Add module_ctx.fail_on_user_error

The new function allows module extensions to fail in a way that is more
appropriate for user errors: No Starlark stack trace is prepended to a
failure, instead the optionally provided tags and their locations are
appended to the error message.

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleExtensionContext.java

@@ -14,6 +14,7 @@
 
 package com.google.devtools.build.lib.bazel.bzlmod;
 
+import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.devtools.build.docgen.annot.DocCategory;
 import com.google.devtools.build.lib.bazel.repository.downloader.DownloadManager;
@@ -22,11 +23,16 @@
 import com.google.devtools.build.lib.runtime.RepositoryRemoteExecutor;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.skyframe.SkyFunction.Environment;
+import java.util.List;
 import java.util.Map;
+import java.util.StringJoiner;
 import javax.annotation.Nullable;
+import net.starlark.java.annot.Param;
+import net.starlark.java.annot.ParamType;
 import net.starlark.java.annot.StarlarkBuiltin;
 import net.starlark.java.annot.StarlarkMethod;
 import net.starlark.java.eval.EvalException;
+import net.starlark.java.eval.Sequence;
 import net.starlark.java.eval.StarlarkList;
 import net.starlark.java.eval.StarlarkSemantics;
 
@@ -99,4 +105,62 @@ protected ImmutableMap<String, String> getRemoteExecProperties() throws EvalExce
   public StarlarkList<StarlarkBazelModule> getModules() {
     return modules;
   }
+
+  @StarlarkMethod(
+      name = "fail_on_user_error",
+      doc = "Fails the module extension with the given error message in reponse to a user error "
+          + "such as a semantically invalid tag. Compared to <a "
+          + "href=\"globals.html#fail\">fail</a>, the error message will not include a Starlark "
+          + "stack trace, but instead emits information about the given tags and their locations "
+          + "after the message.",
+      parameters = {
+          @Param(
+              name = "message",
+              allowedTypes = {@ParamType(type = String.class)},
+              doc = "The error message to display to the user."),
+      },
+      extraPositionals = @Param(
+          name = "tags",
+          allowedTypes = {@ParamType(type = Sequence.class, generic1 = TypeCheckedTag.class)}
+      )
+  )
+  public void fail(String message, Sequence<?> tags) throws EvalException {
+    Sequence<TypeCheckedTag> checkedTags = Sequence.cast(tags, TypeCheckedTag.class, "tags");
+    throw new InvalidTagEvalException(message, checkedTags, extensionId);
+  }
+
+  static final class InvalidTagEvalException extends EvalException {
+
+    private final ImmutableList<TypeCheckedTag> tags;
+    private final ModuleExtensionId extensionId;
+
+    InvalidTagEvalException(String message, List<TypeCheckedTag> tags,
+        ModuleExtensionId extensionId) {
+      super(message);
+      this.tags = ImmutableList.copyOf(tags);
+      this.extensionId = extensionId;
+    }
+
+    public String getMessageWithTags() {
+      String tagInfo = tags.stream()
+          .map(this::formatTag)
+          .collect(
+              () -> new StringJoiner("\n", "\n\n", "").setEmptyValue(""),
+              StringJoiner::add,
+              StringJoiner::merge)
+          .toString();
+      return String.format(
+          "Reported by module extension %s in %s:\n%s%s",
+          extensionId.getExtensionName(), extensionId.getBzlFileLabel(), getMessage(), tagInfo);
+    }
+
+    private String formatTag(TypeCheckedTag tag) {
+      // Tag instantiations are expected to be top-level statements, so the column number isn't
+      // useful.
+      return String.format("File \"%s\", line %d, evaluated to:\n%s",
+          tag.getLocation().file(),
+          tag.getLocation().line(),
+          extensionId.getExtensionName() + "." + tag.prettyPrint());
+    }
+  }
 }

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/SingleExtensionEvalFunction.java

@@ -20,6 +20,7 @@
 
 import com.google.common.collect.ImmutableSet;
 import com.google.devtools.build.lib.analysis.BlazeDirectories;
+import com.google.devtools.build.lib.bazel.bzlmod.ModuleExtensionContext.InvalidTagEvalException;
 import com.google.devtools.build.lib.bazel.repository.downloader.DownloadManager;
 import com.google.devtools.build.lib.cmdline.BazelModuleContext;
 import com.google.devtools.build.lib.cmdline.LabelConstants;
@@ -195,6 +196,16 @@ public SkyValue compute(SkyKey skyKey, Environment env)
           throw new SingleExtensionEvalFunctionException(e1, Transience.TRANSIENT);
         }
         return null;
+      } catch (InvalidTagEvalException e) {
+        env.getListener().handle(Event.error(e.getMessageWithTags()));
+        throw new SingleExtensionEvalFunctionException(
+            ExternalDepsException.withCauseAndMessage(
+                Code.BAD_MODULE,
+                e,
+                "Error evaluating module extension %s in %s",
+                extensionId.getExtensionName(),
+                extensionId.getBzlFileLabel()),
+            Transience.TRANSIENT);
       } catch (EvalException e) {
         env.getListener().handle(Event.error(e.getMessageWithStack()));
         throw new SingleExtensionEvalFunctionException(

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/StarlarkBazelModule.java

@@ -128,7 +128,7 @@ public static StarlarkBazelModule create(
       // (for example, String to Label).
       typeCheckedTags
           .get(tag.getTagName())
-          .add(TypeCheckedTag.create(tagClass, tag, labelConverter));
+          .add(TypeCheckedTag.create(tagClass, tag.getTagName(), tag, labelConverter));
     }
     return new StarlarkBazelModule(
         module.getName(),

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/TypeCheckedTag.java

@@ -23,25 +23,36 @@
 import javax.annotation.Nullable;
 import net.starlark.java.annot.StarlarkBuiltin;
 import net.starlark.java.eval.EvalException;
+import net.starlark.java.eval.Starlark;
 import net.starlark.java.eval.Structure;
 import net.starlark.java.spelling.SpellChecker;
+import net.starlark.java.syntax.Location;
 
 /**
  * A {@link Tag} whose attribute values have been type-checked against the attribute schema define
  * in the {@link TagClass}.
  */
 @StarlarkBuiltin(name = "bazel_module_tag", documented = false)
 public class TypeCheckedTag implements Structure {
+
   private final TagClass tagClass;
+  private final String tagClassName;
   private final Object[] attrValues;
+  private final Location location;
 
-  private TypeCheckedTag(TagClass tagClass, Object[] attrValues) {
+  private TypeCheckedTag(TagClass tagClass, String tagClassName, Object[] attrValues,
+      Location location) {
     this.tagClass = tagClass;
+    this.tagClassName = tagClassName;
     this.attrValues = attrValues;
+    this.location = location;
   }
 
-  /** Creates a {@link TypeCheckedTag}. */
-  public static TypeCheckedTag create(TagClass tagClass, Tag tag, LabelConverter labelConverter)
+  /**
+   * Creates a {@link TypeCheckedTag}.
+   */
+  public static TypeCheckedTag create(TagClass tagClass, String tagClassName, Tag tag,
+      LabelConverter labelConverter)
       throws ExternalDepsException {
     Object[] attrValues = new Object[tagClass.getAttributes().size()];
     for (Map.Entry<String, Object> attrValue : tag.getAttributeValues().entrySet()) {
@@ -95,7 +106,7 @@ public static TypeCheckedTag create(TagClass tagClass, Tag tag, LabelConverter l
         attrValues[i] = Attribute.valueToStarlark(attr.getDefaultValueUnchecked());
       }
     }
-    return new TypeCheckedTag(tagClass, attrValues);
+    return new TypeCheckedTag(tagClass, tagClassName, attrValues, tag.getLocation());
   }
 
   @Override
@@ -123,4 +134,24 @@ public ImmutableCollection<String> getFieldNames() {
   public String getErrorMessageForUnknownField(String field) {
     return "unknown attribute " + field;
   }
+
+  public Location getLocation() {
+    return location;
+  }
+
+  public String prettyPrint() {
+    StringBuilder sb = new StringBuilder();
+    sb.append(tagClassName);
+    sb.append("(\n");
+    for (int i = 0; i < attrValues.length; i++) {
+      Attribute attr = tagClass.getAttributes().get(i);
+      sb.append("    ");
+      sb.append(attr.getPublicName());
+      sb.append(" = ");
+      sb.append(Starlark.repr(attrValues[i]));
+      sb.append(",\n");
+    }
+    sb.append(")");
+    return sb.toString();
+  }
 }

src/test/java/com/google/devtools/build/lib/bazel/bzlmod/TypeCheckedTagTest.java

@@ -60,9 +60,9 @@ private static Object getattr(Structure structure, String fieldName) throws Exce
   public void basic() throws Exception {
     TypeCheckedTag typeCheckedTag =
         TypeCheckedTag.create(
-            createTagClass(attr("foo", Type.INTEGER).build()),
-            buildTag("tag_name").addAttr("foo", StarlarkInt.of(3)).build(),
-            /*labelConverter=*/ null);
+            createTagClass(attr("foo", Type.INTEGER).build()), "tag_name",
+            /*labelConverter=*/ buildTag("tag_name").addAttr("foo", StarlarkInt.of(3)).build(),
+            null);
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo");
     assertThat(getattr(typeCheckedTag, "foo")).isEqualTo(StarlarkInt.of(3));
   }
@@ -73,11 +73,11 @@ public void label() throws Exception {
         TypeCheckedTag.create(
             createTagClass(
                 attr("foo", BuildType.LABEL_LIST).allowedFileTypes(FileTypeSet.ANY_FILE).build()),
+            "tag_name",
             buildTag("tag_name")
                 .addAttr(
                     "foo", StarlarkList.immutableOf(":thing1", "//pkg:thing2", "@repo//pkg:thing3"))
-                .build(),
-            new LabelConverter(
+                .build(), new LabelConverter(
                 PackageIdentifier.parse("@myrepo//mypkg"),
                 createRepositoryMapping(createModuleKey("test", "1.0"), "repo", "other_repo")));
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo");
@@ -95,8 +95,8 @@ public void label_withoutDefaultValue() throws Exception {
         TypeCheckedTag.create(
             createTagClass(
                 attr("foo", BuildType.LABEL).allowedFileTypes(FileTypeSet.ANY_FILE).build()),
-            buildTag("tag_name").build(),
-            new LabelConverter(
+            "tag_name",
+            buildTag("tag_name").build(), new LabelConverter(
                 PackageIdentifier.parse("@myrepo//mypkg"),
                 createRepositoryMapping(createModuleKey("test", "1.0"), "repo", "other_repo")));
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo");
@@ -110,9 +110,8 @@ public void stringListDict_default() throws Exception {
             createTagClass(
                 attr("foo", Type.STRING_LIST_DICT)
                     .value(ImmutableMap.of("key", ImmutableList.of("value1", "value2")))
-                    .build()),
-            buildTag("tag_name").build(),
-            null);
+                    .build()), "tag_name",
+            buildTag("tag_name").build(), null);
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo");
     assertThat(getattr(typeCheckedTag, "foo"))
         .isEqualTo(
@@ -128,12 +127,11 @@ public void multipleAttributesAndDefaults() throws Exception {
             createTagClass(
                 attr("foo", Type.STRING).mandatory().build(),
                 attr("bar", Type.INTEGER).value(StarlarkInt.of(3)).build(),
-                attr("quux", Type.STRING_LIST).build()),
-            buildTag("tag_name")
+                attr("quux", Type.STRING_LIST).build()), "tag_name",
+            /*labelConverter=*/ buildTag("tag_name")
                 .addAttr("foo", "fooValue")
                 .addAttr("quux", StarlarkList.immutableOf("quuxValue1", "quuxValue2"))
-                .build(),
-            /*labelConverter=*/ null);
+                .build(), null);
     assertThat(typeCheckedTag.getFieldNames()).containsExactly("foo", "bar", "quux");
     assertThat(getattr(typeCheckedTag, "foo")).isEqualTo("fooValue");
     assertThat(getattr(typeCheckedTag, "bar")).isEqualTo(StarlarkInt.of(3));
@@ -148,9 +146,8 @@ public void mandatory() throws Exception {
             ExternalDepsException.class,
             () ->
                 TypeCheckedTag.create(
-                    createTagClass(attr("foo", Type.STRING).mandatory().build()),
-                    buildTag("tag_name").build(),
-                    /*labelConverter=*/ null));
+                    createTagClass(attr("foo", Type.STRING).mandatory().build()), "tag_name",
+                    /*labelConverter=*/ buildTag("tag_name").build(), null));
     assertThat(e).hasMessageThat().contains("mandatory attribute foo isn't being specified");
   }
 
@@ -164,9 +161,9 @@ public void allowedValues() throws Exception {
                     createTagClass(
                         attr("foo", Type.STRING)
                             .allowedValues(new AllowedValueSet("yes", "no"))
-                            .build()),
-                    buildTag("tag_name").addAttr("foo", "maybe").build(),
-                    /*labelConverter=*/ null));
+                            .build()), "tag_name",
+                    /*labelConverter=*/ buildTag("tag_name").addAttr("foo", "maybe").build(),
+                    null));
     assertThat(e)
         .hasMessageThat()
         .contains("the value for attribute foo has to be one of 'yes' or 'no' instead of 'maybe'");
@@ -179,9 +176,9 @@ public void unknownAttr() throws Exception {
             ExternalDepsException.class,
             () ->
                 TypeCheckedTag.create(
-                    createTagClass(attr("foo", Type.STRING).build()),
-                    buildTag("tag_name").addAttr("bar", "maybe").build(),
-                    /*labelConverter=*/ null));
+                    createTagClass(attr("foo", Type.STRING).build()), "tag_name",
+                    /*labelConverter=*/ buildTag("tag_name").addAttr("bar", "maybe").build(),
+                    null));
     assertThat(e).hasMessageThat().contains("unknown attribute bar provided");
   }
 }