Go 1.12 is not yet released. These are work-in-progress release notes. Go 1.12 is expected to be released in February 2019.
The latest Go release, version 1.12, arrives six months after Go 1.11. Most of its changes are in the implementation of the toolchain, runtime, and libraries. As always, the release maintains the Go 1 promise of compatibility. We expect almost all Go programs to continue to compile and run as before.
There is a known issue in the garbage collector that can cause rare crashes. It is being investigated. Please report any issues you encounter.
There are no changes to the language specification.
The race detector is now supported on linux/arm64.
Go 1.12 is the last release that is supported on FreeBSD 10.x, which has already reached end-of-life. Go 1.13 will require FreeBSD 11.2+ or FreeBSD 12.0+.
cgo is now supported on linux/ppc64.
hurd is now a recognized value for GOOS, reserved
for the GNU/Hurd system for use with gccgo.
Go's new windows/arm port supports running Go on Windows 10
IoT Core on 32-bit ARM chips such as the Raspberry Pi 3.
Go now supports AIX 7.2 and later on POWER8 architectures (aix/ppc64). External linking, cgo, pprof and the race detector aren't yet supported.
Go 1.12 is the last release that will run on macOS 10.10 Yosemite. Go 1.13 will require macOS 10.11 El Capitan or later.
libSystem is now used when making syscalls on Darwin,
ensuring forward-compatibility with future versions of macOS and iOS.
The switch to libSystem triggered additional App Store
checks for private API usage. Since it is considered private,
syscall.Getdirentries now always fails with
ENOSYS on iOS.
go tool vet no longer supported
The go vet command has been rewritten to serve as the
base for a range of different source code analysis tools. See
the golang.org/x/tools/go/analysis
package for details. A side-effect is that go tool vet
is no longer supported. External tools that use go tool
vet must be changed to use go
vet. Using go vet instead of go tool
vet should work with all supported versions of Go.
As part of this change, the experimental -shadow option
is no longer available with go vet. Checking for
variable shadowing may now be done using
go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
go vet -vettool=$(which shadow)
The build cache is now
required as a step toward eliminating
$GOPATH/pkg. Setting the environment variable
GOCACHE=off will cause go commands that write to the
cache to fail.
Go 1.12 is the last release that will support binary-only packages.
When GO111MODULE is set to on, the go
command now supports module-aware operations outside of a module directory,
provided that those operations do not need to resolve import paths relative to
the current directory or explicitly edit the go.mod file.
Commands such as go get,
go list, and
go mod download behave as if in a
module with initially-empty requirements.
In this mode, go env GOMOD reports
the system's null device (/dev/null or NUL).
go commands that download and extract modules are now safe to
invoke concurrently.
The module cache (GOPATH/pkg/mod) must reside in a filesystem that
supports file locking.
The go directive in a go.mod file now indicates the
version of the language used by the files within that module, and
go mod tidy sets it to the
current release (go 1.12) if no existing
version is present.
If the go directive for a module specifies a
version newer than the toolchain in use, the go command
will attempt to build the packages regardless, and will note the mismatch only if
that build fails.
When an import cannot be resolved using the active modules,
the go command will now try to use the modules mentioned in the
main module's replace directives before consulting the module
cache and the usual network sources.
If a matching replacement is found but the replace directive does
not specify a version, the go command uses a pseudo-version
derived from the zero time.Time (such
as v0.0.0-00010101000000-000000000000).
The compiler's live variable analysis has improved. This may mean that
finalizers will be executed sooner in this release than in previous
releases. If that is a problem, consider the appropriate addition of a
runtime.KeepAlive call.
More functions are now eligible for inlining by default, including
functions that do nothing but call another function.
This extra inlining makes it additionally important to use
runtime.CallersFrames
instead of iterating over the result of
runtime.Callers directly.
// Old code which no longer works correctly (it will miss inlined call frames).
var pcs [10]uintptr
n := runtime.Callers(1, pcs[:])
for _, pc := range pcs[:n] {
f := runtime.FuncForPC(pc)
if f != nil {
fmt.Println(f.Name())
}
}
// New code which will work correctly.
var pcs [10]uintptr
n := runtime.Callers(1, pcs[:])
frames := runtime.CallersFrames(pcs[:n])
for {
frame, more := frames.Next()
fmt.Println(frame.Function)
if !more {
break
}
}
The compiler now accepts a -lang flag to set the Go language
version to use. For example, -lang=go1.8 causes the compiler to
emit an error if the program uses type aliases, which were added in Go 1.9.
Language changes made before Go 1.12 are not consistently enforced.
The compiler toolchain now uses different conventions to call Go functions and assembly functions. This should be invisible to users, except for calls that simultaneously cross between Go and assembly and cross a package boundary. If linking results in an error like "relocation target not defined for ABIInternal (but is defined for ABI0)", please refer to help section of the ABI design document.
There have been many improvements to the DWARF debug information produced by the compiler, including improvements to argument printing and variable location information.
Go programs now also maintain stack frame pointers on linux/arm64
for the benefit of profiling tools like perf. The frame pointer
maintenance has a small run-time overhead that varies but averages around 3%.
To build a toolchain that does not use frame pointers, set
GOEXPERIMENT=noframepointer when running make.bash.
The obsolete "safe" compiler mode (enabled by the -u gcflag) has been removed.
godoc and go doc
In Go 1.12, godoc no longer has a command-line interface and
is only a web server. Users should use go doc
for command-line help output instead.
go doc now supports the -all flag,
which will cause it to print all exported APIs and their documentation,
similarly to what the godoc command line used to do.
The trace tool now supports plotting mutator utilization curves, including cross-references to the execution trace. These are useful for analyzing the impact of the garbage collector on application latency and throughput.
On arm64, the platform register was renamed from
R18 to R18_PLATFORM to prevent accidental
use, as the OS could choose to reserve this register.
Go 1.12 significantly improves the performance of sweeping when a large fraction of the heap remains live. This reduces allocation latency immediately following a garbage collection.
The Go runtime now releases memory back to the operating system more aggressively, particularly in response to large allocations that can't reuse existing heap space.
On Linux, the Go runtime now releases memory back to the operating system only when the OS is under memory pressure. This is more efficient, but means a process's RSS (resident set size) won't decrease unless the OS is running out of memory.
Adding cpu.extension=off to the GODEBUG environment variable now disables the use of optional CPU instruction set extensions in the standard library and runtime. This is not yet supported on Windows.
Go 1.12 adds support in the crypto/tls package for TLS 1.3 as
specified in RFC 8446.
Programs that did not set an explicit MaxVersion in
Config will automatically negotiate
TLS 1.3 if available. All TLS 1.2 features except TLSUnique in
ConnectionState
and renegotiation are available in TLS 1.3 and provide equivalent or
better security and performance.
TLS 1.3 cipher suites are not configurable. All supported cipher suites are
safe, and if PreferServerCipherSuites is set in
Config the preference order
is based on the available hardware.
Early data (also called "0-RTT mode") is not currently supported as a client or server. Additionally, a Go 1.12 server does not support skipping unexpected early data if a client sends it. Since TLS 1.3 0-RTT mode involves clients keeping state regarding which servers support 0-RTT, a Go 1.12 server cannot be part of a load-balancing pool where some other servers do support 0-RTT. If switching a domain from a server that supported 0-RTT to a Go 1.12 server, 0-RTT would have to be disabled for at least the lifetime of the issued session tickets before the switch to ensure uninterrupted operation.
In TLS 1.3 the client is the last one to speak in the handshake, so if it causes
an error to occur on the server, it will be returned on the client by the first
Read, not by
Handshake. For
example, that will be the case if the server rejects the client certificate.
Similarly, session tickets are now post-handshake messages, so are only
received by the client upon its first
Read.
As always, there are various minor changes and updates to the library, made with the Go 1 promise of compatibility in mind.
Reader's UnreadRune and
UnreadByte methods will now return an error
if they are called after Peek.
The new function ReplaceAll returns a copy of
a byte slice with all non-overlapping instances of a value replaced by another.
A pointer to a zero-value Reader is now
functionally equivalent to NewReader(nil).
Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.
A warning will now be printed to standard error the first time
Reader.Read is blocked for more than 60 seconds waiting
to read entropy from the kernel.
On FreeBSD, Reader now uses the getrandom
system call if available, /dev/urandom otherwise.
This release removes the optimized assembly implementations. RC4 is insecure and should only be used for compatibility with legacy systems.
If a client sends an initial message that does not look like TLS, the server
will now not reply with an alert, and it will expose the underlying
net.Conn in the new field Conn of
RecordHeaderError.
Maps are now printed in key-sorted order to ease testing. The ordering rules are:
reflect.Type describing the concrete type
and then by concrete value as described in the previous rules.
When printing maps, non-reflexive key values like NaN were previously
displayed as <nil>. As of this release, the correct values are printed.
To address some outstanding issues in cmd/doc,
this package has a new Mode bit,
PreserveAST, which controls whether AST data is cleared.
The File type has a new
LineStart field,
which returns the position of the start of a given line. This is especially useful
in programs that occassionally handle non-Go files, such as assembly, but wish to use
the token.Pos mechanism to identify file positions.
The RegisterFormat function is now safe for concurrent use.
Paletted images with fewer than 16 colors now encode to smaller outputs.
The new StringWriter interface wraps the
WriteString function.
The time zone database in $GOROOT/lib/time/zoneinfo.zip has been updated to version 2018g. Note that this ZIP file is only used if a time zone database is not provided by the operating system.
New extended precision operations Add, Sub, Mul, and Div are available in int, int32, and int64 versions.
The
Dialer.DualStack setting is now ignored and deprecated;
RFC 6555 Fast Fallback ("Happy Eyeballs") is now enabled by default. To disable, set
Dialer.FallbackDelay to a negative value.
Similarly, TCP keep-alives are now enabled by default if
Dialer.KeepAlive is zero.
To disable, set it to a negative value.
On Linux, the splice system call is now used when copying from a
UnixConn to a
TCPConn.
The HTTP server now rejects misdirected HTTP requests to HTTPS servers with a plaintext "400 Bad Request" response.
The new Client.CloseIdleConnections
method calls the Client's underlying Transport's CloseIdleConnections
if it has one.
The Transport no longer rejects HTTP responses which declare
HTTP Trailers but don't use chunked encoding. Instead, the declared trailers are now just ignored.
The Transport no longer handles MAX_CONCURRENT_STREAMS values
advertised from HTTP/2 servers as strictly as it did during Go 1.10 and Go 1.11. The default behavior is now back
to how it was in Go 1.9: each connection to a server can have up to MAX_CONCURRENT_STREAMS requests
active and then new TCP connections are created as needed. In Go 1.10 and Go 1.11 the http2 package
would block and wait for requests to finish instead of creating new connections.
To get the stricter behavior back, import the
golang.org/x/net/http2 package
directly and set
Transport.StrictMaxConcurrentStreams to
true.
The ReverseProxy now automatically
proxies WebSocket requests.
The new ProcessState.ExitCode method
returns the process's exit code.
ModeCharDevice has been added to the ModeType bitmask, allowing for
ModeDevice | ModeCharDevice to be recovered when masking a
FileMode with ModeType.
The new function UserHomeDir returns the
current user's home directory.
RemoveAll now supports paths longer than 4096 characters
on most Unix systems.
File.Sync now uses F_FULLFSYNC on macOS
to properly flush content to permanent storage. Note that this might have a negative performance impact.
The IsAbs function now returns true when passed
a reserved filename on Windows such as NUL.
List of reserved names.
A new MapIter type is
an iterator for ranging over a map. This type is exposed through the
Value type's new
MapRange method.
This follows the same iteration semantics as a range statment, with Next
to advance the iterator, and Key/Value to access each entry.
Copy is no longer necessary
to avoid lock contention, so it has been given a partial deprecation comment.
Copy
may still be appropriate if the reason for its use is to make two copies with
different Longest settings.
A new BuildInfo type
exposes the build information read from the running binary, available only in
binaries built with module support. This includes the main package path, main
module information, and the module dependencies. This type is given though the
ReadBuildInfo function
on BuildInfo.
The new function ReplaceAll returns a copy of
a string with all non-overlapping instances of a value replaced by another.
A pointer to a zero-value Reader is now
functionally equivalent to NewReader(nil).
Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.
The new Builder.Cap method returns the capacity of the builder's underlying byte slice.
The character mapping functions Map,
Title,
ToLower,
ToLowerSpecial,
ToTitle,
ToTitleSpecial,
ToUpper, and
ToUpperSpecial
now always guarantee to return valid UTF-8. In earlier releases, if the input was invalid UTF-8 but no character replacements
needed to be applied, these routines incorrectly returned the invalid UTF-8 unmodified.
64-bit inodes are now supported on FreeBSD 12. Some types have been adjusted accordingly.
The Unix socket
(AF_UNIX)
address family is now supported for compatible versions of Windows.
The new function Syscall18
has been introduced for Windows, allowing for calls with up to 18 arguments.
The Callback type and NewCallback function have been renamed;
they are now called
Func and
FuncOf, respectively.
This is a breaking change, but WebAssembly support is still experimental
and not yet subject to the
Go 1 compatibility promise. Any code using the
old names will need to be updated.
If a type implements the new
Wrapper
interface,
ValueOf
will use it to return the JavaScript value for that type.
The meaning of the zero
Value
has changed. It now represents the JavaScript undefined value
instead of the number zero.
This is a breaking change, but WebAssembly support is still experimental
and not yet subject to the
Go 1 compatibility promise. Any code relying on
the zero Value
to mean the number zero will need to be updated.
The new
Value.Truthy
method reports the
JavaScript "truthiness"
of a given value.
The -benchtime flag now supports setting an explicit iteration count instead of a time when the value ends with an "x". For example, -benchtime=100x runs the benchmark 100 times.
When executing a template, long context values are no longer truncated in errors.
executing "tmpl" at <.very.deep.context.v...>: map has no entry for key "notpresent"
is now
executing "tmpl" at <.very.deep.context.value.notpresent>: map has no entry for key "notpresent"
It is invalid to convert a nil unsafe.Pointer to uintptr and back with arithmetic.
(This was already invalid, but will now cause the compiler to misbehave.)