Interceptors
============

Interceptors are a powerful mechanism that can monitor, rewrite, and retry calls. Here's a simple interceptor that logs the outgoing request and the incoming response.

```java
class LoggingInterceptor implements Interceptor {
  @Override public Response intercept(Interceptor.Chain chain) throws IOException {
    Request request = chain.request();

    long t1 = System.nanoTime();

<p>
Note that if there are no read-write or write-write data races on memory location <i>x</i>,
then any read <i>r</i> on <i>x</i> has only one possible <i>W</i>(<i>r</i>):
the single <i>w</i> that immediately precedes it in the happens before order.
</p>

<p>
More generally, it can be shown that any Go program that is data-race-free,
meaning it has no program executions with read-write or write-write data races,

provided

	mpz_t zero;

then cgo would rewrite a reference to C.zero by introducing

	var _C_zero *C.mpz_t

and then replacing all instances of C.zero with (*_C_zero).

Cgo's most interesting translation is for functions.  If xxx is a C
function, then cgo rewrites C.xxx into a new function _C_xxx that
calls the C xxx in a standard pthread.  The new function translates

    // the `keys` and `values` arrays do not need to change. But because the "short hash" now has a
    // different number of bits, we must rewrite each element of `entries` so that its contribution
    // to the full hashcode reflects the change, and so that its `next` link corresponds to the new
    // linked list of entries with the new short hash.

    /** Name of the search engine cluster */
    protected String clusterName = "fesen";

    /** List of rewrite rules for document settings */
    protected final List<UnaryOperator<String>> docSettingRewriteRuleList = new ArrayList<>();

    /** List of rewrite rules for document mappings */
    protected final List<UnaryOperator<String>> docMappingRewriteRuleList = new ArrayList<>();

}

// RenameDataResp - RenameData()'s response.
// Provides information about the final state of Rename()
//   - on xl.meta (array of versions) on disk to check for version disparity
//   - on rewrite dataDir on disk that must be additionally purged
//     only after as a 2-phase call, allowing the older dataDir to
//     hang-around in-case we need some form of recovery.
type RenameDataResp struct {

    // the `keys` and `values` arrays do not need to change. But because the "short hash" now has a
    // different number of bits, we must rewrite each element of `entries` so that its contribution
    // to the full hashcode reflects the change, and so that its `next` link corresponds to the new
    // linked list of entries with the new short hash.

    even for a single call.

    Interceptors that rewrite or replace the request body may now inadvertently interfere with
    duplex request bodies. Such interceptors should check `RequestBody.isDuplex()` and avoid
    accessing the request body when it is.

    Duplex calls require HTTP/2. If HTTP/1 is established instead the duplex call will fail. The

	}

	// TODO: It seems like we may have to be prepared to rewrite and sort trailing headers:
	// https://docs.aws.amazon.com/IAM/latest/UserGuide/create-signed-request.html

	// Any value must end with a newline.
	// Not all clients send that.
	trailerRaw := valueBuffer.Bytes()
	if len(trailerRaw) > 0 && trailerRaw[len(trailerRaw)-1] != '\n' {
		valueBuffer.Write([]byte{'\n'})
	}
	sig = sig[len("x-amz-trailer-signature:"):]

                "- Otherwise, preserve the original translation LINE-BY-LINE, AS-IS.",
                "Do not:",
                "- rephrase or rewrite correct lines just to improve the style.",
                "- add or remove line breaks, unless the original English content changed.",
                "- change formatting or whitespace unless absolutely required.",