Presize the inputs array when reading an action cache entry.

This generally saves at least one copy.

Closes #6181.

PiperOrigin-RevId: 214382842

src/main/java/com/google/devtools/build/lib/actions/cache/CompactPersistentActionCache.java

@@ -418,26 +418,26 @@ private static ActionCache.Entry decode(StringIndexer indexer, byte[] data) thro
       Md5Digest md5Digest = DigestUtils.read(source);
 
       int count = VarInt.getVarInt(source);
-      ImmutableList.Builder<String> builder = new ImmutableList.Builder<>();
-      for (int i = 0; i < count; i++) {
-        int id = VarInt.getVarInt(source);
-        String filename = (id >= 0 ? indexer.getStringForIndex(id) : null);
-        if (filename == null) {
-          throw new IOException("Corrupted file index");
+      ImmutableList<String> files = null;
+      if (count != NO_INPUT_DISCOVERY_COUNT) {
+        ImmutableList.Builder<String> builder = ImmutableList.builderWithExpectedSize(count);
+        for (int i = 0; i < count; i++) {
+          int id = VarInt.getVarInt(source);
+          String filename = (id >= 0 ? indexer.getStringForIndex(id) : null);
+          if (filename == null) {
+            throw new IOException("Corrupted file index");
+          }
+          builder.add(filename);
         }
-        builder.add(filename);
+        files = builder.build();
       }
 
       Md5Digest usedClientEnvDigest = DigestUtils.read(source);
 
       if (source.remaining() > 0) {
         throw new IOException("serialized entry data has not been fully decoded");
       }
-      return new ActionCache.Entry(
-          actionKey,
-          usedClientEnvDigest,
-          count == NO_INPUT_DISCOVERY_COUNT ? null : builder.build(),
-          md5Digest);
+      return new ActionCache.Entry(actionKey, usedClientEnvDigest, files, md5Digest);
     } catch (BufferUnderflowException e) {
       throw new IOException("encoded entry data is incomplete", e);
     }