bazel sync: support --loading_phase_threads instead of using Skyframe…

…Executor.DEFAULT_THREAD_COUNT

The number of threads during the loading (i.e., the only thing that bazel sync does)
is an important tuneable to obtain good performance and there is no "one size fits
all" for this. So make `bazel sync` honor the already existing flag.

Related to bazelbuild#6502.

Change-Id: Iaf38315b4a6640c7011fee99da235d7c9bbba6b1
PiperOrigin-RevId: 222051926

src/main/java/com/google/devtools/build/lib/BUILD

@@ -831,6 +831,7 @@ java_library(
         ":events",
         ":exitcode-external",
         ":keep-going-option",
+        ":loading-phase-threads-option",
         ":packages-internal",
         ":runtime",
         ":util",

src/main/java/com/google/devtools/build/lib/bazel/commands/SyncCommand.java

@@ -31,6 +31,7 @@
 import com.google.devtools.build.lib.runtime.Command;
 import com.google.devtools.build.lib.runtime.CommandEnvironment;
 import com.google.devtools.build.lib.runtime.KeepGoingOption;
+import com.google.devtools.build.lib.runtime.LoadingPhaseThreadsOption;
 import com.google.devtools.build.lib.skyframe.PackageLookupValue;
 import com.google.devtools.build.lib.skyframe.PrecomputedValue;
 import com.google.devtools.build.lib.skyframe.SkyframeExecutor;
@@ -50,7 +51,7 @@
 /** Syncs all repositories specifed in the workspace file */
 @Command(
     name = SyncCommand.NAME,
-    options = {PackageCacheOptions.class, KeepGoingOption.class},
+    options = {PackageCacheOptions.class, KeepGoingOption.class, LoadingPhaseThreadsOption.class},
     help = "resource:sync.txt",
     shortDescription = "Syncs all repositories specifed in the workspace file",
     allowResidue = false)
@@ -94,9 +95,10 @@ public BlazeCommandResult exec(CommandEnvironment env, OptionsParsingResult opti
 
       // Obtain the key for the top-level WORKSPACE file
       SkyKey packageLookupKey = PackageLookupValue.key(Label.EXTERNAL_PACKAGE_IDENTIFIER);
+      LoadingPhaseThreadsOption threadsOption = options.getOptions(LoadingPhaseThreadsOption.class);
       EvaluationContext evaluationContext =
           EvaluationContext.newBuilder()
-              .setNumThreads(SkyframeExecutor.DEFAULT_THREAD_COUNT)
+              .setNumThreads(threadsOption.threads)
               .setEventHander(env.getReporter())
               .build();
       EvaluationResult<SkyValue> packageLookupValue =