Make repo marker file parsing robust to version changes

src/main/java/com/google/devtools/build/lib/rules/repository/RepositoryDelegatorFunction.java

@@ -621,6 +621,10 @@ static String unescape(String str) {
   }
 
   private static class DigestWriter {
+    // Input value map to force repo invalidation.
+    private static final ImmutableMap<RepoRecordedInput, String> NOT_UP_TO_DATE = ImmutableMap.of(
+        RepoRecordedInput.NEVER_UP_TO_DATE, "");
+
     private final BlazeDirectories directories;
     private final Path markerPath;
     private final Rule rule;
@@ -681,43 +685,41 @@ byte[] areRepositoryAndMarkerFileConsistent(
         return null;
       }
 
-      Map<RepoRecordedInput, String> recordedInputValues = new TreeMap<>();
-      String content;
       try {
-        content = FileSystemUtils.readContent(markerPath, UTF_8);
-        String markerRuleKey = readMarkerFile(content, recordedInputValues);
-        boolean verified = false;
-        if (Preconditions.checkNotNull(ruleKey).equals(markerRuleKey)) {
-          verified = handler.verifyRecordedInputs(rule, directories, recordedInputValues, env);
-          if (env.valuesMissing()) {
-            return null;
-          }
+        String content = FileSystemUtils.readContent(markerPath, UTF_8);
+        Map<RepoRecordedInput, String> recordedInputValues = readMarkerFile(
+            content, Preconditions.checkNotNull(ruleKey));
+        if (!handler.verifyRecordedInputs(rule, directories, recordedInputValues, env)) {
+          return null;
         }
-
-        if (verified) {
-          return new Fingerprint().addString(content).digestAndReset();
-        } else {
+        if (env.valuesMissing()) {
           return null;
         }
+        return new Fingerprint().addString(content).digestAndReset();
       } catch (IOException e) {
         throw new RepositoryFunctionException(e, Transience.TRANSIENT);
       }
     }
 
     @Nullable
-    private static String readMarkerFile(
-        String content, Map<RepoRecordedInput, String> recordedInputValues) {
-      String markerRuleKey = null;
+    private static Map<RepoRecordedInput, String> readMarkerFile(
+        String content, String expectedRuleKey) {
       Iterable<String> lines = Splitter.on('\n').split(content);
 
-      boolean firstLine = true;
+      @Nullable Map<RepoRecordedInput, String> recordedInputValues = null;
+      boolean firstLineVerified = false;
       for (String line : lines) {
         if (line.isEmpty()) {
           continue;
         }
-        if (firstLine) {
-          markerRuleKey = line;
-          firstLine = false;
+        if (!firstLineVerified) {
+          if (!line.equals(expectedRuleKey)) {
+            // Break early, need to reload anyway. This also detects marker file version changes
+            // so that unknown formats are not parsed.
+            return NOT_UP_TO_DATE;
+          }
+          firstLineVerified = true;
+          recordedInputValues = new TreeMap<>();
         } else {
           int sChar = line.indexOf(' ');
           if (sChar > 0) {
@@ -728,12 +730,13 @@ private static String readMarkerFile(
             }
           }
           // On parse failure, just forget everything else and mark the whole input out of date.
-          recordedInputValues.clear();
-          recordedInputValues.put(RepoRecordedInput.NEVER_UP_TO_DATE, "");
-          break;
+          return NOT_UP_TO_DATE;
         }
       }
-      return markerRuleKey;
+      if (!firstLineVerified) {
+        return NOT_UP_TO_DATE;
+      }
+      return Preconditions.checkNotNull(recordedInputValues);
     }
 
     private String computeRuleKey(Rule rule, StarlarkSemantics starlarkSemantics) {

src/test/shell/bazel/starlark_repository_test.sh

@@ -2902,6 +2902,14 @@ EOF
   bazel shutdown
   bazel build @foo >& $TEST_log || fail "expected bazel to succeed"
   expect_log "I see: nothing"
+
+  # Test to clear the marker file.
+  echo > ${marker_file}
+
+  # Running Bazel again shouldn't crash, and should result in a refetch.
+  bazel shutdown
+  bazel build @foo >& $TEST_log || fail "expected bazel to succeed"
+  expect_log "I see: nothing"
 }
 
 function test_file_watching_in_undefined_repo() {