dag: add fail points on opening namedag

Summary:
It turns out that the namedag was opened multiple times. Add a fail point to
help figure out the callsite.

The `fail` crate allows something like:

  FAILPOINTS="dag-namedag-open=1*sleep(1)->return"
  FAILPOINTS="dag-namedag-open=1*sleep(1)->panic"

Meaning that the first open causes 1ms sleep, and the second
causes an error (turns into a Python backtrace), or a panic (turns into a Rust
backtrace with RUST_BACKTRACE=1).

Reviewed By: andll

Differential Revision: D29888937

fbshipit-source-id: b1644d7196f68262523ab9a5fc4fb110a4cc0062

eden/scm/lib/dag/Cargo.toml

@@ -27,6 +27,7 @@ bitflags = "1.2"
 byteorder = "1.3"
 dag-types = { path = "dag-types" }
 drawdag = { path = "../drawdag" }
+fail = { version = "0.4", features = ["failpoints"] }
 fs2 = { version = "0.4", optional = true }
 futures = { version = "0.3.13", features = ["async-await", "compat"] }
 indexedlog = { path = "../indexedlog", optional = true }

eden/scm/lib/dag/src/lib.rs

@@ -74,5 +74,17 @@ pub type Result<T> = std::result::Result<T, Error>;
 pub use indexedlog::Repair;
 pub use nonblocking;
 
+#[macro_export]
+macro_rules! failpoint {
+    ($name:literal) => {
+        ::fail::fail_point!($name, |_| {
+            let msg = format!("failpoint injected by FAILPOINTS: {}", $name);
+            Err($crate::errors::DagError::from(
+                $crate::errors::BackendError::Generic(msg),
+            ))
+        })
+    };
+}
+
 #[cfg(test)]
 dev_logger::init!();

eden/scm/lib/dag/src/namedag/indexedlog_namedag.rs

@@ -42,6 +42,7 @@ impl Open for IndexedLogNameDagPath {
     type OpenTarget = NameDag;
 
     fn open(&self) -> Result<Self::OpenTarget> {
+        crate::failpoint!("dag-namedag-open");
         let path = &self.0;
         let opts = NameDag::default_open_options();
         let mut mlog = opts.open(path)?;