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        "Path":  "/doc/go1.14",
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<h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.14</h2>

<p>
  <strong>
    Go 1.14 is not yet released. These are work-in-progress
    release notes. Go 1.14 is expected to be released in February 2020.
  </strong>
</p>

<h2 id="language">Changes to the language</h2>

<p>
TODO
</p>

<p><!-- CL 187519 -->
  TODO: <a href="https://golang.org/cl/187519">https://golang.org/cl/187519</a>: allow embedding overlapping interfaces
</p>

<h2 id="ports">Ports</h2>

<p>
  TODO: is Dragonfly passing? On both Dragonfly release & tip? (ABI
  change happened) Does the net package's interface APIs work on both?
  https://golang.org/issue/34368.
</p>

<p>
  TODO: is Illumos up with a builder and passing?
  https://golang.org/issue/15581.
</p>

<p>
  TODO: announce something about the Go Solaris port? Solaris itself
  is unmaintained? The builder is still running at Oracle, but the
  employee who set it up left the company and we have no way to
  maintain it.
</p>

<h3 id="darwin">Darwin</h3>

<p><!-- golang.org/issue/34749 -->
  Go 1.14 is the last Go release to support 32-bit binaries on
  macOS (the <code>darwin/386</code> port). They are no longer
  supported by macOS, starting with macOS 10.15 (Catalina).
  Go continues to support the 64-bit <code>darwin/amd64</code> port.
</p>

<p><!-- golang.org/issue/34751 -->
  Go 1.14 will likely be the last Go release to support 32-bit
  binaries on iOS, iPadOS, watchOS, and tvOS
  (the <code>darwin/arm</code> port). Go continues to support the
  64-bit <code>darwin/arm64</code> port.
</p>

<h3 id="windows">Windows</h3>

<p><!-- CL 203601 -->
  Go binaries on Windows now
  have <a href="https://docs.microsoft.com/en-us/windows/win32/memory/data-execution-prevention">DEP
  (Data Execution Prevention)</a> enabled.
</p>

<h3 id="wasm">WebAssembly</h3>

<p><!-- CL 203600 -->
  JavaScript values referenced from Go via <code>js.Value</code>
  objects can now be garbage collected.
</p>

<p><!-- CL 203600 -->
  <code>js.Value</code> values can no longer be compared using
  the <code>==</code> operator, and instead must be compared using
  their <code>Equal</code> method.
</p>

<p><!-- CL 203600 -->
  <code>js.Value</code> now
  has <code>IsUndefined</code>, <code>IsNull</code>,
  and <code>IsNaN</code> methods.
</p>

<h3 id="freebsd">FreeBSD</h3>

<p><!-- CL 199919 -->
  Go now supports the 64-bit ARM architecture on FreeBSD (the
  <code>freebsd/arm64</code> port).
</p>

<h3 id="nacl">Native Client (NaCl)</h3>

<p><!-- golang.org/issue/30439 -->
  As <a href="go1.13#ports">announced</a> in the Go 1.13 release notes,
  Go 1.14 drops support for the Native Client platform (<code>GOOS=nacl</code>).
</p>

<p><!-- CL 203758 -->
  The runtime now respects zone CPU caps
  (the <code>zone.cpu-cap</code> resource control)
  for <code>runtime.NumCPU</code> and the default value
  of <code>GOMAXPROCS</code>.
</p>

<h2 id="tools">Tools</h2>

<p>
TODO
</p>

<h3 id="go-command">Go command</h3>

<h4 id="vendor">Vendoring</h4>
<!-- golang.org/issue/33848 -->

<p>
  When the main module contains a top-level <code>vendor</code> directory and
  its <code>go.mod</code> file specifies <code>go</code> <code>1.14</code> or
  higher, the <code>go</code> command now defaults to <code>-mod=vendor</code>
  for operations that accept that flag. A new value for that flag,
  <code>-mod=mod</code>, causes the <code>go</code> command to instead load
  modules from the module cache (as when no <code>vendor</code> directory is
  present).
</p>

<p>
  When <code>-mod=vendor</code> is set (explicitly or by default), the
  <code>go</code> command now verifies that the main module's
  <code>vendor/modules.txt</code> file is consistent with its
  <code>go.mod</code> file.
</p>

<p>
  <code>go</code> <code>list</code> <code>-m</code> no longer silently omits
  transitive dependencies that do not provide packages in
  the <code>vendor</code> directory. It now fails explicitly if
  <code>-mod=vendor</code> is set.
</p>

<h4 id="go-flags">Flags</h4>

<p><!-- golang.org/issue/32502, golang.org/issue/30345 -->
  The <code>go</code> <code>get</code> command no longer accepts
  the <code>-mod</code> flag. Previously, the flag's setting either
  <a href="https://golang.org/issue/30345">was ignored</a> or
  <a href="https://golang.org/issue/32502">caused the build to fail</a>.
</p>

<p><!-- golang.org/issue/33326 -->
  <code>-mod=readonly</code> is now set by default when the <code>go.mod</code>
  file is read-only and no top-level <code>vendor</code> directory is present.
</p>

<p><!-- golang.org/issue/31481 -->
  <code>-modcacherw</code> is a new flag that instructs the <code>go</code>
  command to leave newly-created directories in the module cache at their
  default permissions rather than making them read-only.
  The use of this flag makes it more likely that tests or other tools will
  accidentally add files not included in the module's verified checksum.
  However, it allows the use of <code>rm</code> <code>-rf</code>
  (instead of <code>go</code> <code>clean</code> <code>-modcache</code>)
  to remove the module cache.
</p>

<p><!-- golang.org/issue/34506 -->
  <code>-modfile=file</code> is a new flag that instructs the <code>go</code>
  command to read (and possibly write) an alternate go.mod file instead of the
  one in the module root directory. A file named "go.mod" must still be present
  in order to determine the module root directory, but it is not
  accessed. When <code>-modfile</code> is specified, an alternate go.sum file
  is also used: its path is derived from the <code>-modfile</code> flag by
  trimming the ".mod" extension and appending ".sum".
</p>

<h4 id="incompatible-versions"><code>+incompatible</code> versions</h4>
<!-- golang.org/issue/34165 -->

<p>
  If the latest version of a module contains a <code>go.mod</code> file,
  <code>go</code> <code>get</code> will no longer upgrade to an
  <a href="/cmd/go/#hdr-Module_compatibility_and_semantic_versioning">incompatible</a>
  major version of that module unless such a version is requested explicitly
  or is already required.
  <code>go</code> <code>list</code> also omits incompatible major versions
  for such a module when fetching directly from version control, but may
  include them if reported by a proxy.
</p>


<h4 id="go.mod"><code>go.mod</code> file maintenance</h4>
<!-- golang.org/issue/34822 -->

<p>
  <code>go</code> commands other than
  <code>go</code> <code>mod</code> <code>tidy</code> no longer
  remove a <code>require</code> directive that specifies a version of an indirect dependency
  that is already implied by other (transitive) dependencies of the main
  module.
</p>

<p>
  <code>go</code> commands other than
  <code>go</code> <code>mod</code> <code>tidy</code> no longer
  edit the <code>go.mod</code> file if the changes are only cosmetic.
</p>

<p>
  When <code>-mod=readonly</code> is set, <code>go</code> commands will no
  longer fail due to a missing <code>go</code> directive or erroneous
  <code>// indirect</code> comment.
</p>

<h4 id="module-downloading">Module downloading</h4>

<p><!-- golang.org/issue/26092 -->
  The <code>go</code> command now supports Subversion repositories in module mode.
</p>

<p><!-- golang.org/issue/30748 -->
  The <code>go</code> command now includes snippets of plain-text error messages
  from module proxies and other HTTP servers.
  An error message will only be shown if it is valid UTF-8 and consists of only
  graphic characters and spaces.
</p>

<h2 id="runtime">Runtime</h2>

<p><!-- CL 190098 -->
  This release improves the performance of most uses
  of <code>defer</code> to incur almost zero overhead compared to
  calling the deferred function directly.
  As a result, <code>defer</code> can now be used in
  performance-critical code without overhead concerns.
</p>

<p><!-- CL 201760, CL 201762 and many others -->
  Goroutines are now asynchronously preemptible.
  As a result, loops without function calls no longer potentially
  deadlock the scheduler or significantly delay garbage collection.
  This is supported on all platforms except <code>windows/arm</code>,
  <code>darwin/arm</code>, <code>js/wasm</code>, and
  <code>plan9/*</code>.
</p>

<p>
  A consequence of the implementation of preemption is that on Unix
  systems, including Linux and macOS systems, programs built with Go
  1.14 will receive more signals than programs built with earlier
  releases.
  This means that programs that use packages
  like <a href="/pkg/syscall/"><code>syscall</code></a>
  or <a href="https://godoc.org/golang.org/x/sys/unix"><code>golang.org/x/sys/unix</code></a>
  will see more slow system calls fail with <code>EINTR</code> errors.
  Those programs will have to handle those errors in some way, most
  likely looping to try the system call again.  For more
  information about this
  see <a href="http://man7.org/linux/man-pages/man7/signal.7.html"><code>man
  7 signal</code></a> for Linux systems or similar documentation for
  other systems.
</p>

<p><!-- CL 201765, CL 195701 and many others -->
  The page allocator is more efficient and incurs significantly less
  lock contention at high values of <code>GOMAXPROCS</code>.
  This is most noticeable as lower latency and higher throughput for
  large allocations being done in parallel and at a high rate.
</p>

<p><!-- CL 171844 and many others -->
  Internal timers, used by
  <a href="/pkg/time/#After"><code>time.After</code></a>,
  <a href="/pkg/time/#Tick"><code>time.Tick</code></a>,
  <a href="/pkg/net/#Conn"><code>net.Conn.SetDeadline</code></a>,
  and friends, are more efficient, with less lock contention and fewer
  context switches.
  This is a performance improvement that should not cause any user
  visible changes.
</p>

<!-- TODO: Maybe CL 200439? -->

<h2 id="compiler">Compiler</h2>

<p><!-- CL 162237 -->
  This release adds <code>-d=checkptr</code> as a compile-time option
  for adding instrumentation to check that Go code is following
  <code>unsafe.Pointer</code> safety rules dynamically.
  This option is enabled by default with the <code>-race</code>
  or <code>-msan</code> flags, and can be disabled
  with <code>-gcflags=-all=-d=checkptr=0</code>.
  Specifically, <code>-d=checkptr</code> checks the following:
</p>

<ol>
  <li>
    When converting <code>unsafe.Pointer</code> to <code>*T</code>,
    the resulting pointer must be aligned appropriately
    for <code>T</code>.
  </li>
  <li>
    If the result of pointer arithmetic points into a Go heap object,
    one of the <code>unsafe.Pointer</code>-typed operands must point
    into the same object.
  </li>
</ol>

<p><!-- CL 204338 -->
  The compiler can now emit machine-readable logs of key optimizations
  using the <code>-json</code> flag, including inlining, escape
  analysis, bounds-check elimination, and nil-check elimination
</p>

<p><!-- CL 196959 -->
  Detailed escape analysis diagnostics (<code>-m=2</code>) now work again.
  This had been dropped from the new escape analysis implementation in
  the previous release.
</p>

<p><!-- CL 196217 -->
  All Go symbols in macOS binaries now begin with an underscore,
  following platform conventions.
</p>

<p><!-- CL 202117 -->
  This release includes experimental support for compiler-inserted
  coverage instrumentation for fuzzing.
  See <a href="https://golang.org/issue/14565">the issue</a> for more
  details.
  This API may change in future releases.
</p>

<h2 id="library">Core library</h2>

<p>
TODO
</p>

<dl id="hash/maphash"><dt><a href="/pkg/hash/maphash">hash/maphash</a></dt>
  <dd>
    <p><!-- CL 186877 -->
      TODO: <a href="https://golang.org/cl/186877">https://golang.org/cl/186877</a>: add hashing package for bytes and strings
    </p>

</dl><!-- hash/maphash -->

<dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
  <dd>
    <p><!-- CL 191976 -->
      TODO: <a href="https://golang.org/cl/191976">https://golang.org/cl/191976</a>: remove SSLv3 support
    </p>

    <p><!-- CL 191999 -->
      TODO: <a href="https://golang.org/cl/191999">https://golang.org/cl/191999</a>: remove TLS 1.3 opt-out
    </p>

    <p><!-- CL 174329 -->
      The <code>tls</code> package no longer supports NPN and now only
      supports ALPN. In previous releases it supported both. There are
      no API changes and code should function identically as before.
      Most other clients & servers have already removed NPN support in
      favor of the standardized ALPN.
    </p>

</dl><!-- crypto/tls -->

<dl id="encoding/asn1"><dt><a href="/pkg/encoding/asn1/">encoding/asn1</a></dt>
  <dd>
    <p><!-- CL 126624 -->
      TODO: <a href="https://golang.org/cl/126624">https://golang.org/cl/126624</a>: handle ASN1&#39;s string type BMPString
    </p>

</dl><!-- encoding/asn1 -->

<dl id="mime"><dt><a href="/pkg/mime/">mime</a></dt>
  <dd>
    <p><!-- CL 186927 -->
      TODO: <a href="https://golang.org/cl/186927">https://golang.org/cl/186927</a>: update type of .js and .mjs files to text/javascript
    </p>

</dl><!-- mime -->

<dl id="math"><dt><a href="/pkg/math/">math</a></dt>
  <dd>
    <p><!-- CL 127458 -->
      The new <a href="/pkg/math/#FMA"><code>FMA</code></a> function
      computes <code>x*y+z</code> in floating point with no
      intermediate rounding of the <code>x*y</code>
      computation. Several architectures implement this computation
      using dedicated hardware instructions for additional performance.
    </p>

</dl><!-- math -->

<dl id="plugin"><dt><a href="/pkg/plugin/">plugin</a></dt>
  <dd>
    <p><!-- CL 191617 -->
      The <code>plugin</code> package now supports <code>freebsd/amd64</code>.
    </p>

</dl><!-- plugin -->

<dl id="reflect">

<dt><a href="/pkg/reflect/">reflect</a></dt>
  <dd>
    <p><!-- CL 85661 -->
      <a href="/pkg/reflect#StructOf"><code>StructOf</code></a> now
      supports creating struct types with unexported fields, by
      setting the <code>PkgPath</code> field in
      a <code>StructField</code> element.
    </p>

</dl><!-- reflect -->

<dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
  <dd>
    <p><!-- CL 200081 -->
      <code>runtime.Goexit</code> can no longer be aborted by a
      recursive <code>panic</code>/<code>recover</code>.
    </p>

    <p><!-- CL 188297, CL 191785 -->
      On macOS, <code>SIGPIPE</code> is no longer forwarded to signal
      handlers installed before the Go runtime is initialized.
      This is necessary because macOS delivers <code>SIGPIPE</code>
      <a href="https://golang.org/issue/33384">to the main thread</a>
      rather than the thread writing to the closed pipe.
    </p>

</dl><!-- runtime -->

<dl id="signal"><dt><a href="/pkg/signal/">signal</a></dt>
  <dd>
    <p><!-- CL 187739 -->
      On Windows,
      the <code>CTRL_CLOSE_EVENT</code>, <code>CTRL_LOGOFF_EVENT</code>,
      and <code>CTRL_SHUTDOWN_EVENT</code> events now generate
      a <code>syscall.SIGTERM</code> signal, similar to how Control-C
      and Control-Break generate a <code>syscall.SIGINT</code> signal.
    </p>

</dl><!-- signal -->

<dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
  <dd>
    <p><!-- CL 200577 -->
      Unlocking a highly contended <code>Mutex</code> now directly
      yields the CPU to the next goroutine waiting for
      that <code>Mutex</code>. This significantly improves the
      performance of highly contended mutexes on high CPU count
      machines.
    </p>

</dl><!-- sync -->

<dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
  <dd>
    <p><!-- CL 201359 -->
       The testing package now supports cleanup functions, called after
       a test or benchmark has finished, by calling
       <a href="/pkg/testing#T.Cleanup"><code>T.Cleanup</code></a> or
       <a href="/pkg/testing#B.Cleanup"><code>B.Cleanup</code></a> respectively.
    </p>
</dl><!-- testing -->

<h3 id="minor_library_changes">Minor changes to the library</h3>

<p>
  As always, there are various minor changes and updates to the library,
  made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
  in mind.
</p>

<p>
TODO
</p>