Keep `weighted_distribution_` in sync with `records_` when pruning all-zero weights.

When all corpus inputs have weight zero, `WeightedDistribution::RemoveRandomWeightedSubset` removes every entry from `weighted_distribution_` before returning `subset_to_remove`. `Corpus::Prune` then erases one index from `subset_to_remove` so `records_` retains one random input, leaving `weighted_distribution_` empty and causing check failures on subsequent corpus operations.

Restore one zero weight in `weighted_distribution_` when keeping that fallback input, and add a unit test covering `Corpus::Prune` with all-zero weights.

PiperOrigin-RevId: 1002585725

centipede/corpus.cc

diff --git a/centipede/corpus.cc b/centipede/corpus.cc
index 5c194b5..0b216f4 100644
--- a/centipede/corpus.cc
+++ b/centipede/corpus.cc
@@ -213,6 +213,8 @@
     subset_to_remove.erase(
         subset_to_remove.begin() +
         absl::Uniform<size_t>(rng, 0, subset_to_remove.size()));
+    // Restore one weight since RemoveRandomWeightedSubset removed all of them.
+    weighted_distribution_.AddWeight(0);
   }
   RemoveSubset(subset_to_remove, records_);
 

centipede/corpus_test.cc

diff --git a/centipede/corpus_test.cc b/centipede/corpus_test.cc
index 46e4d4e..5187a60 100644
--- a/centipede/corpus_test.cc
+++ b/centipede/corpus_test.cc
@@ -338,6 +338,23 @@
   corpus.Prune(fs, coverage_frontier, max_corpus_size, rng);
 }
 
+TEST(Corpus, PruneCorpusWithAllZeroWeights) {
+  Corpus corpus;
+  Rng rng(0);
+
+  // Newly added inputs have weight 0 before UpdateWeights is called.
+  corpus.Add({1}, {}, {}, {}, FeatureSet(1, {}),
+             CoverageFrontier(BinaryInfo{}));
+  corpus.Add({2}, {}, {}, {}, FeatureSet(1, {}),
+             CoverageFrontier(BinaryInfo{}));
+
+  EXPECT_EQ(corpus.Prune(FeatureSet(1, {}), CoverageFrontier(BinaryInfo{}),
+                         /*max_corpus_size=*/1000, rng),
+            1);
+  EXPECT_EQ(corpus.NumActive(), 1);
+  EXPECT_EQ(corpus.WeightedRandom(rng), 0);
+}
+
 // Regression test for a crash in Corpus::Prune().
 TEST(Corpus, PruneRegressionTest1) {
   PCTable pc_table(100);