AVRO-4241: [Java] BinaryDecoder should verify available bytes before reading

Add ensureAvailableBytes() pre-check in readString, readBytes,
readArrayStart, arrayNext, readMapStart, and mapNext to verify the
source has sufficient data before proceeding.

Byte-array-backed sources return an exact remaining count.
Stream-backed sources return buffered bytes plus InputStream.available(),
which is reliable for the finite streams used by DataFileReader and
DataFileStream.

Includes regression tests and updated array/map limit tests.

Author: iemejia

lang/java/avro/src/main/java/org/apache/avro/generic/GenericDatumReader.java

@@ -17,12 +17,15 @@
  */
 package org.apache.avro.generic;
 
+import java.io.EOFException;
 import java.io.IOException;
 import java.lang.reflect.Constructor;
 import java.nio.ByteBuffer;
 import java.util.Collection;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Map;
+import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Function;
 
@@ -291,6 +294,7 @@ protected Object readArray(Object old, Schema expected, ResolvingDecoder in) thr
     long l = in.readArrayStart();
     long base = 0;
     if (l > 0) {
+      ensureAvailableCollectionBytes(in, l, expectedType);
       LogicalType logicalType = expectedType.getLogicalType();
       Conversion<?> conversion = getData().getConversionFor(logicalType);
       Object array = newArray(old, (int) l, expected);
@@ -306,13 +310,25 @@ protected Object readArray(Object old, Schema expected, ResolvingDecoder in) thr
           }
         }
         base += l;
-      } while ((l = in.arrayNext()) > 0);
+      } while ((l = arrayNext(in, expectedType)) > 0);
       return pruneArray(array);
     } else {
       return pruneArray(newArray(old, 0, expected));
     }
   }
 
+  /**
+   * Reads the next array block count and validates remaining bytes before the
+   * caller allocates storage.
+   */
+  private long arrayNext(ResolvingDecoder in, Schema elementType) throws IOException {
+    long l = in.arrayNext();
+    if (l > 0) {
+      ensureAvailableCollectionBytes(in, l, elementType);
+    }
+    return l;
+  }
+
   private Object pruneArray(Object object) {
     if (object instanceof GenericArray<?>) {
       ((GenericArray<?>) object).prune();
@@ -348,6 +364,9 @@ protected Object readMap(Object old, Schema expected, ResolvingDecoder in) throw
     long l = in.readMapStart();
     LogicalType logicalType = eValue.getLogicalType();
     Conversion<?> conversion = getData().getConversionFor(logicalType);
+    if (l > 0) {
+      ensureAvailableMapBytes(in, l, eValue);
+    }
     Object map = newMap(old, (int) l);
     if (l > 0) {
       do {
@@ -361,11 +380,40 @@ protected Object readMap(Object old, Schema expected, ResolvingDecoder in) throw
             addToMap(map, readMapKey(null, expected, in), readWithoutConversion(null, eValue, in));
           }
         }
-      } while ((l = in.mapNext()) > 0);
+      } while ((l = mapNext(in, eValue)) > 0);
     }
     return map;
   }
 
+  /**
+   * Reads the next map block count and validates remaining bytes before the
+   * caller allocates storage.
+   */
+  private long mapNext(ResolvingDecoder in, Schema valueType) throws IOException {
+    long l = in.mapNext();
+    if (l > 0) {
+      ensureAvailableMapBytes(in, l, valueType);
+    }
+    return l;
+  }
+
+  /**
+   * Validates remaining bytes for a map block. Each map entry has a string key
+   * (at least 1 byte for the length varint) plus a value, so the minimum bytes
+   * per entry is {@code 1 + minBytesPerElement(valueSchema)}.
+   */
+  private static void ensureAvailableMapBytes(Decoder decoder, long count, Schema valueSchema) throws EOFException {
+    // Map keys are always strings: at least 1 byte for the length varint
+    int minBytesPerEntry = 1 + minBytesPerElement(valueSchema);
+    if (count > 0) {
+      int remaining = decoder.remainingBytes();
+      if (remaining >= 0 && count * (long) minBytesPerEntry > remaining) {
+        throw new EOFException("Map claims " + count + " entries with at least " + minBytesPerEntry
+            + " bytes each, but only " + remaining + " bytes are available");
+      }
+    }
+  }
+
   /**
    * Called by the default implementation of {@link #readMap} to read a key value.
    * The default implementation returns delegates to
@@ -384,6 +432,73 @@ protected void addToMap(Object map, Object key, Object value) {
     ((Map) map).put(key, value);
   }
 
+  /**
+   * Returns the minimum number of bytes required to encode a single value of the
+   * given schema in Avro binary format. Used to validate that the decoder has
+   * enough data remaining before allocating collection backing arrays.
+   * <p>
+   * Returns 0 for types whose binary encoding is empty ({@code null}, zero-length
+   * {@code fixed}, records with only zero-byte fields). Returns a positive value
+   * for all other types.
+   */
+  static int minBytesPerElement(Schema schema) {
+    return minBytesPerElement(schema, new HashSet<>());
+  }
+
+  private static int minBytesPerElement(Schema schema, Set<String> visited) {
+    switch (schema.getType()) {
+    case NULL:
+      return 0;
+    case FIXED:
+      return schema.getFixedSize();
+    case FLOAT:
+      return 4;
+    case DOUBLE:
+      return 8;
+    case RECORD:
+      String fullName = schema.getFullName();
+      if (!visited.add(fullName)) {
+        return 0; // break recursion for self-referencing schemas
+      }
+      int sum = 0;
+      for (Schema.Field f : schema.getFields()) {
+        sum += minBytesPerElement(f.schema(), visited);
+        if (sum > 0) {
+          break; // at least 1 byte is enough to confirm non-zero
+        }
+      }
+      visited.remove(fullName);
+      return sum;
+    case UNION:
+      // The branch index varint is always at least 1 byte
+      return 1;
+    default:
+      // BOOLEAN, INT, LONG, ENUM, STRING, BYTES, ARRAY, MAP are all >= 1 byte
+      return 1;
+    }
+  }
+
+  /**
+   * Validates that the decoder has enough remaining bytes to hold {@code count}
+   * elements of the given schema, assuming each element requires at least
+   * {@link #minBytesPerElement} bytes. Throws {@link EOFException} if the decoder
+   * reports fewer remaining bytes than required.
+   * <p>
+   * This check prevents out-of-memory errors from pre-allocating huge backing
+   * arrays when the source data is truncated or malicious.
+   */
+  private static void ensureAvailableCollectionBytes(Decoder decoder, long count, Schema elementSchema)
+      throws EOFException {
+    int minBytes = minBytesPerElement(elementSchema);
+    if (minBytes > 0 && count > 0) {
+      int remaining = decoder.remainingBytes();
+      if (remaining >= 0 && count * (long) minBytes > remaining) {
+        throw new EOFException("Collection claims " + count + " elements with at least " + minBytes
+            + " bytes each, but only " + remaining + " bytes are available");
+      }
+    }
+  }
+
   /**
    * Called to read a fixed value. May be overridden for alternate fixed
    * representations. By default, returns {@link GenericFixed}.

lang/java/avro/src/main/java/org/apache/avro/io/BinaryDecoder.java

@@ -295,6 +295,7 @@ public double readDouble() throws IOException {
   @Override
   public Utf8 readString(Utf8 old) throws IOException {
     int length = SystemLimitException.checkMaxStringLength(readLong());
+    ensureAvailableBytes(length);
     Utf8 result = (old != null ? old : new Utf8());
     result.setByteLength(length);
     if (0 != length) {
@@ -318,6 +319,7 @@ public void skipString() throws IOException {
   @Override
   public ByteBuffer readBytes(ByteBuffer old) throws IOException {
     int length = SystemLimitException.checkMaxBytesLength(readLong());
+    ensureAvailableBytes(length);
     final ByteBuffer result;
     if (old != null && length <= old.capacity()) {
       result = old;
@@ -508,6 +510,23 @@ public boolean isEnd() throws IOException {
     return (0 == read);
   }
 
+  /**
+   * Returns the total number of bytes remaining that can be read from this
+   * decoder (including any buffered bytes), or {@code -1} if the total is
+   * unknown.
+   * <p>
+   * Byte-array-backed decoders return an exact count. Stream-backed decoders
+   * return the sum of buffered bytes and {@link InputStream#available()}, which
+   * is a reliable lower bound for the finite, in-memory or file-backed streams
+   * used by {@code DataFileReader} and {@code DataFileStream}.
+   * <p>
+   * {@link DirectBinaryDecoder} always returns {@code -1}.
+   */
+  @Override
+  public int remainingBytes() {
+    return source != null ? source.remainingBytes() : -1;
+  }
+
   /**
    * Ensures that buf[pos + num - 1] is not out of the buffer array bounds.
    * However, buf[pos + num -1] may be >= limit if there is not enough data left
@@ -530,6 +549,29 @@ private void ensureBounds(int num) throws IOException {
     }
   }
 
+  /**
+   * Validates that the source has at least {@code length} bytes remaining before
+   * proceeding. Throws early if the declared length is inconsistent with the
+   * available data.
+   * <p>
+   * For byte-array-backed sources the remaining count is exact, so this check is
+   * definitive. For stream-backed sources the check uses buffered bytes plus
+   * {@link InputStream#available()}, which is a reliable lower bound for the
+   * finite streams used by {@code DataFileReader} and {@code DataFileStream}.
+   *
+   * @param length the number of bytes expected to be available
+   * @throws EOFException if the source is known to have fewer bytes remaining
+   */
+  private void ensureAvailableBytes(int length) throws EOFException {
+    if (source != null && length > 0) {
+      int remaining = source.remainingBytes();
+      if (remaining >= 0 && length > remaining) {
+        throw new EOFException(
+            "Attempted to read " + length + " bytes, but only " + remaining + " bytes are available");
+      }
+    }
+  }
+
   /**
    * Returns an {@link java.io.InputStream} that is aware of any buffering that
    * may occur in this BinaryDecoder. Readers that need to interleave decoding
@@ -664,6 +706,17 @@ protected ByteSource() {
 
     abstract boolean isEof();
 
+    /**
+     * Returns the total number of bytes remaining that can be read from this source
+     * (including any buffered bytes), or {@code -1} if the total is unknown.
+     * Byte-array-backed sources return an exact count. Stream-backed sources return
+     * the sum of buffered bytes and {@link InputStream#available()}, which is a
+     * reliable lower bound for the finite, in-memory or file-backed streams used by
+     * {@code DataFileReader} and {@code DataFileStream}. Returns {@code -1} only
+     * when {@code available()} throws.
+     */
+    protected abstract int remainingBytes();
+
     protected void attach(int bufferSize, BinaryDecoder decoder) {
       decoder.buf = new byte[bufferSize];
       decoder.pos = 0;
@@ -910,6 +963,15 @@ public boolean isEof() {
       return isEof;
     }
 
+    @Override
+    protected int remainingBytes() {
+      try {
+        return (ba.getLim() - ba.getPos()) + in.available();
+      } catch (IOException e) {
+        return -1;
+      }
+    }
+
     @Override
     public void close() throws IOException {
       in.close();
@@ -1028,5 +1090,10 @@ public boolean isEof() {
       int remaining = ba.getLim() - ba.getPos();
       return (remaining == 0);
     }
+
+    @Override
+    protected int remainingBytes() {
+      return ba.getLim() - ba.getPos();
+    }
   }
 }

lang/java/avro/src/main/java/org/apache/avro/io/Decoder.java

@@ -299,4 +299,14 @@ public void readFixed(byte[] bytes) throws IOException {
    *                           type of the next value to be read
    */
   public abstract int readIndex() throws IOException;
+
+  /**
+   * Returns the total number of bytes remaining that can be read from this
+   * decoder, or {@code -1} if the total is unknown. Implementations that can
+   * determine remaining capacity (for example, byte-array-backed decoders) should
+   * override this method. The default returns {@code -1}.
+   */
+  public int remainingBytes() {
+    return -1;
+  }
 }

lang/java/avro/src/main/java/org/apache/avro/io/ValidatingDecoder.java

@@ -246,4 +246,9 @@ public int readIndex() throws IOException {
   public Symbol doAction(Symbol input, Symbol top) throws IOException {
     return null;
   }
+
+  @Override
+  public int remainingBytes() {
+    return in != null ? in.remainingBytes() : -1;
+  }
 }