Rewrite workspace creation to use a builder class.

An instance of the builder is passed to all modules, which can each add / set
things on the builder. This reduces the BlazeModule API surface, while also
being more flexible for future changes.

--
MOS_MIGRATED_REVID=126613981

7 files changed, 244 insertions, 231 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/bazel/BazelDiffAwarenessModule.java b/src/main/java/com/google/devtools/build/lib/bazel/BazelDiffAwarenessModule.java
index aabf86c440..4140a7d5e7 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/BazelDiffAwarenessModule.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/BazelDiffAwarenessModule.java
@@ -14,7 +14,9 @@
 package com.google.devtools.build.lib.bazel;
 
 import com.google.common.collect.ImmutableList;
+import com.google.devtools.build.lib.analysis.BlazeDirectories;
 import com.google.devtools.build.lib.runtime.BlazeModule;
+import com.google.devtools.build.lib.runtime.WorkspaceBuilder;
 import com.google.devtools.build.lib.skyframe.DiffAwareness;
 import com.google.devtools.build.lib.skyframe.LocalDiffAwareness;
 
@@ -23,11 +25,9 @@ import com.google.devtools.build.lib.skyframe.LocalDiffAwareness;
  */
 public class BazelDiffAwarenessModule extends BlazeModule {
   @Override
-  public Iterable<DiffAwareness.Factory> getDiffAwarenessFactories(boolean watchFS) {
-    ImmutableList.Builder<DiffAwareness.Factory> builder = ImmutableList.builder();
-    if (watchFS) {
-      builder.add(new LocalDiffAwareness.Factory(ImmutableList.<String>of()));
+  public void workspaceInit(BlazeDirectories directories, WorkspaceBuilder builder) {
+    if (builder.enableWatchFs()) {
+      builder.addDiffAwarenessFactory(new LocalDiffAwareness.Factory(ImmutableList.<String>of()));
     }
-    return builder.build();
   }
 }
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java b/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java
index 3c51885c60..12c4bd208e 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java
@@ -56,12 +56,11 @@ import com.google.devtools.build.lib.runtime.BlazeCommand;
 import com.google.devtools.build.lib.runtime.BlazeModule;
 import com.google.devtools.build.lib.runtime.Command;
 import com.google.devtools.build.lib.runtime.CommandEnvironment;
+import com.google.devtools.build.lib.runtime.WorkspaceBuilder;
 import com.google.devtools.build.lib.skyframe.SkyFunctions;
 import com.google.devtools.build.lib.skyframe.SkyValueDirtinessChecker;
 import com.google.devtools.build.lib.util.AbruptExitException;
 import com.google.devtools.build.lib.util.io.TimestampGranularityMonitor;
-import com.google.devtools.build.skyframe.SkyFunction;
-import com.google.devtools.build.skyframe.SkyFunctionName;
 import com.google.devtools.build.skyframe.SkyKey;
 import com.google.devtools.build.skyframe.SkyValue;
 import com.google.devtools.common.options.OptionsProvider;
@@ -133,8 +132,12 @@ public class BazelRepositoryModule extends BlazeModule {
       };
 
   @Override
-  public Iterable<SkyValueDirtinessChecker> getCustomDirtinessCheckers() {
-    return ImmutableList.of(REPOSITORY_VALUE_CHECKER);
+  public void workspaceInit(BlazeDirectories directories, WorkspaceBuilder builder) {
+    builder.addCustomDirtinessChecker(REPOSITORY_VALUE_CHECKER);
+    // Create the repository function everything flows through.
+    builder.addSkyFunction(SkyFunctions.REPOSITORY, new RepositoryLoaderFunction());
+    builder.addSkyFunction(SkyFunctions.REPOSITORY_DIRECTORY, delegator);
+    builder.addSkyFunction(MavenServerFunction.NAME, new MavenServerFunction());
   }
 
   @Override
@@ -164,18 +167,6 @@ public class BazelRepositoryModule extends BlazeModule {
   }
 
   @Override
-  public ImmutableMap<SkyFunctionName, SkyFunction> getSkyFunctions(BlazeDirectories directories) {
-    ImmutableMap.Builder<SkyFunctionName, SkyFunction> builder = ImmutableMap.builder();
-
-    // Create the repository function everything flows through.
-    builder.put(SkyFunctions.REPOSITORY, new RepositoryLoaderFunction());
-
-    builder.put(SkyFunctions.REPOSITORY_DIRECTORY, delegator);
-    builder.put(MavenServerFunction.NAME, new MavenServerFunction());
-    return builder.build();
-  }
-
-  @Override
   public void beforeCommand(Command command, CommandEnvironment env) throws AbruptExitException {
     delegator.setClientEnvironment(env.getClientEnv());
     skylarkRepositoryFunction.setCommandEnvironment(env);
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/BazelWorkspaceStatusModule.java b/src/main/java/com/google/devtools/build/lib/bazel/BazelWorkspaceStatusModule.java
index 4704497d4f..894a27c1d4 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/BazelWorkspaceStatusModule.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/BazelWorkspaceStatusModule.java
@@ -32,6 +32,7 @@ import com.google.devtools.build.lib.actions.Executor;
 import com.google.devtools.build.lib.actions.ResourceSet;
 import com.google.devtools.build.lib.actions.Root;
 import com.google.devtools.build.lib.actions.SimpleActionContextProvider;
+import com.google.devtools.build.lib.analysis.BlazeDirectories;
 import com.google.devtools.build.lib.analysis.BuildInfo;
 import com.google.devtools.build.lib.analysis.WorkspaceStatusAction;
 import com.google.devtools.build.lib.analysis.WorkspaceStatusAction.Key;
@@ -41,6 +42,7 @@ import com.google.devtools.build.lib.runtime.BlazeModule;
 import com.google.devtools.build.lib.runtime.Command;
 import com.google.devtools.build.lib.runtime.CommandEnvironment;
 import com.google.devtools.build.lib.runtime.GotOptionsEvent;
+import com.google.devtools.build.lib.runtime.WorkspaceBuilder;
 import com.google.devtools.build.lib.shell.CommandException;
 import com.google.devtools.build.lib.shell.CommandResult;
 import com.google.devtools.build.lib.util.CommandBuilder;
@@ -286,7 +288,7 @@ public class BazelWorkspaceStatusModule extends BlazeModule {
   }
 
   @Override
-  public WorkspaceStatusAction.Factory getWorkspaceStatusActionFactory() {
-    return new BazelStatusActionFactory();
+  public void workspaceInit(BlazeDirectories directories, WorkspaceBuilder builder) {
+    builder.setWorkspaceStatusActionFactory(new BazelStatusActionFactory());
   }
 }
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/rules/BazelRulesModule.java b/src/main/java/com/google/devtools/build/lib/bazel/rules/BazelRulesModule.java
index e5fb01ad7c..a169f0c5a0 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/rules/BazelRulesModule.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/rules/BazelRulesModule.java
@@ -40,10 +40,9 @@ import com.google.devtools.build.lib.runtime.BlazeModule;
 import com.google.devtools.build.lib.runtime.Command;
 import com.google.devtools.build.lib.runtime.CommandEnvironment;
 import com.google.devtools.build.lib.runtime.GotOptionsEvent;
+import com.google.devtools.build.lib.runtime.WorkspaceBuilder;
 import com.google.devtools.build.lib.skyframe.PrecomputedValue;
 import com.google.devtools.build.lib.util.ResourceFileLoader;
-import com.google.devtools.build.skyframe.SkyFunction;
-import com.google.devtools.build.skyframe.SkyFunctionName;
 import com.google.devtools.common.options.Converters.AssignmentConverter;
 import com.google.devtools.common.options.Option;
 import com.google.devtools.common.options.OptionsBase;
@@ -190,8 +189,9 @@ public class BazelRulesModule extends BlazeModule {
   }
 
   @Override
-  public Iterable<PrecomputedValue.Injected> getPrecomputedSkyframeValues() {
-    return ImmutableList.of(PrecomputedValue.injected(
+  public void workspaceInit(BlazeDirectories directories, WorkspaceBuilder builder) {
+    builder.addSkyFunction(FdoSupportValue.SKYFUNCTION, new FdoSupportFunction());
+    builder.addPrecomputedValue(PrecomputedValue.injected(
         GenQuery.QUERY_OUTPUT_FORMATTERS,
         new Supplier<ImmutableList<OutputFormatter>>() {
           @Override
@@ -200,10 +200,4 @@ public class BazelRulesModule extends BlazeModule {
           }
         }));
   }
-
-  @Override
-  public ImmutableMap<SkyFunctionName, SkyFunction> getSkyFunctions(BlazeDirectories directories) {
-    return ImmutableMap.<SkyFunctionName, SkyFunction>of(
-        FdoSupportValue.SKYFUNCTION, new FdoSupportFunction());
-  }
 }
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/BlazeModule.java b/src/main/java/com/google/devtools/build/lib/runtime/BlazeModule.java
index 79d0c935c8..03580b2c3a 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/BlazeModule.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/BlazeModule.java
@@ -14,7 +14,6 @@
 package com.google.devtools.build.lib.runtime;
 
 import com.google.common.base.Function;
-import com.google.common.base.Predicate;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.devtools.build.lib.actions.ActionContextConsumer;
@@ -23,14 +22,12 @@ import com.google.devtools.build.lib.actions.ActionInputFileCache;
 import com.google.devtools.build.lib.analysis.BlazeDirectories;
 import com.google.devtools.build.lib.analysis.BlazeVersionInfo;
 import com.google.devtools.build.lib.analysis.ConfiguredRuleClassProvider;
-import com.google.devtools.build.lib.analysis.WorkspaceStatusAction;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.exec.OutputService;
 import com.google.devtools.build.lib.packages.AttributeContainer;
 import com.google.devtools.build.lib.packages.NoSuchThingException;
 import com.google.devtools.build.lib.packages.Package;
 import com.google.devtools.build.lib.packages.PackageFactory;
-import com.google.devtools.build.lib.packages.Preprocessor;
 import com.google.devtools.build.lib.packages.RuleClass;
 import com.google.devtools.build.lib.packages.RuleClassProvider;
 import com.google.devtools.build.lib.query2.AbstractBlazeQueryEnvironment;
@@ -40,18 +37,11 @@ import com.google.devtools.build.lib.query2.output.OutputFormatter;
 import com.google.devtools.build.lib.rules.test.CoverageReportActionFactory;
 import com.google.devtools.build.lib.runtime.commands.InfoItem;
 import com.google.devtools.build.lib.runtime.proto.InvocationPolicyOuterClass.InvocationPolicy;
-import com.google.devtools.build.lib.skyframe.DiffAwareness;
-import com.google.devtools.build.lib.skyframe.PrecomputedValue.Injected;
-import com.google.devtools.build.lib.skyframe.SkyValueDirtinessChecker;
-import com.google.devtools.build.lib.skyframe.SkyframeExecutor;
-import com.google.devtools.build.lib.skyframe.SkyframeExecutorFactory;
 import com.google.devtools.build.lib.util.AbruptExitException;
 import com.google.devtools.build.lib.util.Clock;
 import com.google.devtools.build.lib.vfs.FileSystem;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.PathFragment;
-import com.google.devtools.build.skyframe.SkyFunction;
-import com.google.devtools.build.skyframe.SkyFunctionName;
 import com.google.devtools.common.options.OptionsBase;
 import com.google.devtools.common.options.OptionsProvider;
 
@@ -110,17 +100,10 @@ public abstract class BlazeModule {
   }
 
   /**
-   * May yield a supplier that provides factories for the Preprocessor to apply. Only one of the
-   * configured modules may return non-null.
-   *
-   * <p>The factory yielded by the supplier will be checked with
-   * {@link Preprocessor.Factory#isStillValid} at the beginning of each incremental build. This
-   * allows modules to have preprocessors customizable by flags.
-   *
-   * <p>This method will be called during Blaze startup (after #blazeStartup).
+   * Called when Blaze initializes a new workspace.
    */
-  public Preprocessor.Factory.Supplier getPreprocessorFactorySupplier() {
-    return null;
+  @SuppressWarnings("unused")
+  public void workspaceInit(BlazeDirectories directories, WorkspaceBuilder builder) {
   }
 
   /**
@@ -151,26 +134,6 @@ public abstract class BlazeModule {
   }
 
   /**
-   * Returns the {@link DiffAwareness} strategies this module contributes. These will be used to
-   * determine which files, if any, changed between Blaze commands.
-   *
-   * <p>This method will be called during Blaze startup (after #blazeStartup).
-   */
-  @SuppressWarnings("unused")
-  public Iterable<? extends DiffAwareness.Factory> getDiffAwarenessFactories(boolean watchFS) {
-    return ImmutableList.of();
-  }
-
-  /**
-   * Returns the workspace status action factory contributed by this module.
-   *
-   * <p>There should always be exactly one of these in a Blaze instance.
-   */
-  public WorkspaceStatusAction.Factory getWorkspaceStatusActionFactory() {
-    return null;
-  }
-
-  /**
    * PlatformSet is a group of platforms characterized by a regular expression.  For example, the
    * entry "oldlinux": "i[34]86-libc[345]-linux" might define a set of platforms representing
    * certain older linux releases.
@@ -189,10 +152,6 @@ public abstract class BlazeModule {
     return ImmutableMap.<String, String>of();
   }
 
-  public Iterable<SkyValueDirtinessChecker> getCustomDirtinessCheckers() {
-    return ImmutableList.of();
-  }
-
   @Nullable
   protected Function<RuleClass, AttributeContainer> getAttributeContainerSupplier() {
     return null;
@@ -341,13 +300,6 @@ public abstract class BlazeModule {
   }
 
   /**
-   * Action inputs are allowed to be missing for all inputs where this predicate returns true.
-   */
-  public Predicate<PathFragment> getAllowedMissingInputs() {
-    return null;
-  }
-
-  /**
    * Perform module specific check of current command environment.
    */
   public void checkEnvironment(CommandEnvironment env) {
@@ -388,47 +340,6 @@ public abstract class BlazeModule {
   }
 
   /**
-   * Returns a factory for creating {@link SkyframeExecutor} objects. If the module does not
-   * provide any SkyframeExecutorFactory, it returns null. Note that only one factory per
-   * Bazel/Blaze runtime is allowed.
-   *
-   * @param directories the workspace directories
-   */
-  public SkyframeExecutorFactory getSkyframeExecutorFactory(BlazeDirectories directories) {
-    return null;
-  }
-
-  /** Returns a map of "extra" SkyFunctions for SkyValues that this module may want to build. */
-  public ImmutableMap<SkyFunctionName, SkyFunction> getSkyFunctions(BlazeDirectories directories) {
-    return ImmutableMap.of();
-  }
-
-  /**
-   * Returns the extra precomputed values that the module makes available in Skyframe.
-   *
-   * <p>This method is called once per Blaze instance at the very beginning of its life.
-   * If it creates the injected values by using a {@code com.google.common.base.Supplier},
-   * that supplier is asked for the value it contains just before the loading phase begins. This
-   * functionality can be used to implement precomputed values that are not constant during the
-   * lifetime of a Blaze instance (naturally, they must be constant over the course of a build)
-   *
-   * <p>The following things must be done in order to define a new precomputed values:
-   * <ul>
-   * <li> Create a public static final variable of type
-   * {@link com.google.devtools.build.lib.skyframe.PrecomputedValue.Precomputed}
-   * <li> Set its value by adding an {@link Injected} in this method (it can be created using the
-   * aforementioned variable and the value or a supplier of the value)
-   * <li> Reference the value in Skyframe functions by calling get {@code get} method on the
-   * {@link com.google.devtools.build.lib.skyframe.PrecomputedValue.Precomputed} variable. This
-   * will never return null, because its value will have been injected before most of the Skyframe
-   * values are computed.
-   * </ul>
-   */
-  public Iterable<Injected> getPrecomputedSkyframeValues() {
-    return ImmutableList.of();
-  }
-
-  /**
    * Optionally returns a provider for project files that can be used to bundle targets and
    * command-line options.
    */
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java b/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java
index f95aecd3e3..ec7c7a0a6c 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java
@@ -16,7 +16,6 @@ package com.google.devtools.build.lib.runtime;
 
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Function;
-import com.google.common.base.Predicate;
 import com.google.common.base.Predicates;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
@@ -30,7 +29,6 @@ import com.google.common.util.concurrent.Uninterruptibles;
 import com.google.devtools.build.lib.analysis.BlazeDirectories;
 import com.google.devtools.build.lib.analysis.BlazeVersionInfo;
 import com.google.devtools.build.lib.analysis.ConfiguredRuleClassProvider;
-import com.google.devtools.build.lib.analysis.WorkspaceStatusAction;
 import com.google.devtools.build.lib.analysis.config.BinTools;
 import com.google.devtools.build.lib.analysis.config.BuildOptions;
 import com.google.devtools.build.lib.analysis.config.ConfigurationFactory;
@@ -40,7 +38,6 @@ import com.google.devtools.build.lib.flags.CommandNameCache;
 import com.google.devtools.build.lib.packages.AttributeContainer;
 import com.google.devtools.build.lib.packages.Package;
 import com.google.devtools.build.lib.packages.PackageFactory;
-import com.google.devtools.build.lib.packages.Preprocessor;
 import com.google.devtools.build.lib.packages.RuleClass;
 import com.google.devtools.build.lib.packages.RuleClassProvider;
 import com.google.devtools.build.lib.profiler.AutoProfiler;
@@ -74,12 +71,6 @@ import com.google.devtools.build.lib.server.signal.InterruptSignalHandler;
 import com.google.devtools.build.lib.shell.JavaSubprocessFactory;
 import com.google.devtools.build.lib.shell.Subprocess;
 import com.google.devtools.build.lib.shell.SubprocessBuilder;
-import com.google.devtools.build.lib.skyframe.DiffAwareness;
-import com.google.devtools.build.lib.skyframe.PrecomputedValue;
-import com.google.devtools.build.lib.skyframe.SequencedSkyframeExecutorFactory;
-import com.google.devtools.build.lib.skyframe.SkyValueDirtinessChecker;
-import com.google.devtools.build.lib.skyframe.SkyframeExecutor;
-import com.google.devtools.build.lib.skyframe.SkyframeExecutorFactory;
 import com.google.devtools.build.lib.util.AbruptExitException;
 import com.google.devtools.build.lib.util.BlazeClock;
 import com.google.devtools.build.lib.util.Clock;
@@ -97,8 +88,6 @@ import com.google.devtools.build.lib.vfs.PathFragment;
 import com.google.devtools.build.lib.vfs.UnixFileSystem;
 import com.google.devtools.build.lib.vfs.WindowsFileSystem;
 import com.google.devtools.build.lib.windows.WindowsSubprocessFactory;
-import com.google.devtools.build.skyframe.SkyFunction;
-import com.google.devtools.build.skyframe.SkyFunctionName;
 import com.google.devtools.common.options.Option;
 import com.google.devtools.common.options.OptionPriority;
 import com.google.devtools.common.options.OptionsBase;
@@ -218,103 +207,14 @@ public final class BlazeRuntime {
 
   public void initWorkspace(BlazeDirectories directories, BinTools binTools)
       throws AbruptExitException {
-    SkyframeExecutorFactory skyframeExecutorFactory = null;
+    boolean watchFS = startupOptionsProvider != null
+        && startupOptionsProvider.getOptions(BlazeServerStartupOptions.class).watchFS;
+    WorkspaceBuilder builder = new WorkspaceBuilder(directories, binTools, watchFS);
     for (BlazeModule module : blazeModules) {
-      SkyframeExecutorFactory skyFactory = module.getSkyframeExecutorFactory(directories);
-      if (skyFactory != null) {
-        Preconditions.checkState(skyframeExecutorFactory == null,
-            "At most one Skyframe factory supported. But found two: %s and %s", skyFactory,
-            skyframeExecutorFactory);
-        skyframeExecutorFactory = skyFactory;
-      }
-    }
-    if (skyframeExecutorFactory == null) {
-      skyframeExecutorFactory = new SequencedSkyframeExecutorFactory();
-    }
-
-    WorkspaceStatusAction.Factory workspaceStatusActionFactory = null;
-    for (BlazeModule module : blazeModules) {
-      WorkspaceStatusAction.Factory candidate = module.getWorkspaceStatusActionFactory();
-      if (candidate != null) {
-        Preconditions.checkState(workspaceStatusActionFactory == null,
-            "more than one module defines a workspace status action factory");
-        workspaceStatusActionFactory = candidate;
-      }
-    }
-
-    Iterable<DiffAwareness.Factory> diffAwarenessFactories;
-    {
-      ImmutableList.Builder<DiffAwareness.Factory> builder = new ImmutableList.Builder<>();
-      boolean watchFS = startupOptionsProvider != null
-          && startupOptionsProvider.getOptions(BlazeServerStartupOptions.class).watchFS;
-      for (BlazeModule module : blazeModules) {
-        builder.addAll(module.getDiffAwarenessFactories(watchFS));
-      }
-      diffAwarenessFactories = builder.build();
-    }
-
-    // Merge filters from Blaze modules that allow some action inputs to be missing.
-    Predicate<PathFragment> allowedMissingInputs = null;
-    for (BlazeModule module : blazeModules) {
-      Predicate<PathFragment> modulePredicate = module.getAllowedMissingInputs();
-      if (modulePredicate != null) {
-        Preconditions.checkArgument(allowedMissingInputs == null,
-            "More than one Blaze module allows missing inputs.");
-        allowedMissingInputs = modulePredicate;
-      }
-    }
-    if (allowedMissingInputs == null) {
-      allowedMissingInputs = Predicates.alwaysFalse();
+      module.workspaceInit(directories, builder);
     }
-
-    Preprocessor.Factory.Supplier preprocessorFactorySupplier = null;
-    for (BlazeModule module : blazeModules) {
-      Preprocessor.Factory.Supplier modulePreprocessorFactorySupplier =
-          module.getPreprocessorFactorySupplier();
-      if (modulePreprocessorFactorySupplier != null) {
-        Preconditions.checkState(preprocessorFactorySupplier == null,
-            "more than one module defines a preprocessor factory supplier");
-        preprocessorFactorySupplier = modulePreprocessorFactorySupplier;
-      }
-    }
-    if (preprocessorFactorySupplier == null) {
-      preprocessorFactorySupplier = Preprocessor.Factory.Supplier.NullSupplier.INSTANCE;
-    }
-
-    // We use an immutable map builder for the nice side effect that it throws if a duplicate key
-    // is inserted.
-    ImmutableMap.Builder<SkyFunctionName, SkyFunction> skyFunctions = ImmutableMap.builder();
-    for (BlazeModule module : blazeModules) {
-      skyFunctions.putAll(module.getSkyFunctions(directories));
-    }
-
-    ImmutableList.Builder<PrecomputedValue.Injected> precomputedValues = ImmutableList.builder();
-    for (BlazeModule module : blazeModules) {
-      precomputedValues.addAll(module.getPrecomputedSkyframeValues());
-    }
-
-    ImmutableList.Builder<SkyValueDirtinessChecker> customDirtinessCheckers =
-        ImmutableList.builder();
-    for (BlazeModule module : blazeModules) {
-      customDirtinessCheckers.addAll(module.getCustomDirtinessCheckers());
-    }
-
-    SkyframeExecutor skyframeExecutor = skyframeExecutorFactory.create(
-        packageFactory,
-        directories,
-        binTools,
-        workspaceStatusActionFactory,
-        ruleClassProvider.getBuildInfoFactories(),
-        diffAwarenessFactories,
-        allowedMissingInputs,
-        preprocessorFactorySupplier,
-        skyFunctions.build(),
-        precomputedValues.build(),
-        customDirtinessCheckers.build(),
-        getProductName());
-    this.workspace = new BlazeWorkspace(
-        this, directories, skyframeExecutor, eventBusExceptionHandler, workspaceStatusActionFactory,
-        binTools);
+    this.workspace = builder.build(
+        this, packageFactory, ruleClassProvider, getProductName(), eventBusExceptionHandler);
   }
 
   @Nullable public CoverageReportActionFactory getCoverageReportActionFactory() {
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/WorkspaceBuilder.java b/src/main/java/com/google/devtools/build/lib/runtime/WorkspaceBuilder.java
new file mode 100644
index 0000000000..d7f0bfd2f2
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/runtime/WorkspaceBuilder.java
@@ -0,0 +1,215 @@
+// Copyright 2016 The Bazel Authors. All rights reserved.
+//
+// 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.devtools.build.lib.runtime;
+
+import com.google.common.base.Predicate;
+import com.google.common.base.Predicates;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.common.eventbus.SubscriberExceptionHandler;
+import com.google.devtools.build.lib.analysis.BlazeDirectories;
+import com.google.devtools.build.lib.analysis.ConfiguredRuleClassProvider;
+import com.google.devtools.build.lib.analysis.WorkspaceStatusAction;
+import com.google.devtools.build.lib.analysis.config.BinTools;
+import com.google.devtools.build.lib.packages.PackageFactory;
+import com.google.devtools.build.lib.packages.Preprocessor;
+import com.google.devtools.build.lib.skyframe.DiffAwareness;
+import com.google.devtools.build.lib.skyframe.PrecomputedValue;
+import com.google.devtools.build.lib.skyframe.PrecomputedValue.Injected;
+import com.google.devtools.build.lib.skyframe.SequencedSkyframeExecutorFactory;
+import com.google.devtools.build.lib.skyframe.SkyValueDirtinessChecker;
+import com.google.devtools.build.lib.skyframe.SkyframeExecutor;
+import com.google.devtools.build.lib.skyframe.SkyframeExecutorFactory;
+import com.google.devtools.build.lib.util.AbruptExitException;
+import com.google.devtools.build.lib.util.Preconditions;
+import com.google.devtools.build.lib.vfs.PathFragment;
+import com.google.devtools.build.skyframe.SkyFunction;
+import com.google.devtools.build.skyframe.SkyFunctionName;
+
+import java.util.Map;
+
+/**
+ * Builder class to create a {@link BlazeWorkspace} instance. This class is part of the module API,
+ * which allows modules to affect how the workspace is initialized.
+ */
+public final class WorkspaceBuilder {
+  private final BlazeDirectories directories;
+  private final BinTools binTools;
+  private final boolean watchFs;
+
+  private SkyframeExecutorFactory skyframeExecutorFactory;
+  private WorkspaceStatusAction.Factory workspaceStatusActionFactory;
+  private final ImmutableList.Builder<DiffAwareness.Factory> diffAwarenessFactories =
+      ImmutableList.builder();
+  private Predicate<PathFragment> allowedMissingInputs;
+  private Preprocessor.Factory.Supplier preprocessorFactorySupplier;
+  // We use an immutable map builder for the nice side effect that it throws if a duplicate key
+  // is inserted.
+  private final ImmutableMap.Builder<SkyFunctionName, SkyFunction> skyFunctions =
+      ImmutableMap.builder();
+  private final ImmutableList.Builder<PrecomputedValue.Injected> precomputedValues =
+      ImmutableList.builder();
+  private final ImmutableList.Builder<SkyValueDirtinessChecker> customDirtinessCheckers =
+      ImmutableList.builder();
+
+  WorkspaceBuilder(BlazeDirectories directories, BinTools binTools, boolean watchFs) {
+    this.directories = directories;
+    this.binTools = binTools;
+    this.watchFs = watchFs;
+  }
+
+  BlazeWorkspace build(
+      BlazeRuntime runtime,
+      PackageFactory packageFactory,
+      ConfiguredRuleClassProvider ruleClassProvider,
+      String productName,
+      SubscriberExceptionHandler eventBusExceptionHandler) throws AbruptExitException {
+    // Set default values if none are set.
+    if (skyframeExecutorFactory == null) {
+      skyframeExecutorFactory = new SequencedSkyframeExecutorFactory();
+    }
+    if (allowedMissingInputs == null) {
+      allowedMissingInputs = Predicates.alwaysFalse();
+    }
+    if (preprocessorFactorySupplier == null) {
+      preprocessorFactorySupplier = Preprocessor.Factory.Supplier.NullSupplier.INSTANCE;
+    }
+
+    SkyframeExecutor skyframeExecutor = skyframeExecutorFactory.create(
+        packageFactory,
+        directories,
+        binTools,
+        workspaceStatusActionFactory,
+        ruleClassProvider.getBuildInfoFactories(),
+        diffAwarenessFactories.build(),
+        allowedMissingInputs,
+        preprocessorFactorySupplier,
+        skyFunctions.build(),
+        precomputedValues.build(),
+        customDirtinessCheckers.build(),
+        productName);
+    return new BlazeWorkspace(
+        runtime, directories, skyframeExecutor, eventBusExceptionHandler,
+        workspaceStatusActionFactory, binTools);
+  }
+
+  public boolean enableWatchFs() {
+    return watchFs;
+  }
+
+  /**
+   * Sets a factory for creating {@link SkyframeExecutor} objects. Note that only one factory per
+   * workspace is allowed.
+   */
+  public WorkspaceBuilder setSkyframeExecutorFactory(
+      SkyframeExecutorFactory skyframeExecutorFactory) {
+    Preconditions.checkState(this.skyframeExecutorFactory == null,
+        "At most one Skyframe factory supported. But found two: %s and %s",
+        this.skyframeExecutorFactory, skyframeExecutorFactory);
+    this.skyframeExecutorFactory = Preconditions.checkNotNull(skyframeExecutorFactory);
+    return this;
+  }
+
+  /**
+   * Sets the workspace status action factory contributed by this module. Only one factory per
+   * workspace is allowed.
+   */
+  public WorkspaceBuilder setWorkspaceStatusActionFactory(
+      WorkspaceStatusAction.Factory workspaceStatusActionFactory) {
+    Preconditions.checkState(this.workspaceStatusActionFactory == null,
+        "At most one workspace status action factory supported. But found two: %s and %s",
+        this.workspaceStatusActionFactory, workspaceStatusActionFactory);
+    this.workspaceStatusActionFactory = Preconditions.checkNotNull(workspaceStatusActionFactory);
+    return this;
+  }
+
+  /**
+   * Add a {@link DiffAwareness} factory. These will be used to determine which files, if any,
+   * changed between Blaze commands.
+   */
+  public WorkspaceBuilder addDiffAwarenessFactory(DiffAwareness.Factory factory) {
+    this.diffAwarenessFactories.add(Preconditions.checkNotNull(factory));
+    return this;
+  }
+
+  /**
+   * Action inputs are allowed to be missing for all inputs where this predicate returns true. Only
+   * one predicate may be set per workspace.
+   */
+  public WorkspaceBuilder setAllowedMissingInputs(Predicate<PathFragment> allowedMissingInputs) {
+    Preconditions.checkArgument(this.allowedMissingInputs == null,
+        "At most one module may set allowed missing inputs. But found two: %s and %s",
+        this.allowedMissingInputs, allowedMissingInputs);
+    this.allowedMissingInputs = Preconditions.checkNotNull(allowedMissingInputs);
+    return this;
+  }
+
+  /**
+   * Sets a supplier that provides factories for the Preprocessor to apply. Only one factory per
+   * workspace is allowed.
+   *
+   * <p>The factory yielded by the supplier will be checked with
+   * {@link Preprocessor.Factory#isStillValid} at the beginning of each incremental build. This
+   * allows modules to have preprocessors customizable by flags.
+   */
+  public WorkspaceBuilder setPreprocessorFactorySupplier(
+      Preprocessor.Factory.Supplier preprocessorFactorySupplier) {
+    Preconditions.checkState(this.preprocessorFactorySupplier == null,
+        "At most one module defines a preprocessor factory supplier. But found two: %s and %s",
+        this.preprocessorFactorySupplier, preprocessorFactorySupplier);
+    this.preprocessorFactorySupplier = Preconditions.checkNotNull(preprocessorFactorySupplier);
+    return this;
+  }
+
+  /** Add an "extra" SkyFunction for SkyValues. */
+  public WorkspaceBuilder addSkyFunction(SkyFunctionName name, SkyFunction skyFunction) {
+    Preconditions.checkNotNull(name);
+    Preconditions.checkNotNull(skyFunction);
+    this.skyFunctions.put(name, skyFunction);
+    return this;
+  }
+
+  /** Add "extra" SkyFunctions for SkyValues. */
+  public WorkspaceBuilder addSkyFunctions(Map<SkyFunctionName, SkyFunction> skyFunctions) {
+    this.skyFunctions.putAll(Preconditions.checkNotNull(skyFunctions));
+    return this;
+  }
+
+  /**
+   * Adds an extra precomputed value to Skyframe.
+   *
+   * <p>This functionality can be used to implement precomputed values that are not constant during
+   * the lifetime of a Blaze instance (naturally, they must be constant over the course of a build).
+   *
+   * <p>The following things must be done in order to define a new precomputed values:
+   * <ul>
+   * <li> Create a public static final variable of type
+   *     {@link com.google.devtools.build.lib.skyframe.PrecomputedValue.Precomputed}.
+   * <li> Set its value by adding an {@link Injected} via this method (it can be created using the
+   *     aforementioned variable and the value or a supplier of the value).
+   * <li> Reference the value in Skyframe functions by calling the {@code get} method on the
+   *     {@link com.google.devtools.build.lib.skyframe.PrecomputedValue.Precomputed} variable.
+   * </ul>
+   */
+  public WorkspaceBuilder addPrecomputedValue(PrecomputedValue.Injected precomputedValue) {
+    this.precomputedValues.add(Preconditions.checkNotNull(precomputedValue));
+    return this;
+  }
+
+  public WorkspaceBuilder addCustomDirtinessChecker(
+      SkyValueDirtinessChecker customDirtinessChecker) {
+    this.customDirtinessCheckers.add(Preconditions.checkNotNull(customDirtinessChecker));
+    return this;
+  }
+}
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