref_mut() = blackbox;
 }