Fix internal error in CssCompiler caused by source-mapping of empty i…

…mport block at start of file.

Trying to fix this by making the DefaultVisitController skip empty
import block nodes breaks another test, so a CL that fixes it by
tweaking the DefaultGssSourceMapGenerator is forthcoming.

It looks like the DefaultGssSourceMapGenerator can already generate
zero-length mappings, as when an empty import block is written after a
copyright header comment so clients should not be confused.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=126835623

src/com/google/common/css/compiler/passes/DefaultGssSourceMapGenerator.java

@@ -154,7 +154,10 @@ public void startSourceMapping(CssNode node, int startLine, int startCharIndex)
   public void endSourceMapping(CssNode node, int endLine, int endCharIndex) {
     Preconditions.checkState(node != null);
     Preconditions.checkState(endLine >= 0);
-    Preconditions.checkState(endCharIndex >= 0);
+    // -1 when a node contributes no content at the start of the buffer,
+    // as when a CssImportBlockNode is encountered, and there is no
+    // copyright comment.
+    Preconditions.checkState(endCharIndex >= -1);
     if (!mappings.isEmpty() && mappings.peek().node == node) {
       Mapping mapping = mappings.pop();
       mapping.end = new FilePosition(endLine, endCharIndex);

src/com/google/common/css/compiler/passes/GssSourceMapGenerator.java

@@ -51,6 +51,8 @@ public interface GssSourceMapGenerator {
    * @param node the {@link CssNode} to be processed
    * @param endLine the last character's line number when it ends writing output
    * @param endCharIndex the last character's character index when it ends writing output
+   *     or one less than the corresponding {@link #startSourceMapping startCharIndex} if
+   *     a source mapping is empty.
    */
   public void endSourceMapping(CssNode node, int endLine, int endCharIndex);
 

tests/com/google/common/css/compiler/commandline/ClosureCommandLineCompilerTest.java

@@ -21,9 +21,12 @@
 import com.google.common.css.SourceCode;
 import com.google.common.css.compiler.ast.ErrorManager;
 import com.google.common.css.compiler.ast.testing.NewFunctionalTestBase;
+import com.google.common.io.Files;
 
 import junit.framework.TestCase;
 
+import java.io.File;
+
 public class ClosureCommandLineCompilerTest extends TestCase {
 
   static final ExitCodeHandler EXIT_CODE_HANDLER =
@@ -64,4 +67,30 @@ public void testAllowDefPropagationDefaultsToTrue() throws Exception {
     JobDescription jobDescription = flags.createJobDescription();
     assertTrue(jobDescription.allowDefPropagation);
   }
+
+
+  public void testEmptyImportBlocks() throws Exception {
+    // See b/29995881
+    ErrorManager errorManager = new NewFunctionalTestBase.TestErrorManager(new String[0]);
+
+    JobDescription job = new JobDescriptionBuilder()
+      .addInput(new SourceCode("main.css", "@import 'common.css';"))
+      .addInput(new SourceCode("common.css", "/* common */"))
+      .setOptimizeStrategy(JobDescription.OptimizeStrategy.SAFE)
+      .setCreateSourceMap(true)
+      .getJobDescription();
+
+    File outputDir = Files.createTempDir();
+    File sourceMapFile = new File(outputDir, "sourceMap");
+
+    String compiledCss = new ClosureCommandLineCompiler(
+        job, EXIT_CODE_HANDLER, errorManager)
+        .execute(null /*renameFile*/, sourceMapFile);
+
+    // The symptom was an IllegalStateException trapped by the compiler that
+    // resulted in the exit handler being called which causes fail(...),
+    // so if control reaches here, we're ok.
+
+    assertNotNull(compiledCss);
+  }
 }