Limit number of reductions made by Prolog

Allow site administrators to place an upper bound on how much CPU
time will be spent evaluating rules for a single change. This covers
not just the project rules, but also all filters up to All-Projects.

Reduction counting and not wall-clock time is used to be fair to
rules that are running on a temporarily overloaded system. It also
permits builtin predicates that are expensive like commit_edits/2
to count only once per result made to Prolog, and not charge time
waiting for JGit to populate the PatchListCache.

The default is configured to be 100,000 reductions. This value was
experimentally determined on a Intel Core i7 running at 2.3 GHz and
Java 1.6.0_37. On that system reaching the limit of reductions and
aborting an infinite loop in a user predicate takes about 14 ms.

Any rules taking 14 ms to run will add ~350 ms to the latency of the
All > Open changes screen. Because of this, Administrators already
have sufficient incentives to keep rule running time low.

Change-Id: I2cd5660fc556e6aeeb61606a850ce45c98157c96

Documentation/config-gerrit.txt

@@ -2800,6 +2800,31 @@ only the default internal rules will be used.
 +
 Default is true, to execute project specific rules.
 
+[[rules.reductionLimit]]rules.reductionLimit::
++
+Maximum number of Prolog reductions that can be performed when
+evaluating rules for a single change. Each function call made
+in user rule code, internal Gerrit Prolog code, or the Prolog
+interpreter counts against this limit.
++
+Sites using very complex rules that need many reductions should
+compile Prolog to Java bytecode with link:pgm-rulec.html[rulec].
+This eliminates the dynamic Prolog interpreter from charging its
+own reductions against the limit, enabling more logic to execute
+within the same bounds.
++
+A reductionLimit of 0 is nearly infinite, implemented by setting
+the internal limit to 2^31-1.
++
+Default is 100,000 reductions (about 14 ms on Intel Core i7 CPU).
+
+[[rules.compileReductionLimit]]rules.compileReductionLimit::
++
+Maximum number of Prolog reductions that can be performed when
+compiling source code to internal Prolog machine code.
++
+Default is 10x reductionLimit (1,000,000).
+
 [[execution]]
 === Section execution
 

gerrit-server/BUCK

@@ -149,8 +149,10 @@ java_test(
     '//gerrit-common:server',
     '//gerrit-reviewdb:server',
     '//gerrit-server/src/main/prolog:common',
+    '//lib:guava',
     '//lib:gwtorm',
     '//lib:junit',
+    '//lib:truth',
     '//lib/jgit:jgit',
     '//lib/guice:guice',
     '//lib/prolog:prolog-cafe',

gerrit-server/src/main/java/com/google/gerrit/rules/PrologEnvironment.java

@@ -16,6 +16,7 @@ package com.google.gerrit.rules;
 
 import com.google.gerrit.server.AnonymousUser;
 import com.google.gerrit.server.IdentifiedUser;
+import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.git.GitRepositoryManager;
 import com.google.gerrit.server.patch.PatchListCache;
 import com.google.gerrit.server.patch.PatchSetInfoFactory;
@@ -26,9 +27,11 @@ import com.google.inject.Singleton;
 import com.google.inject.assistedinject.Assisted;
 
 import com.googlecode.prolog_cafe.lang.BufferingPrologControl;
+import com.googlecode.prolog_cafe.lang.Predicate;
 import com.googlecode.prolog_cafe.lang.Prolog;
 import com.googlecode.prolog_cafe.lang.PrologMachineCopy;
 
+import org.eclipse.jgit.lib.Config;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -65,6 +68,8 @@ public class PrologEnvironment extends BufferingPrologControl {
 
   private final Args args;
   private final Map<StoredValue<Object>, Object> storedValues;
+  private int reductionLimit;
+  private int reductionsRemaining;
   private List<Runnable> cleanup;
 
   @Inject
@@ -74,6 +79,8 @@ public class PrologEnvironment extends BufferingPrologControl {
     setEnabled(EnumSet.allOf(Prolog.Feature.class), false);
     args = a;
     storedValues = new HashMap<>();
+    reductionLimit = a.reductionLimit;
+    reductionsRemaining = reductionLimit;
     cleanup = new LinkedList<>();
   }
 
@@ -81,6 +88,28 @@ public class PrologEnvironment extends BufferingPrologControl {
     return args;
   }
 
+  @Override
+  public boolean isEngineStopped() {
+    if (super.isEngineStopped()) {
+      return true;
+    } else if (--reductionsRemaining <= 0) {
+      throw new ReductionLimitException(reductionLimit);
+    }
+    return false;
+  }
+
+  @Override
+  public void setPredicate(Predicate goal) {
+    super.setPredicate(goal);
+    reductionLimit = args.reductionLimit(goal);
+    reductionsRemaining = reductionLimit;
+  }
+
+  /** @return number of reductions during execution. */
+  public int getReductions() {
+    return reductionLimit - reductionsRemaining;
+  }
+
   /**
    * Lookup a stored value in the interpreter's hash manager.
    *
@@ -154,6 +183,8 @@ public class PrologEnvironment extends BufferingPrologControl {
     private final PatchSetInfoFactory patchSetInfoFactory;
     private final IdentifiedUser.GenericFactory userFactory;
     private final Provider<AnonymousUser> anonymousUser;
+    private final int reductionLimit;
+    private final int compileLimit;
 
     @Inject
     Args(ProjectCache projectCache,
@@ -161,13 +192,29 @@ public class PrologEnvironment extends BufferingPrologControl {
         PatchListCache patchListCache,
         PatchSetInfoFactory patchSetInfoFactory,
         IdentifiedUser.GenericFactory userFactory,
-        Provider<AnonymousUser> anonymousUser) {
+        Provider<AnonymousUser> anonymousUser,
+        @GerritServerConfig Config config) {
       this.projectCache = projectCache;
       this.repositoryManager = repositoryManager;
       this.patchListCache = patchListCache;
       this.patchSetInfoFactory = patchSetInfoFactory;
       this.userFactory = userFactory;
       this.anonymousUser = anonymousUser;
+
+      int limit = config.getInt("rules", null, "reductionLimit", 100000);
+      reductionLimit = limit <= 0 ? Integer.MAX_VALUE : limit;
+
+      limit = config.getInt("rules", null, "compileReductionLimit",
+          (int) Math.min(10L * limit, Integer.MAX_VALUE));
+      compileLimit = limit <= 0 ? Integer.MAX_VALUE : limit;
+    }
+
+    private int reductionLimit(Predicate goal) {
+      if ("com.googlecode.prolog_cafe.builtin.PRED_consult_stream_2"
+          .equals(goal.getClass().getName())) {
+        return compileLimit;
+      }
+      return reductionLimit;
     }
 
     public ProjectCache getProjectCache() {

gerrit-server/src/main/java/com/google/gerrit/rules/ReductionLimitException.java

@@ -0,0 +1,24 @@
+// Copyright (C) 2014 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.rules;
+
+/** Thrown by {@link PrologEnvironment} if a script runs too long. */
+public class ReductionLimitException extends RuntimeException {
+  private static final long serialVersionUID = 1L;
+
+  ReductionLimitException(int limit) {
+    super(String.format("exceeded reduction limit of %d", limit));
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/server/change/TestSubmitRule.java

@@ -91,6 +91,9 @@ public class TestSubmitRule implements RestModifyView<RevisionResource, Input> {
     for (SubmitRecord r : records) {
       out.add(new Record(r, accounts));
     }
+    if (!out.isEmpty()) {
+      out.get(0).prologReductionCount = evaluator.getReductionsConsumed();
+    }
     accounts.fill();
     return out;
   }
@@ -103,6 +106,7 @@ public class TestSubmitRule implements RestModifyView<RevisionResource, Input> {
     Map<String, None> need;
     Map<String, AccountInfo> may;
     Map<String, None> impossible;
+    Integer prologReductionCount;
 
     Record(SubmitRecord r, AccountLoader accounts) {
       this.status = r.status;

gerrit-server/src/main/java/com/google/gerrit/server/project/SubmitRuleEvaluator.java

@@ -27,6 +27,7 @@ import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.PatchSet;
 import com.google.gerrit.rules.PrologEnvironment;
+import com.google.gerrit.rules.ReductionLimitException;
 import com.google.gerrit.rules.StoredValues;
 import com.google.gerrit.server.CurrentUser;
 import com.google.gerrit.server.query.change.ChangeData;
@@ -36,7 +37,6 @@ import com.googlecode.prolog_cafe.compiler.CompileException;
 import com.googlecode.prolog_cafe.lang.IntegerTerm;
 import com.googlecode.prolog_cafe.lang.ListTerm;
 import com.googlecode.prolog_cafe.lang.Prolog;
-import com.googlecode.prolog_cafe.lang.PrologException;
 import com.googlecode.prolog_cafe.lang.StructureTerm;
 import com.googlecode.prolog_cafe.lang.SymbolTerm;
 import com.googlecode.prolog_cafe.lang.Term;
@@ -107,6 +107,7 @@ public class SubmitRuleEvaluator {
   private boolean skipFilters;
   private String rule;
   private boolean logErrors = true;
+  private int reductionsConsumed;
 
   private Term submitRule;
 
@@ -183,6 +184,11 @@ public class SubmitRuleEvaluator {
     return this;
   }
 
+  /** @return Prolog reductions consumed during evaluation. */
+  public int getReductionsConsumed() {
+    return reductionsConsumed;
+  }
+
   /**
    * Evaluate the submit rules.
    *
@@ -457,10 +463,16 @@ public class SubmitRuleEvaluator {
               new VariableTerm())) {
           results.add(template[1]);
         }
+      } catch (ReductionLimitException err) {
+        throw new RuleEvalException(String.format(
+            "%s on change %d of %s",
+            err.getMessage(), cd.getId().get(), getProjectName()));
       } catch (RuntimeException err) {
-        throw new RuleEvalException("Exception calling " + sr
-            + " on change " + cd.getId() + " of " + getProjectName(),
-            err);
+        throw new RuleEvalException(String.format(
+            "Exception calling %s on change %d of %s",
+            sr, cd.getId().get(), getProjectName()), err);
+      } finally {
+        reductionsConsumed = env.getReductions();
       }
 
       Term resultsTerm = toListTerm(results);
@@ -539,14 +551,16 @@ public class SubmitRuleEvaluator {
             parentEnv.once("gerrit", filterRuleWrapperName, filterRule,
                 results, new VariableTerm());
         results = template[2];
-      } catch (PrologException err) {
-        throw new RuleEvalException("Exception calling " + filterRule
-            + " on change " + cd.getId() + " of "
-            + parentState.getProject().getName(), err);
+      } catch (ReductionLimitException err) {
+        throw new RuleEvalException(String.format(
+            "%s on change %d of %s",
+            err.getMessage(), cd.getId().get(), parentState.getProject().getName()));
       } catch (RuntimeException err) {
-        throw new RuleEvalException("Exception calling " + filterRule
-            + " on change " + cd.getId() + " of "
-            + parentState.getProject().getName(), err);
+        throw new RuleEvalException(String.format(
+            "Exception calling %s on change %d of %s",
+            filterRule, cd.getId().get(), parentState.getProject().getName()), err);
+      } finally {
+        reductionsConsumed += env.getReductions();
       }
       childEnv = parentEnv;
     }

gerrit-server/src/test/java/com/google/gerrit/rules/GerritCommonTest.java

@@ -18,6 +18,8 @@ import static com.google.gerrit.common.data.Permission.LABEL;
 import static com.google.gerrit.server.project.Util.allow;
 import static com.google.gerrit.server.project.Util.category;
 import static com.google.gerrit.server.project.Util.value;
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.fail;
 
 import com.google.gerrit.common.TimeUtil;
 import com.google.gerrit.common.data.LabelType;
@@ -31,9 +33,18 @@ import com.google.gerrit.server.project.Util;
 import com.google.gerrit.testutil.InMemoryRepositoryManager;
 import com.google.inject.AbstractModule;
 
+import com.googlecode.prolog_cafe.compiler.CompileException;
+import com.googlecode.prolog_cafe.lang.JavaObjectTerm;
+import com.googlecode.prolog_cafe.lang.Prolog;
+import com.googlecode.prolog_cafe.lang.StructureTerm;
+import com.googlecode.prolog_cafe.lang.SymbolTerm;
+
+import org.eclipse.jgit.lib.Config;
 import org.junit.Before;
 import org.junit.Test;
 
+import java.io.PushbackReader;
+import java.io.StringReader;
 import java.util.Arrays;
 
 public class GerritCommonTest extends PrologTestCase {
@@ -56,6 +67,9 @@ public class GerritCommonTest extends PrologTestCase {
     load("gerrit", "gerrit_common_test.pl", new AbstractModule() {
       @Override
       protected void configure() {
+        Config cfg = new Config();
+        cfg.setInt("rules", null, "reductionLimit", 1300);
+        cfg.setInt("rules", null, "compileReductionLimit", (int) 1e6);
         bind(PrologEnvironment.Args.class).toInstance(
             new PrologEnvironment.Args(
                 null,
@@ -63,7 +77,8 @@ public class GerritCommonTest extends PrologTestCase {
                 null,
                 null,
                 null,
-                null));
+                null,
+                cfg));
       }
     });
 
@@ -92,4 +107,30 @@ public class GerritCommonTest extends PrologTestCase {
   public void testGerritCommon() {
     runPrologBasedTests();
   }
+
+  @Test
+  public void testReductionLimit() throws CompileException {
+    PrologEnvironment env = envFactory.create(machine);
+    setUpEnvironment(env);
+    env.setEnabled(Prolog.Feature.IO, true);
+
+    String script = "loopy :- b(5).\n"
+        + "b(N) :- N > 0, !, S = N - 1, b(S).\n"
+        + "b(_) :- true.\n";
+
+    SymbolTerm nameTerm = SymbolTerm.create("testReductionLimit");
+    JavaObjectTerm inTerm = new JavaObjectTerm(
+        new PushbackReader(new StringReader(script), Prolog.PUSHBACK_SIZE));
+    if (!env.execute(Prolog.BUILTIN, "consult_stream", nameTerm, inTerm)) {
+      throw new CompileException("Cannot consult " + nameTerm);
+    }
+
+    try {
+      env.once(Prolog.BUILTIN, "call", new StructureTerm(":",
+          SymbolTerm.create("user"), SymbolTerm.create("loopy")));
+      fail("long running loop did not abort with ReductionLimitException");
+    } catch (ReductionLimitException e) {
+      assertThat(e.getMessage()).isEqualTo("exceeded reduction limit of 1300");
+    }
+  }
 }

gerrit-server/src/test/java/com/google/gerrit/rules/PrologTestCase.java

@@ -52,8 +52,8 @@ public abstract class PrologTestCase {
   private boolean hasSetup;
   private boolean hasTeardown;
   private List<Term> tests;
-  private PrologMachineCopy machine;
-  private PrologEnvironment.Factory envFactory;
+  protected PrologMachineCopy machine;
+  protected PrologEnvironment.Factory envFactory;
 
   protected void load(String pkg, String prologResource, Module... modules)
       throws CompileException, IOException {