Make --watchfs a common command option.

Adding an options parameter to DiffAwareness#getCurrentView seems like the
simplest way to achieve that.

Alternatives considered:
1. Making the diff awareness modules stateful. However, I did not want to do so
as I've also been working on improving the module API to reduce state, or at
least to have a proper lifecycle management for any necessary state.

2. Making the watchFs flag a constructor parameter. However, that would also
invalidate any implementations that don't use the flag (of which we have
several).

3. Only passing in a single boolean flag instead of an options class provider;
however, this is a more principled, futureproof API, which allows other
modules / awareness implementations to use their own options.

RELNOTES: --watchfs is now a command option; the startup option of the same
    name is deprecated. I.e., use bazel build --watchfs, not blaze --watchfs
    build.

--
MOS_MIGRATED_REVID=136026835

1 files changed, 10 insertions, 1 deletions

diff --git a/src/main/java/com/google/devtools/common/options/OptionsClassProvider.java b/src/main/java/com/google/devtools/common/options/OptionsClassProvider.java
index 90e7d48d1c..b28c29d3dc 100644
--- a/src/main/java/com/google/devtools/common/options/OptionsClassProvider.java
+++ b/src/main/java/com/google/devtools/common/options/OptionsClassProvider.java
@@ -13,11 +13,20 @@
 // limitations under the License.
 package com.google.devtools.common.options;
 
+import javax.annotation.Nullable;
+
 /**
  * A read-only interface for options parser results, which only allows to query the options of
  * a specific class, but not e.g. the residue any other information pertaining to the command line.
  */
 public interface OptionsClassProvider {
+  public static final OptionsClassProvider EMPTY = new OptionsClassProvider() {
+    @Override @Nullable
+    public <O extends OptionsBase> O getOptions(Class<O> optionsClass) {
+      return null;
+    }
+  };
+
   /**
    * Returns the options instance for the given {@code optionsClass}, that is,
    * the parsed options, or null if it is not among those available.
@@ -25,5 +34,5 @@ public interface OptionsClassProvider {
    * <p>The returned options should be treated by library code as immutable and
    * a provider is permitted to return the same options instance multiple times.
    */
-  <O extends OptionsBase> O getOptions(Class<O> optionsClass);
+  @Nullable <O extends OptionsBase> O getOptions(Class<O> optionsClass);
 }