Go 1.11 Release Notes

Introduction to Go 1.11

The latest Go release, version 1.11, arrives six months after Go 1.10. 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.

Changes to the language

There are no changes to the language specification.

Ports

As announced in the Go 1.10 release notes, Go 1.11 now requires OpenBSD 6.2 or later, macOS 10.10 Yosemite or later, or Windows 7 or later; support for previous versions of these operating systems has been removed.

Go 1.11 supports the upcoming OpenBSD 6.4 release. Due to changes in the OpenBSD kernel, older versions of Go will not work on OpenBSD 6.4.

There are known issues with NetBSD on i386 hardware.

The race detector is now supported on linux/ppc64le and, to a lesser extent, on netbsd/amd64. The NetBSD race detector support has known issues.

The memory sanitizer (-msan) is now supported on linux/arm64.

The build modes c-shared and c-archive are now supported on freebsd/amd64.

On 64-bit MIPS systems, the new environment variable settings GOMIPS64=hardfloat (the default) and GOMIPS64=softfloat select whether to use hardware instructions or software emulation for floating-point computations. For 32-bit systems, the environment variable is still GOMIPS, as added in Go 1.10.

On soft-float ARM systems (GOARM=5), Go now uses a more efficient software floating point interface. This is transparent to Go code, but ARM assembly that uses floating-point instructions not guarded on GOARM will break and must be ported to the new interface.

Go 1.11 on ARMv7 no longer requires a Linux kernel configured with KUSER_HELPERS. This setting is enabled in default kernel configurations, but is sometimes disabled in stripped-down configurations.

WebAssembly

Go 1.11 adds an experimental port to WebAssembly (js/wasm).

Go programs currently compile to one WebAssembly module that includes the Go runtime for goroutine scheduling, garbage collection, maps, etc. As a result, the resulting size is at minimum around 2 MB, or 500 KB compressed. Go programs can call into JavaScript using the new experimental syscall/js package. Binary size and interop with other languages has not yet been a priority but may be addressed in future releases.

As a result of the addition of the new GOOS value “js” and GOARCH value “wasm”, Go files named *_js.go or *_wasm.go will now be ignored by Go tools except when those GOOS/GOARCH values are being used. If you have existing filenames matching those patterns, you will need to rename them.

More information can be found on the WebAssembly wiki page.

RISC-V GOARCH values reserved

The main Go compiler does not yet support the RISC-V architecture but we’ve reserved the GOARCH values “riscv” and “riscv64”, as used by Gccgo, which does support RISC-V. This means that Go files named *_riscv.go will now also be ignored by Go tools except when those GOOS/GOARCH values are being used.

Tools

Modules, package versioning, and dependency management

Go 1.11 adds preliminary support for a new concept called “modules,” an alternative to GOPATH with integrated support for versioning and package distribution. Using modules, developers are no longer confined to working inside GOPATH, version dependency information is explicit yet lightweight, and builds are more reliable and reproducible.

Module support is considered experimental. Details are likely to change in response to feedback from Go 1.11 users, and we have more tools planned. Although the details of module support may change, projects that convert to modules using Go 1.11 will continue to work with Go 1.12 and later. If you encounter bugs using modules, please file issues so we can fix them. For more information, see the go command documentation.

Import path restriction

Because Go module support assigns special meaning to the @ symbol in command line operations, the go command now disallows the use of import paths containing @ symbols. Such import paths were never allowed by go get, so this restriction can only affect users building custom GOPATH trees by other means.

Package loading

The new package golang.org/x/tools/go/packages provides a simple API for locating and loading packages of Go source code. Although not yet part of the standard library, for many tasks it effectively replaces the go/build package, whose API is unable to fully support modules. Because it runs an external query command such as go list to obtain information about Go packages, it enables the construction of analysis tools that work equally well with alternative build systems such as Bazel and Buck.

Build cache requirement

Go 1.11 will be the last release to support setting the environment variable GOCACHE=off to disable the build cache, introduced in Go 1.10. Starting in Go 1.12, the build cache will be required, as a step toward eliminating $GOPATH/pkg. The module and package loading support described above already require that the build cache be enabled. If you have disabled the build cache to avoid problems you encountered, please file an issue to let us know about them.

Compiler toolchain

More functions are now eligible for inlining by default, including functions that call panic.

The compiler toolchain now supports column information in line directives.

A new package export data format has been introduced. This should be transparent to end users, except for speeding up build times for large Go projects. If it does cause problems, it can be turned off again by passing -gcflags=all=-iexport=false to the go tool when building a binary.

The compiler now rejects unused variables declared in a type switch guard, such as x in the following example:

func f(v interface{}) {
    switch x := v.(type) {
    }
}

This was already rejected by both gccgo and go/types.

Assembler

The assembler for amd64 now accepts AVX512 instructions.

Debugging

The compiler now produces significantly more accurate debug information for optimized binaries, including variable location information, line numbers, and breakpoint locations. This should make it possible to debug binaries compiled without -N -l. There are still limitations to the quality of the debug information, some of which are fundamental, and some of which will continue to improve with future releases.

DWARF sections are now compressed by default because of the expanded and more accurate debug information produced by the compiler. This is transparent to most ELF tools (such as debuggers on Linux and *BSD) and is supported by the Delve debugger on all platforms, but has limited support in the native tools on macOS and Windows. To disable DWARF compression, pass -ldflags=-compressdwarf=false to the go tool when building a binary.

Go 1.11 adds experimental support for calling Go functions from within a debugger. This is useful, for example, to call String methods when paused at a breakpoint. This is currently only supported by Delve (version 1.1.0 and up).

Test

Since Go 1.10, the go test command runs go vet on the package being tested, to identify problems before running the test. Since vet typechecks the code with go/types before running, tests that do not typecheck will now fail. In particular, tests that contain an unused variable inside a closure compiled with Go 1.10, because the Go compiler incorrectly accepted them (Issue #3059), but will now fail, since go/types correctly reports an “unused variable” error in this case.

The -memprofile flag to go test now defaults to the “allocs” profile, which records the total bytes allocated since the test began (including garbage-collected bytes).

Vet

The go vet command now reports a fatal error when the package under analysis does not typecheck. Previously, a type checking error simply caused a warning to be printed, and vet to exit with status 1.

Additionally, go vet has become more robust when format-checking printf wrappers. Vet now detects the mistake in this example:

func wrapper(s string, args ...interface{}) {
    fmt.Printf(s, args...)
}

func main() {
    wrapper("%s", 42)
}

Trace

With the new runtime/trace package’s user annotation API, users can record application-level information in execution traces and create groups of related goroutines. The go tool trace command visualizes this information in the trace view and the new user task/region analysis page.

Cgo

Since Go 1.10, cgo has translated some C pointer types to the Go type uintptr. These types include the CFTypeRef hierarchy in Darwin’s CoreFoundation framework and the jobject hierarchy in Java’s JNI interface. In Go 1.11, several improvements have been made to the code that detects these types. Code that uses these types may need some updating. See the Go 1.10 release notes for details.

Go command

The environment variable GOFLAGS may now be used to set default flags for the go command. This is useful in certain situations. Linking can be noticeably slower on underpowered systems due to DWARF, and users may want to set -ldflags=-w by default. For modules, some users and CI systems will want vendoring always, so they should set -mod=vendor by default. For more information, see the go command documentation.

Godoc

Go 1.11 will be the last release to support godoc’s command-line interface. In future releases, godoc will only be a web server. Users should use go doc for command-line help output instead.

The godoc web server now shows which version of Go introduced new API features. The initial Go version of types, funcs, and methods are shown right-aligned. For example, see UserCacheDir, with “1.11” on the right side. For struct fields, inline comments are added when the struct field was added in a Go version other than when the type itself was introduced. For a struct field example, see ClientTrace.Got1xxResponse.

Gofmt

One minor detail of the default formatting of Go source code has changed. When formatting expression lists with inline comments, the comments were aligned according to a heuristic. However, in some cases the alignment would be split up too easily, or introduce too much whitespace. The heuristic has been changed to behave better for human-written code.

Note that these kinds of minor updates to gofmt are expected from time to time. In general, systems that need consistent formatting of Go source code should use a specific version of the gofmt binary. See the go/format package documentation for more information.

Run

The go run command now allows a single import path, a directory name or a pattern matching a single package. This allows go run pkg or go run dir, most importantly go run .

Runtime

The runtime now uses a sparse heap layout so there is no longer a limit to the size of the Go heap (previously, the limit was 512GiB). This also fixes rare “address space conflict” failures in mixed Go/C binaries or binaries compiled with -race.

On macOS and iOS, the runtime now uses libSystem.dylib instead of calling the kernel directly. This should make Go binaries more compatible with future versions of macOS and iOS. The syscall package still makes direct system calls; fixing this is planned for a future release.

Performance

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 better generated code and optimizations in the core library.

There were multiple performance changes to the math/big package as well as many changes across the tree specific to GOARCH=arm64.

Compiler toolchain

The compiler now optimizes map clearing operations of the form:

for k := range m {
    delete(m, k)
}

The compiler now optimizes slice extension of the form append(s, make([]T, n)...).

The compiler now performs significantly more aggressive bounds-check and branch elimination. Notably, it now recognizes transitive relations, so if i<j and j<len(s), it can use these facts to eliminate the bounds check for s[i]. It also understands simple arithmetic such as s[i-10] and can recognize more inductive cases in loops. Furthermore, the compiler now uses bounds information to more aggressively optimize shift operations.

Standard library

All of the changes to the standard library are minor.

Minor changes to the library

As always, there are various minor changes and updates to the library, made with the Go 1 promise of compatibility in mind.

crypto

Certain crypto operations, including ecdsa.Sign, rsa.EncryptPKCS1v15 and rsa.GenerateKey, now randomly read an extra byte of randomness to ensure tests don’t rely on internal behavior.

crypto/cipher

The new function NewGCMWithTagSize implements Galois Counter Mode with non-standard tag lengths for compatibility with existing cryptosystems.

crypto/rsa

PublicKey now implements a Size method that returns the modulus size in bytes.

crypto/tls

ConnectionState’s new ExportKeyingMaterial method allows exporting keying material bound to the connection according to RFC 5705.

crypto/x509

The deprecated, legacy behavior of treating the CommonName field as a hostname when no Subject Alternative Names are present is now disabled when the CN is not a valid hostname. The CommonName can be completely ignored by adding the experimental value x509ignoreCN=1 to the GODEBUG environment variable. When the CN is ignored, certificates without SANs validate under chains with name constraints instead of returning NameConstraintsWithoutSANs.

Extended key usage restrictions are again checked only if they appear in the KeyUsages field of VerifyOptions, instead of always being checked. This matches the behavior of Go 1.9 and earlier.

The value returned by SystemCertPool is now cached and might not reflect system changes between invocations.

debug/elf

More ELFOSABI and EM constants have been added.

encoding/asn1

Marshal and Unmarshal now support “private” class annotations for fields.

encoding/base32

The decoder now consistently returns io.ErrUnexpectedEOF for an incomplete chunk. Previously it would return io.EOF in some cases.

encoding/csv

The Reader now rejects attempts to set the Comma field to a double-quote character, as double-quote characters already have a special meaning in CSV.

html/template

The package has changed its behavior when a typed interface value is passed to an implicit escaper function. Previously such a value was written out as (an escaped form) of <nil>. Now such values are ignored, just as an untyped nil value is (and always has been) ignored.

image/gif

Non-looping animated GIFs are now supported. They are denoted by having a LoopCount of -1.

io/ioutil

The TempFile function now supports specifying where the random characters in the filename are placed. If the prefix argument includes a “*”, the random string replaces the “*”. For example, a prefix argument of “myname.*.bat” will result in a random filename such as “myname.123456.bat”. If no “*” is included the old behavior is retained, and the random digits are appended to the end.

math/big

ModInverse now returns nil when g and n are not relatively prime. The result was previously undefined.

mime/multipart

The handling of form-data with missing/empty file names has been restored to the behavior in Go 1.9: in the Form for the form-data part the value is available in the Value field rather than the File field. In Go releases 1.10 through 1.10.3 a form-data part with a missing/empty file name and a non-empty “Content-Type” field was stored in the File field. This change was a mistake in 1.10 and has been reverted to the 1.9 behavior.

mime/quotedprintable

To support invalid input found in the wild, the package now permits non-ASCII bytes but does not validate their encoding.

net

The new ListenConfig type and the new Dialer.Control field permit setting socket options before accepting and creating connections, respectively.

The syscall.RawConn Read and Write methods now work correctly on Windows.

The net package now automatically uses the splice system call on Linux when copying data between TCP connections in TCPConn.ReadFrom, as called by io.Copy. The result is faster, more efficient TCP proxying.

The TCPConn.File, UDPConn.File, UnixConn.File, and IPConn.File methods no longer put the returned *os.File into blocking mode.

net/http

The Transport type has a new MaxConnsPerHost option that permits limiting the maximum number of connections per host.

The Cookie type has a new SameSite field (of new type also named SameSite) to represent the new cookie attribute recently supported by most browsers. The net/http’s Transport does not use the SameSite attribute itself, but the package supports parsing and serializing the attribute for browsers to use.

It is no longer allowed to reuse a Server after a call to Shutdown or Close. It was never officially supported in the past and had often surprising behavior. Now, all future calls to the server’s Serve methods will return errors after a shutdown or close.

The constant StatusMisdirectedRequest is now defined for HTTP status code 421.

The HTTP server will no longer cancel contexts or send on CloseNotifier channels upon receiving pipelined HTTP/1.1 requests. Browsers do not use HTTP pipelining, but some clients (such as Debian’s apt) may be configured to do so.

ProxyFromEnvironment, which is used by the DefaultTransport, now supports CIDR notation and ports in the NO_PROXY environment variable.

net/http/httputil

The ReverseProxy has a new ErrorHandler option to permit changing how errors are handled.

The ReverseProxy now also passes “TE: trailers” request headers through to the backend, as required by the gRPC protocol.

os

The new UserCacheDir function returns the default root directory to use for user-specific cached data.

The new ModeIrregular is a FileMode bit to represent that a file is not a regular file, but nothing else is known about it, or that it’s not a socket, device, named pipe, symlink, or other file type for which Go has a defined mode bit.

Symlink now works for unprivileged users on Windows 10 on machines with Developer Mode enabled.

When a non-blocking descriptor is passed to NewFile, the resulting *File will be kept in non-blocking mode. This means that I/O for that *File will use the runtime poller rather than a separate thread, and that the SetDeadline methods will work.

os/signal

The new Ignored function reports whether a signal is currently ignored.

os/user

The os/user package can now be built in pure Go mode using the build tag “osusergo”, independent of the use of the environment variable CGO_ENABLED=0. Previously the only way to use the package’s pure Go implementation was to disable cgo support across the entire program.

runtime

Setting the GODEBUG=tracebackancestors=N environment variable now extends tracebacks with the stacks at which goroutines were created, where N limits the number of ancestor goroutines to report.

runtime/pprof

This release adds a new “allocs” profile type that profiles total number of bytes allocated since the program began (including garbage-collected bytes). This is identical to the existing “heap” profile viewed in -alloc_space mode. Now go test -memprofile=... reports an “allocs” profile instead of “heap” profile.

sync

The mutex profile now includes reader/writer contention for RWMutex. Writer/writer contention was already included in the mutex profile.

syscall

On Windows, several fields were changed from uintptr to a new Pointer type to avoid problems with Go’s garbage collector. The same change was made to the golang.org/x/sys/windows package. For any code affected, users should first migrate away from the syscall package to the golang.org/x/sys/windows package, and then change to using the Pointer, while obeying the unsafe.Pointer conversion rules.

On Linux, the flags parameter to Faccessat is now implemented just as in glibc. In earlier Go releases the flags parameter was ignored.

On Linux, the flags parameter to Fchmodat is now validated. Linux’s fchmodat doesn’t support the flags parameter so we now mimic glibc’s behavior and return an error if it’s non-zero.

text/scanner

The Scanner.Scan method now returns the RawString token instead of String for raw string literals.

text/template

Modifying template variables via assignments is now permitted via the = token:

  {{ $v := "init" }}
  {{ if true }}
    {{ $v = "changed" }}
  {{ end }}
  v: {{ $v }} {{/* "changed" */}}

In previous versions untyped nil values passed to template functions were ignored. They are now passed as normal arguments.

time

Parsing of timezones denoted by sign and offset is now supported. In previous versions, numeric timezone names (such as +03) were not considered valid, and only three-letter abbreviations (such as MST) were accepted when expecting a timezone name.