The latest Go release, version 1.9, arrives six months after Go 1.8 and is the tenth release in the Go 1.x series. There are two changes to the language: adding support for type aliases and defining when implementations may fuse floating point operations. Most of the 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.
The release adds transparent monotonic time support, parallelizes compilation of functions within a package, better supports test helper functions, includes a new bit manipulation package, and has a new concurrent map type.
There are two changes to the language.
Go now supports type aliases to support gradual code repair while moving a type between packages. The type alias design document and an article on refactoring cover the problem in detail. In short, a type alias declaration has the form:
type T1 = T2
This declaration introduces an alias name T1—an
alternate spelling—for the type denoted by T2; that is,
both T1 and T2 denote the same type.
A smaller language change is that the
language specification
now states when implementations are allowed to fuse floating
point operations together, such as by using an architecture's "fused
multiply and add" (FMA) instruction to compute x*y + z
without rounding the intermediate result x*y.
To force the intermediate rounding, write float64(x*y) + z.
There are no new supported operating systems or processor architectures in this release.
Both GOARCH=ppc64 and GOARCH=ppc64le now
require at least POWER8 support. In previous releases,
only GOARCH=ppc64le required POWER8 and the big
endian ppc64 architecture supported older
hardware.
Go 1.9 is the last release that will run on FreeBSD 9.3, which is already unsupported by FreeBSD. Go 1.10 will require FreeBSD 10.3+.
Go 1.9 now enables PT_TLS generation for cgo binaries and thus requires OpenBSD 6.0 or newer. Go 1.9 no longer supports OpenBSD 5.9.
There are some instabilities on FreeBSD that are known but not understood. These can lead to program crashes in rare cases. See issue 15658. Any help in solving this FreeBSD-specific issue would be appreciated.
Go stopped running NetBSD builders during the Go 1.9 development cycle due to NetBSD kernel crashes, up to and including NetBSD 7.1. As Go 1.9 is being released, NetBSD 7.1.1 is being released with a fix. However, at this time we have no NetBSD builders passing our test suite. Any help investigating the various NetBSD issues would be appreciated.
The Go compiler now supports compiling a package's functions in parallel, taking
advantage of multiple cores. This is in addition to the go command's
existing support for parallel compilation of separate packages.
Parallel compilation is on by default, but it can be disabled by setting the
environment variable GO19CONCURRENTCOMPILATION to 0.
By popular request, ./... no longer matches packages
in vendor directories in tools accepting package names,
such as go test. To match vendor
directories, write ./vendor/....
The go tool will now use the path from which it
was invoked to attempt to locate the root of the Go install tree.
This means that if the entire Go installation is moved to a new
location, the go tool should continue to work as usual.
This may be overridden by setting GOROOT in the environment,
which should only be done in unusual circumstances.
Note that this does not affect the result of
the runtime.GOROOT function, which
will continue to report the original installation location;
this may be fixed in later releases.
Complex division is now C99-compatible. This has always been the case in gccgo and is now fixed in the gc toolchain.
The linker will now generate DWARF information for cgo executables on Windows.
The compiler now includes lexical scopes in the generated DWARF if the
-N -l flags are provided, allowing
debuggers to hide variables that are not in scope. The .debug_info
section is now DWARF version 4.
The values of GOARM and GO386 now affect a
compiled package's build ID, as used by the go tool's
dependency caching.
The four-operand ARM MULA instruction is now assembled correctly,
with the addend register as the third argument and the result
register as the fourth and final argument.
In previous releases, the two meanings were reversed.
The three-operand form, in which the fourth argument is implicitly
the same as the third, is unaffected.
Code using four-operand MULA instructions
will need to be updated, but we believe this form is very rarely used.
MULAWT and MULAWB were already
using the correct order in all forms and are unchanged.
The assembler now supports ADDSUBPS/PD, completing the
two missing x86 SSE3 instructions.
Long lists of arguments are now truncated. This improves the readability
of go doc on some generated code.
Viewing documentation on struct fields is now supported.
For example, go doc http.Client.Jar.
The new go env -json flag
enables JSON output, instead of the default OS-specific output
format.
The go test
command accepts a new -list flag, which takes a regular
expression as an argument and prints to stdout the name of any
tests, benchmarks, or examples that match it, without running them.
Profiles produced by the runtime/pprof package now
include symbol information, so they can be viewed
in go tool pprof
without the binary that produced the profile.
The go tool pprof command now
uses the HTTP proxy information defined in the environment, using
http.ProxyFromEnvironment.
The vet command
has been better integrated into the
go tool,
so go vet now supports all standard build
flags while vet's own flags are now available
from go vet as well as
from go tool vet.
Due to the alignment of Go's semiannual release schedule with GCC's annual release schedule, GCC release 7 contains the Go 1.8.3 version of gccgo. We expect that the next release, GCC 8, will contain the Go 1.10 version of gccgo.
Users of
runtime.Callers
should avoid directly inspecting the resulting PC slice and instead use
runtime.CallersFrames
to get a complete view of the call stack, or
runtime.Caller
to get information about a single caller.
This is because an individual element of the PC slice cannot account
for inlined frames or other nuances of the call stack.
Specifically, code that directly iterates over the PC slice and uses
functions such as
runtime.FuncForPC
to resolve each PC individually will miss inlined frames.
To get a complete view of the stack, such code should instead use
CallersFrames.
Likewise, code should not assume that the length returned by
Callers is any indication of the call depth.
It should instead count the number of frames returned by
CallersFrames.
Code that queries a single caller at a specific depth should use
Caller rather than passing a slice of length 1 to
Callers.
runtime.CallersFrames
has been available since Go 1.7, so code can be updated prior to
upgrading to Go 1.9.
As always, the changes are so general and varied that precise statements about performance are difficult to make. Most programs should run a bit faster, due to speedups in the garbage collector, better generated code, and optimizations in the core library.
Library functions that used to trigger stop-the-world garbage
collection now trigger concurrent garbage collection.
Specifically, runtime.GC,
debug.SetGCPercent,
and
debug.FreeOSMemory,
now trigger concurrent garbage collection, blocking only the calling
goroutine until the garbage collection is done.
The
debug.SetGCPercent
function only triggers a garbage collection if one is immediately
necessary because of the new GOGC value.
This makes it possible to adjust GOGC on-the-fly.
Large object allocation performance is significantly improved in applications using large (>50GB) heaps containing many large objects.
The runtime.ReadMemStats
function now takes less than 100µs even for very large heaps.
The time package now transparently
tracks monotonic time in each Time
value, making computing durations between two Time values
a safe operation in the presence of wall clock adjustments.
See the package docs and
design document
for details.
Go 1.9 includes a new package,
math/bits, with optimized
implementations for manipulating bits. On most architectures,
functions in this package are additionally recognized by the
compiler and treated as intrinsics for additional performance.
The
new (*T).Helper
and (*B).Helper
methods mark the calling function as a test helper function. When
printing file and line information, that function will be skipped.
This permits writing test helper functions while still having useful
line numbers for users.
The new Map type
in the sync package
is a concurrent map with amortized-constant-time loads, stores, and
deletes. It is safe for multiple goroutines to call a Map's methods
concurrently.
The runtime/pprof package
now supports adding labels to pprof profiler records.
Labels form a key-value map that is used to distinguish calls of the
same function in different contexts when looking at profiles
with the pprof command.
The pprof package's
new Do function
runs code associated with some provided labels. Other new functions
in the package help work with labels.
As always, there are various minor changes and updates to the library, made with the Go 1 promise of compatibility in mind.
The
ZIP Writer
now sets the UTF-8 bit in
the FileHeader.Flags
when appropriate.
On Linux, Go now calls the getrandom system call
without the GRND_NONBLOCK flag; it will now block
until the kernel has sufficient randomness. On kernels predating
the getrandom system call, Go continues to read
from /dev/urandom.
On Unix systems the environment
variables SSL_CERT_FILE
and SSL_CERT_DIR can now be used to override the
system default locations for the SSL certificate file and SSL
certificate files directory, respectively.
The FreeBSD file /usr/local/etc/ssl/cert.pem is
now included in the certificate search path.
The package now supports excluded domains in name constraints.
In addition to enforcing such constraints,
CreateCertificate
will create certificates with excluded name constraints
if the provided template certificate has the new
field
ExcludedDNSDomains
populated.
If any SAN extension, including with no DNS names, is present
in the certificate, then the Common Name from
Subject is ignored.
In previous releases, the code tested only whether DNS-name SANs were
present in a certificate.
The package will now use a cached Stmt if
available in Tx.Stmt.
This prevents statements from being re-prepared each time
Tx.Stmt is called.
The package now allows drivers to implement their own argument checkers by implementing
driver.NamedValueChecker.
This also allows drivers to support OUTPUT and INOUT parameter types.
Out should be used to return output parameters
when supported by the driver.
Rows.Scan can now scan user-defined string types.
Previously the package supported scanning into numeric types like type Int int64. It now also supports
scanning into string types like type String string.
The new DB.Conn method returns the new
Conn type representing an
exclusive connection to the database from the connection pool. All queries run on
a Conn will use the same underlying
connection until Conn.Close is called
to return the connection to the connection pool.
The new
NullBytes
and
NullRawValue
represent the ASN.1 NULL type.
The new Encoding.WithPadding method adds support for custom padding characters and disabling padding.
The new field
Reader.ReuseRecord
controls whether calls to
Read
may return a slice sharing the backing array of the previous
call's returned slice for improved performance.
The sharp flag ('#') is now supported when printing
floating point and complex numbers. It will always print a
decimal point
for %e, %E, %f, %F, %g
and %G; it will not remove trailing zeros
for %g and %G.
The package now includes 128-bit FNV-1 and FNV-1a hash support with
New128 and
New128a, respectively.
The package now reports an error if a predefined escaper (one of "html", "urlquery" and "js") is found in a pipeline and does not match what the auto-escaper would have decided on its own. This avoids certain security or correctness issues. Now use of one of these escapers is always either a no-op or an error. (The no-op case eases migration from text/template.)
The Rectangle.Intersect
method now returns a zero Rectangle when called on
adjacent but non-overlapping rectangles, as documented. In
earlier releases it would incorrectly return an empty but
non-zero Rectangle.
The YCbCr to RGBA conversion formula has been tweaked to ensure that rounding adjustments span the complete [0, 0xffff] RGBA range.
The new Encoder.BufferPool
field allows specifying an EncoderBufferPool,
that will be used by the encoder to get temporary EncoderBuffer
buffers when encoding a PNG image.
The use of a BufferPool reduces the number of
memory allocations performed while encoding multiple images.
The package now supports the decoding of transparent 8-bit grayscale ("Gray8") images.
The new
IsInt64
and
IsUint64
methods report whether an Int
may be represented as an int64 or uint64
value.
The new
FileHeader.Size
field describes the size of a file in a multipart message.
The new
Resolver.StrictErrors
provides control over how Go's built-in DNS resolver handles
temporary errors during queries composed of multiple sub-queries,
such as an A+AAAA address lookup.
The new
Resolver.Dial
allows a Resolver to use a custom dial function.
JoinHostPort now only places an address in square brackets if the host contains a colon.
In previous releases it would also wrap addresses in square brackets if they contained a percent ('%') sign.
The new methods
TCPConn.SyscallConn,
IPConn.SyscallConn,
UDPConn.SyscallConn,
and
UnixConn.SyscallConn
provide access to the connections' underlying file descriptors.
It is now safe to call Dial with the address obtained from
(*TCPListener).String() after creating the listener with
Listen("tcp", ":0").
Previously it failed on some machines with half-configured IPv6 stacks.
The Cookie.String method, used for
Cookie and Set-Cookie headers, now encloses values in double quotes
if the value contains either a space or a comma.
Server changes:
ServeMux now ignores ports in the host
header when matching handlers. The host is matched unmodified for CONNECT requests.
Server.ServeTLS method wraps
Server.Serve with added TLS support.
Server.WriteTimeout
now applies to HTTP/2 connections and is enforced per-stream.
StripPrefix
now calls its provided handler with a modified clone of the original *http.Request.
Any code storing per-request state in maps keyed by *http.Request should
use
Request.Context,
Request.WithContext,
and
context.WithValue instead.
LocalAddrContextKey now contains
the connection's actual network address instead of the interface address used by the listener.
Client & Transport changes:
Transport
now supports making requests via SOCKS5 proxy when the URL returned by
Transport.Proxy
has the scheme socks5.
The new
ProcessEnv
function returns FastCGI environment variables associated with an HTTP request
for which there are no appropriate
http.Request
fields, such as REMOTE_USER.
The new
Server.Client
method returns an HTTP client configured for making requests to the test server.
The new
Server.Certificate
method returns the test server's TLS certificate, if any.
The ReverseProxy
now proxies all HTTP/2 response trailers, even those not declared in the initial response
header. Such undeclared trailers are used by the gRPC protocol.
The os package now uses the internal runtime poller
for file I/O.
This reduces the number of threads required for read/write
operations on pipes, and it eliminates races when one goroutine
closes a file while another is using the file for I/O.
On Windows,
Args
is now populated without shell32.dll, improving process start-up time by 1-7 ms.
The os/exec package now prevents child processes from being created with
any duplicate environment variables.
If Cmd.Env
contains duplicate environment keys, only the last
value in the slice for each duplicate key is used.
Lookup and
LookupId now
work on Unix systems when CGO_ENABLED=0 by reading
the /etc/passwd file.
LookupGroup and
LookupGroupId now
work on Unix systems when CGO_ENABLED=0 by reading
the /etc/group file.
The new
MakeMapWithSize
function creates a map with a capacity hint.
Tracebacks generated by the runtime and recorded in profiles are
now accurate in the presence of inlining.
To retrieve tracebacks programmatically, applications should use
runtime.CallersFrames
rather than directly iterating over the results of
runtime.Callers.
On Windows, Go no longer forces the system timer to run at high resolution when the program is idle. This should reduce the impact of Go programs on battery life.
On FreeBSD, GOMAXPROCS and
runtime.NumCPU
are now based on the process' CPU mask, rather than the total
number of CPUs.
The runtime has preliminary support for Android O.
Calling
SetGCPercent
with a negative value no longer runs an immediate garbage collection.
The execution trace now displays mark assist events, which indicate when an application goroutine is forced to assist garbage collection because it is allocating too quickly.
"Sweep" events now encompass the entire process of finding free space for an allocation, rather than recording each individual span that is swept. This reduces allocation latency when tracing allocation-heavy programs. The sweep event shows how many bytes were swept and how many were reclaimed.
The new field
Credential.NoSetGroups
controls whether Unix systems make a setgroups system call
to set supplementary groups when starting a new process.
The new field
SysProcAttr.AmbientCaps
allows setting ambient capabilities on Linux 4.3+ when creating
a new process.
On 64-bit x86 Linux, process creation latency has been optimized with
use of CLONE_VFORK and CLONE_VM.
The new
Conn
interface describes some types in the
net
package that can provide access to their underlying file descriptor
using the new
RawConn
interface.
The package now chooses values in the full range when
generating int64 and uint64 random
numbers; in earlier releases generated values were always
limited to the [-262, 262) range.
In previous releases, using a nil
Config.Rand
value caused a fixed deterministic random number generator to be used.
It now uses a random number generator seeded with the current time.
For the old behavior, set Config.Rand to rand.New(rand.NewSource(0)).
The handling of empty blocks, which was broken by a Go 1.8 change that made the result dependent on the order of templates, has been fixed, restoring the old Go 1.7 behavior.
The new methods
Duration.Round
and
Duration.Truncate
handle rounding and truncating durations to multiples of a given duration.
Retrieving the time and sleeping now work correctly under Wine.
If a Time value has a monotonic clock reading, its
string representation (as returned by String) now includes a
final field "m=±value", where value is the
monotonic clock reading formatted as a decimal number of seconds.
The included tzdata timezone database has been
updated to version 2017b. As always, it is only used if the
system does not already have the database available.