Expose ProtoSourcesProvider.transitive_proto_path_flags to Skylark.

Progress on #4544.

RELNOTES: None.
PiperOrigin-RevId: 187179454

src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCommon.java

@@ -100,29 +100,28 @@ public static NestedSet<Artifact> collectDependenciesDescriptorSets(RuleContext
   }
 
   /**
-   * Returns all proto source roots in this lib and in its transitive dependencies, each prefixed
-   * by {@code --proto_path}.
+   * Returns all proto source roots in this lib and in its transitive dependencies.
    *
    * Build will fail if the {@code proto_source_root} of the current lib is different than the
    * package name.
    */
   public static NestedSet<String> collectTransitiveProtoPathFlags(RuleContext ruleContext) {
-    NestedSetBuilder<String> protoPathFlags = NestedSetBuilder.stableOrder();
+    NestedSetBuilder<String> protoPath = NestedSetBuilder.stableOrder();
 
     // first add the protoSourceRoot of the current target, if any
     String protoSourceRoot =
         ruleContext.attributes().get("proto_source_root", Type.STRING);
     if (protoSourceRoot != null && !protoSourceRoot.isEmpty()) {
       checkProtoSourceRootIsTheSameAsPackage(protoSourceRoot, ruleContext);
-      protoPathFlags.add("--proto_path=" + protoSourceRoot);
+      protoPath.add(protoSourceRoot);
     }
 
     for (ProtoSourcesProvider provider : ruleContext.getPrerequisites(
             "deps", Mode.TARGET, ProtoSourcesProvider.class)) {
-      protoPathFlags.addTransitive(provider.getTransitiveProtoPathFlags());
+      protoPath.addTransitive(provider.getTransitiveProtoPathFlags());
     }
 
-    return protoPathFlags.build();
+    return protoPath.build();
   }
 
   private static void checkProtoSourceRootIsTheSameAsPackage(

src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java

@@ -530,7 +530,9 @@ static CustomCommandLine createCommandLineFromToolchains(
       ImmutableList<String> protocOpts) {
     CustomCommandLine.Builder cmdLine = CustomCommandLine.builder();
 
-    cmdLine.addAll(transitiveProtoPathFlags);
+    cmdLine.addAll(
+        VectorArg.of(transitiveProtoPathFlags)
+            .mapped(ProtoCompileActionBuilder::expandTransitiveProtoPathFlags));
 
     // A set to check if there are multiple invocations with the same name.
     HashSet<String> invocationNames = new HashSet<>();
@@ -604,6 +606,10 @@ static void addIncludeMapArguments(
     }
   }
 
+  private static void expandTransitiveProtoPathFlags(String flag, Consumer<String> args) {
+    args.accept("--proto_path=" + flag);
+  }
+
   private static void expandTransitiveImportArg(Artifact artifact, Consumer<String> args) {
     args.accept("-I" + getPathIgnoringRepository(artifact) + "=" + artifact.getExecPathString());
   }

src/main/java/com/google/devtools/build/lib/rules/proto/ProtoSourcesProvider.java

@@ -133,9 +133,14 @@ public static ProtoSourcesProvider create(
   public abstract NestedSet<Artifact> transitiveDescriptorSets();
 
   /**
-   * Directories of .proto sources collected from the transitive closure, each prefixed with
-   * {@code --proto_path}. These flags will be passed to {@code protoc} in the specified oreder.
+   * Directories of .proto sources collected from the transitive closure. These flags will be passed
+   * to {@code protoc} in the specified order, via the {@code --proto_path} flag.
    */
+  @SkylarkCallable(
+      name = "transitive_proto_path",
+      doc = "A set of proto source roots collected from the transitive closure of this rule.",
+      structField = true
+  )
   public abstract NestedSet<String> getTransitiveProtoPathFlags();
 
   ProtoSourcesProvider() {}

src/main/java/com/google/devtools/build/lib/rules/proto/SupportData.java

@@ -52,8 +52,8 @@ public static SupportData create(
   public abstract NestedSet<Artifact> getProtosInDirectDeps();
 
   /**
-   * Directories of .proto sources collected from the transitive closure, each prefixed with
-   * {@code --proto_path}. These flags will be passed to {@code protoc} in the specified oreder.
+   * Directories of .proto sources collected from the transitive closure. These flags will be passed
+   * to {@code protoc} in the specified order, via the {@code --proto_path} flag.
    */
   public abstract NestedSet<String> getTransitiveProtoPathFlags();
 

src/test/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibraryTest.java

@@ -251,11 +251,11 @@ public void testProtoSourceRootWithoutDeps() throws Exception {
     );
     ConfiguredTarget protoTarget = getConfiguredTarget("//x/foo:nodeps");
     ProtoSourcesProvider sourcesProvider = protoTarget.getProvider(ProtoSourcesProvider.class);
-    assertThat(sourcesProvider.getTransitiveProtoPathFlags()).containsExactly("--proto_path=x/foo");
+    assertThat(sourcesProvider.getTransitiveProtoPathFlags()).containsExactly("x/foo");
 
     SupportData supportData =
         protoTarget.getProvider(ProtoSupportDataProvider.class).getSupportData();
-    assertThat(supportData.getTransitiveProtoPathFlags()).containsExactly("--proto_path=x/foo");
+    assertThat(supportData.getTransitiveProtoPathFlags()).containsExactly("x/foo");
 
     assertThat(getGeneratingSpawnAction(getDescriptorOutput("//x/foo:nodeps"))
         .getRemainingArguments())
@@ -304,11 +304,11 @@ public void testProtoSourceRootWithDepsDuplicate() throws Exception {
     );
     ConfiguredTarget protoTarget = getConfiguredTarget("//x/foo:withdeps");
     ProtoSourcesProvider sourcesProvider = protoTarget.getProvider(ProtoSourcesProvider.class);
-    assertThat(sourcesProvider.getTransitiveProtoPathFlags()).containsExactly("--proto_path=x/foo");
+    assertThat(sourcesProvider.getTransitiveProtoPathFlags()).containsExactly("x/foo");
 
     SupportData supportData =
         protoTarget.getProvider(ProtoSupportDataProvider.class).getSupportData();
-    assertThat(supportData.getTransitiveProtoPathFlags()).containsExactly("--proto_path=x/foo");
+    assertThat(supportData.getTransitiveProtoPathFlags()).containsExactly("x/foo");
 
     assertThat(getGeneratingSpawnAction(getDescriptorOutput("//x/foo:withdeps"))
         .getRemainingArguments())
@@ -342,12 +342,12 @@ public void testProtoSourceRootWithDeps() throws Exception {
     ConfiguredTarget protoTarget = getConfiguredTarget("//x/foo:withdeps");
     ProtoSourcesProvider sourcesProvider = protoTarget.getProvider(ProtoSourcesProvider.class);
     assertThat(sourcesProvider.getTransitiveProtoPathFlags())
-        .containsExactly("--proto_path=x/foo", "--proto_path=x/bar");
+        .containsExactly("x/foo", "x/bar");
 
     SupportData supportData =
         protoTarget.getProvider(ProtoSupportDataProvider.class).getSupportData();
     assertThat(supportData.getTransitiveProtoPathFlags())
-        .containsExactly("--proto_path=x/foo", "--proto_path=x/bar");
+        .containsExactly("x/foo", "x/bar");
 
     assertThat(getGeneratingSpawnAction(getDescriptorOutput("//x/foo:withdeps"))
         .getRemainingArguments())