Do local dep file cache lookup before a remote dep file cache lookup

Summary:
I think we should always use local dep files if we have them, as they are faster to access that remote dep files. This changes the order of the lookups.

I'm not sure if this needs a post in "Buck2 Dev", since it's not really an API change but it will affect some metrics. Tbh, I probably want to fix a bunch of metrics around this stuff anyway, so maybe worth a fuller post saying that I want to make this change along with some metrics changes. (E.g. right now I don't think we count identical dep file hits separately from actually dep file lookups in buck2 builds - though we do in buck2 actions).

Reviewed By: blackm00n

Differential Revision: D74064655

fbshipit-source-id: f6ad95f225c338c93f5b31ad3a70b57e90a06b75

Author: Ian Childs

app/buck2_action_impl/src/actions/impls/run.rs

@@ -677,6 +677,17 @@ impl RunAction {
         let result = match action_cache_result {
             ControlFlow::Break(_) => action_cache_result,
             ControlFlow::Continue(manager) => {
+                // If we didn't find anything in the action cache, first do a local dep file cache lookup, and if that fails,
+                // try to find a remote dep file cache hit.
+                if should_fully_check_dep_file_cache {
+                    let lookup = dep_file_bundle
+                        .check_local_dep_file_cache(ctx, self.outputs.as_slice())
+                        .await?;
+                    if let Some((outputs, metadata)) = lookup {
+                        return Ok(ExecuteResult::LocalDepFileHit(outputs, metadata));
+                    }
+                }
+
                 let remote_dep_file_result = ctx
                     .remote_dep_file_cache(manager, &req, &prepared_action)
                     .await;
@@ -697,21 +708,10 @@ impl RunAction {
             }
         };
 
-        // If the cache queries did not yield to a result, fallback to local dep file query (continuation), then execution.
+        // If the cache queries did not yield to a result, then we need to execute the action.
         let mut result = match result {
             ControlFlow::Break(res) => res,
-            ControlFlow::Continue(manager) => {
-                if should_fully_check_dep_file_cache {
-                    let lookup = dep_file_bundle
-                        .check_local_dep_file_cache(ctx, self.outputs.as_slice())
-                        .await?;
-                    if let Some((outputs, metadata)) = lookup {
-                        return Ok(ExecuteResult::LocalDepFileHit(outputs, metadata));
-                    }
-                }
-
-                ctx.exec_cmd(manager, &req, &prepared_action).await
-            }
+            ControlFlow::Continue(manager) => ctx.exec_cmd(manager, &req, &prepared_action).await,
         };
 
         // If the action has a dep file, log the remote dep file key so we can look out for collisions

tests/core/build/test_dep_files.py

@@ -620,10 +620,21 @@ async def test_re_dep_file_query_change_tagged_unused_file(buck: Buck) -> None:
         # Check the MatchDepFiles events
         await check_match_dep_files(buck, expected_dep_file_match)
 
+    # # Change a file that is tracked by a dep file but shows up as unused, we get a local dep file cache hit
+    # # as that is checked first.
+    tagged_unused.write_text("CHANGE")
+    result = await buck.build(*target_upload_enabled)
+    output = result.get_build_report().output_for_target(target).read_text()
+    assert output == "used1\nused2\nused3\n"
+
+    execution_kind = await _get_execution_kind(buck)
+    assert execution_kind == ACTION_EXECUTION_KIND_LOCAL_DEP_FILE
+
     # Change a file that is tracked by a dep file but shows up as unused, this will again result in one of
     # 1. A remote dep file cache hit and a subsequent dep file validation
     # 2. A remote dep file cache miss, fall back to local execution (local dep file cache is flushed)
-    tagged_unused.write_text("CHANGE")
+    await buck.debug("flush-dep-files")
+    tagged_unused.write_text("CHANGE_AGAIN")
     result = await buck.build(*target_upload_enabled)
     output = result.get_build_report().output_for_target(target).read_text()
     assert output == "used1\nused2\nused3\n"