okhttp: avoid unnecessary Header allocation for lowercase names

okhttp/third_party/okhttp/main/java/io/grpc/okhttp/internal/framed/Hpack.java

@@ -490,7 +490,7 @@ void writeHeaders(List<io.grpc.okhttp.internal.framed.Header> headerBlock) throw
           out.writeByte(0x40);
           writeByteString(name);
           writeByteString(value);
-          insertIntoDynamicTable(new io.grpc.okhttp.internal.framed.Header(name, value));
+          insertIntoDynamicTable(canonicalHeader(header, name, value));
         } else if (name.startsWith(PSEUDO_PREFIX)
             && !io.grpc.okhttp.internal.framed.Header.TARGET_AUTHORITY.equals(name)
             && !io.grpc.okhttp.internal.framed.Header.TARGET_PATH.equals(name)) {
@@ -504,11 +504,25 @@ void writeHeaders(List<io.grpc.okhttp.internal.framed.Header> headerBlock) throw
           // Literal Header Field with Incremental Indexing - Indexed Name.
           writeInt(headerNameIndex, PREFIX_6_BITS, 0x40);
           writeByteString(value);
-          insertIntoDynamicTable(new io.grpc.okhttp.internal.framed.Header(name, value));
+          insertIntoDynamicTable(canonicalHeader(header, name, value));
         }
       }
     }
 
+    /**
+     * Returns {@code header} as-is when its name is already the lowercase {@code name} used for
+     * indexing, avoiding an extra Header allocation on the common path where header names are
+     * already lowercase (e.g. gRPC's generated headers and the {@code :path}/{@code :authority}
+     * pseudo headers). A value comparison detects that case while also avoiding an allocation if
+     * an equivalent lowercase name is represented by a different {@link ByteString} instance.
+     */
+    private static io.grpc.okhttp.internal.framed.Header canonicalHeader(
+        io.grpc.okhttp.internal.framed.Header header, ByteString name, ByteString value) {
+      return name.equals(header.name)
+          ? header
+          : new io.grpc.okhttp.internal.framed.Header(name, value);
+    }
+
     // http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-12#section-4.1.1
     void writeInt(int value, int prefixMask, int bits) throws IOException {
       // Write the raw value for a single byte value.

okhttp/third_party/okhttp/test/java/io/grpc/okhttp/internal/framed/HpackTest.java

@@ -18,6 +18,7 @@
 
 import static okio.ByteString.decodeHex;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertSame;
 import static org.junit.Assert.fail;
 
 import java.io.IOException;
@@ -1140,6 +1141,28 @@ public void pseudoHeaderIndexingForPathAndAuthority() throws IOException {
     assertEquals(3, hpackWriter.dynamicTableHeaderCount);
   }
 
+  @Test
+  public void mixedCaseHeaderNameIndexedOnRepeat() throws IOException {
+    hpackWriter.writeHeaders(Arrays.asList(new Header("FoO", "BaR")));
+    assertBytes(0x40, 3, 'f', 'o', 'o', 3, 'B', 'a', 'R');
+    assertEquals(1, hpackWriter.dynamicTableHeaderCount);
+
+    // Even though the header name is mixed case again, the dynamic table entry was stored
+    // with the lowercase name, so the repeat is emitted as an indexed reference.
+    hpackWriter.writeHeaders(Arrays.asList(new Header("FoO", "BaR")));
+    assertBytes(0xbe);
+    assertEquals(1, hpackWriter.dynamicTableHeaderCount);
+  }
+
+  @Test
+  public void lowercaseHeaderIsReusedInDynamicTable() throws IOException {
+    Header header = new Header("custom-key", "custom-value");
+
+    hpackWriter.writeHeaders(Arrays.asList(header));
+
+    assertSame(header, hpackWriter.dynamicTable[hpackWriter.dynamicTable.length - 1]);
+  }
+
   @Test
   public void evictToRecoverBytesDoesNotNpeWhenBytesToRecoverExceedsTable() throws IOException {
     hpackWriter.writeHeaders(headerEntries("custom-key", "custom-value"));