.put("three", 3)
            .put("one", 1)
            .put("five", 5)
            .put("four", 3)
            .put("four", 5)
            .put("four", 4) // this should win because it's last
            .put("two", 2);
    assertMapEquals(
        builder.buildKeepingLast(), "one", 1, "two", 2, "three", 3, "four", 4, "five", 5);

---
title: "Go, Backwards Compatibility, and GODEBUG"
layout: article
---

<!--
This document is kept in the Go repo, not x/website,
because it documents the full list of known GODEBUG settings,
which are tied to a specific release.
-->

## Introduction {#intro}

Go's emphasis on backwards compatibility is one of its key strengths.
There are, however, times when we cannot maintain complete compatibility.

    // lockD -> lockA
    lockD.lock();
    lockA.lock();
    lockA.unlock();
    lockD.unlock();

    // lockA -> lockD should warn but otherwise succeed because lockD was
    // created by a factory with the WARN policy.
    lockA.lock();
    lockD.lock();
  }

  public void testReentrantLock_tryLock() throws Exception {

    // lockD -> lockA
    lockD.lock();
    lockA.lock();
    lockA.unlock();
    lockD.unlock();

    // lockA -> lockD should warn but otherwise succeed because lockD was
    // created by a factory with the WARN policy.
    lockA.lock();
    lockD.lock();
  }

  public void testReentrantLock_tryLock() throws Exception {

		b[0] |= 0x80 // Highest bit indicates binary format
		return
	}

	f.formatOctal(b, 0) // Last resort, just write zero
	f.err = ErrFieldTooLong
}

func (p *parser) parseOctal(b []byte) int64 {
	// Because unused fields are filled with NULs, we need
	// to skip leading NULs. Fields may also be padded with
	// spaces or NULs.
	// So we remove leading and trailing NULs and spaces to
	// be sure.
	b = bytes.Trim(b, " \x00")

    assertThat(server.takeRequest().exchangeIndex).isEqualTo(1)
    assertThat(server.takeRequest().exchangeIndex).isEqualTo(2)
  }

  /**
   * Each OkHttpClient used to get its own instance of NullProxySelector, and because these weren't
   * equal their connections weren't pooled. That's a nasty performance bug!
   *
   * https://github.com/square/okhttp/issues/5519
   */
  @Test
  fun connectionPoolingWithFreshClientSamePool() {

        return;
      }

      /*
       * Reclassify nodes in each list to new Map. Because we are using power-of-two expansion, the
       * elements from each bin must either stay at same index, or move with a power of two offset.
       * We eliminate unnecessary node creation by catching cases where old nodes can be reused
       * because their next fields won't change. Statistically, at the default threshold, only

// are equal. The signatures are expected to be HEX encoded strings
// according to the AWS S3 signature V4 spec.
func compareSignatureV4(sig1, sig2 string) bool {
	// The CTC using []byte(str) works because the hex encoding
	// is unique for a sequence of bytes. See also compareSignatureV2.
	return subtle.ConstantTimeCompare([]byte(sig1), []byte(sig2)) == 1
}

```

</div>

And then you would end up with `harry` version `3` installed in your global Python environment.

And if you try to run `philosophers-stone` again, there's a chance it would **not work** because it needs `harry` version `1`.

```mermaid
flowchart LR
    subgraph global[global env]
        harry-1[<strike>harry v1</strike>]
        style harry-1 fill:#ccc,stroke-dasharray: 5 5
        harry-3[harry v3]

// descriptions for all the error responses.
var errorCodes = errorCodeMap{
	ErrInvalidCopyDest: {
		Code:           "InvalidRequest",
		Description:    "This copy request is illegal because it is trying to copy an object to itself without changing the object's metadata, storage class, website redirect location or encryption attributes.",
		HTTPStatusCode: http.StatusBadRequest,
	},
	ErrInvalidCopySource: {