Reimplement blaze query using an async evaluation model. Use a concurrent backend for SkyQueryEnvironment's implementation in order to achieve parallelism.

Advantages:
-New design has no flaws that the old design had.
-Code is structured so that deadlocks due to thread starvation are impossible (yup!).

Disadvantages:
-The meat of this change needs to all be in a single CL because every single QueryFunction and QueryExpression needs to be rewritten in the async style.

Still TODO:
-Fully embrace the async model in all QueryFunctions (e.g. 'rdeps', 'allpaths').
-Use concurrency in BlazeQueryEnvironment to achieve parallel evaluation for (non SkyQuery) 'blaze query' and genquery.

--
PiperOrigin-RevId: 148690279
MOS_MIGRATED_REVID=148690279

1 files changed, 18 insertions, 7 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/query2/engine/LetExpression.java b/src/main/java/com/google/devtools/build/lib/query2/engine/LetExpression.java
index 64d94da19a..a7c3abeb62 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/engine/LetExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/engine/LetExpression.java
@@ -13,6 +13,8 @@
 // limitations under the License.
 package com.google.devtools.build.lib.query2.engine;
 
+import com.google.common.base.Function;
+import com.google.devtools.build.lib.query2.engine.QueryEnvironment.QueryTaskFuture;
 import java.util.Collection;
 import java.util.Set;
 import java.util.regex.Pattern;
@@ -64,15 +66,24 @@ class LetExpression extends QueryExpression {
   }
 
   @Override
-  protected <T> void evalImpl(
-      QueryEnvironment<T> env, VariableContext<T> context, Callback<T> callback)
-          throws QueryException, InterruptedException {
+  public <T> QueryTaskFuture<Void> eval(
+      final QueryEnvironment<T> env,
+      final VariableContext<T> context,
+      final Callback<T> callback) {
     if (!NAME_PATTERN.matcher(varName).matches()) {
-      throw new QueryException(this, "invalid variable name '" + varName + "' in let expression");
+      return env.immediateFailedFuture(
+          new QueryException(this, "invalid variable name '" + varName + "' in let expression"));
     }
-    Set<T> varValue = QueryUtil.evalAll(env, context, varExpr);
-    VariableContext<T> bodyContext = VariableContext.with(context, varName, varValue);
-    env.eval(bodyExpr, bodyContext, callback);
+    QueryTaskFuture<Set<T>> varValueFuture = QueryUtil.evalAll(env, context, varExpr);
+    Function<Set<T>, QueryTaskFuture<Void>> evalBodyAsyncFunction =
+        new Function<Set<T>, QueryTaskFuture<Void>>() {
+          @Override
+          public QueryTaskFuture<Void> apply(Set<T> varValue) {
+            VariableContext<T> bodyContext = VariableContext.with(context, varName, varValue);
+            return env.eval(bodyExpr, bodyContext, callback);
+          }
+    };
+    return env.transformAsync(varValueFuture, evalBodyAsyncFunction);
   }
 
   @Override