## Introduction to Go 1.15

The latest Go release, version 1.15, arrives six months after Go 1.14. 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.

Go 1.15 includes substantial improvements to the linker, improves allocation for small objects at high core counts, and deprecates X.509 CommonName. `GOPROXY` now supports skipping proxies that return errors and a new embedded tzdata package has been added.

## Changes to the language

There are no changes to the language.

## Ports

### Darwin

As announced in the Go 1.14 release notes, Go 1.15 requires macOS 10.12 Sierra or later; support for previous versions has been discontinued.

As announced in the Go 1.14 release notes, Go 1.15 drops support for 32-bit binaries on macOS, iOS, iPadOS, watchOS, and tvOS (the `darwin/386` and `darwin/arm` ports). Go continues to support the 64-bit `darwin/amd64` and `darwin/arm64` ports.

### Windows

Go now generates Windows ASLR executables when `-buildmode=pie` cmd/link flag is provided. Go command uses `-buildmode=pie` by default on Windows.

The `-race` and `-msan` flags now always enable `-d=checkptr`, which checks uses of `unsafe.Pointer`. This was previously the case on all OSes except Windows.

Go-built DLLs no longer cause the process to exit when it receives a signal (such as Ctrl-C at a terminal).

### Android

When linking binaries for Android, Go 1.15 explicitly selects the `lld` linker available in recent versions of the NDK. The `lld` linker avoids crashes on some devices, and is planned to become the default NDK linker in a future NDK version.

### OpenBSD

Go 1.15 adds support for OpenBSD 6.7 on `GOARCH=arm` and `GOARCH=arm64`. Previous versions of Go already supported OpenBSD 6.7 on `GOARCH=386` and `GOARCH=amd64`.

### RISC-V

There has been progress in improving the stability and performance of the 64-bit RISC-V port on Linux (`GOOS=linux`, `GOARCH=riscv64`). It also now supports asynchronous preemption.

### 386

Go 1.15 is the last release to support x87-only floating-point hardware (`GO386=387`). Future releases will require at least SSE2 support on 386, raising Go's minimum `GOARCH=386` requirement to the Intel Pentium 4 (released in 2000) or AMD Opteron/Athlon 64 (released in 2003).

## Tools

### Go command

The `GOPROXY` environment variable now supports skipping proxies that return errors. Proxy URLs may now be separated with either commas (`,`) or pipe characters (`|`). If a proxy URL is followed by a comma, the `go` command will only try the next proxy in the list after a 404 or 410 HTTP response. If a proxy URL is followed by a pipe character, the `go` command will try the next proxy in the list after any error. Note that the default value of `GOPROXY` remains `https://proxy.golang.org,direct`, which does not fall back to `direct` in case of errors.

#### `go` `test`

Changing the `-timeout` flag now invalidates cached test results. A cached result for a test run with a long timeout will no longer count as passing when `go` `test` is re-invoked with a short one.

#### Flag parsing

Various flag parsing issues in `go` `test` and `go` `vet` have been fixed. Notably, flags specified in `GOFLAGS` are handled more consistently, and the `-outputdir` flag now interprets relative paths relative to the working directory of the `go` command (rather than the working directory of each individual test).

#### Module cache

The location of the module cache may now be set with the `GOMODCACHE` environment variable. The default value of `GOMODCACHE` is `GOPATH[0]/pkg/mod`, the location of the module cache before this change.

A workaround is now available for Windows "Access is denied" errors in `go` commands that access the module cache, caused by external programs concurrently scanning the file system (see issue #36568). The workaround is not enabled by default because it is not safe to use when Go versions lower than 1.14.2 and 1.13.10 are running concurrently with the same module cache. It can be enabled by explicitly setting the environment variable `GODEBUG=modcacheunzipinplace=1`.

### Vet

#### New warning for string(x)

The vet tool now warns about conversions of the form `string(x)` where `x` has an integer type other than `rune` or `byte`. Experience with Go has shown that many conversions of this form erroneously assume that `string(x)` evaluates to the string representation of the integer `x`. It actually evaluates to a string containing the UTF-8 encoding of the value of `x`. For example, `string(9786)` does not evaluate to the string `"9786"`; it evaluates to the string `"\xe2\x98\xba"`, or `"☺"`.

Code that is using `string(x)` correctly can be rewritten to `string(rune(x))`. Or, in some cases, calling `utf8.EncodeRune(buf, x)` with a suitable byte slice `buf` may be the right solution. Other code should most likely use `strconv.Itoa` or `fmt.Sprint`.

This new vet check is enabled by default when using `go` `test`.

We are considering prohibiting the conversion in a future release of Go. That is, the language would change to only permit `string(x)` for integer `x` when the type of `x` is `rune` or `byte`. Such a language change would not be backward compatible. We are using this vet check as a first trial step toward changing the language.

#### New warning for impossible interface conversions

The vet tool now warns about type assertions from one interface type to another interface type when the type assertion will always fail. This will happen if both interface types implement a method with the same name but with a different type signature.

There is no reason to write a type assertion that always fails, so any code that triggers this vet check should be rewritten.

This new vet check is enabled by default when using `go` `test`.

We are considering prohibiting impossible interface type assertions in a future release of Go. Such a language change would not be backward compatible. We are using this vet check as a first trial step toward changing the language.

## Runtime

If `panic` is invoked with a value whose type is derived from any of: `bool`, `complex64`, `complex128`, `float32`, `float64`, `int`, `int8`, `int16`, `int32`, `int64`, `string`, `uint`, `uint8`, `uint16`, `uint32`, `uint64`, `uintptr`, then the value will be printed, instead of just its address. Previously, this was only true for values of exactly these types.

On a Unix system, if the `kill` command or `kill` system call is used to send a `SIGSEGV`, `SIGBUS`, or `SIGFPE` signal to a Go program, and if the signal is not being handled via `os/signal.Notify`, the Go program will now reliably crash with a stack trace. In earlier releases the behavior was unpredictable.

Allocation of small objects now performs much better at high core counts, and has lower worst-case latency.

Converting a small integer value into an interface value no longer causes allocation.

Non-blocking receives on closed channels now perform as well as non-blocking receives on open channels.

## Compiler

Package `unsafe`'s safety rules allow converting an `unsafe.Pointer` into `uintptr` when calling certain functions. Previously, in some cases, the compiler allowed multiple chained conversions (for example, `syscall.Syscall(…,` `uintptr(uintptr(ptr)),` `…)`). The compiler now requires exactly one conversion. Code that used multiple conversions should be updated to satisfy the safety rules.

Go 1.15 reduces typical binary sizes by around 5% compared to Go 1.14 by eliminating certain types of GC metadata and more aggressively eliminating unused type metadata.

The toolchain now mitigates Intel CPU erratum SKX102 on `GOARCH=amd64` by aligning functions to 32 byte boundaries and padding jump instructions. While this padding increases binary sizes, this is more than made up for by the binary size improvements mentioned above.

Go 1.15 adds a `-spectre` flag to both the compiler and the assembler, to allow enabling Spectre mitigations. These should almost never be needed and are provided mainly as a “defense in depth” mechanism. See the Spectre wiki page for details.

The compiler now rejects `//go:` compiler directives that have no meaning for the declaration they are applied to with a "misplaced compiler directive" error. Such misapplied directives were broken before, but were silently ignored by the compiler.

The compiler's `-json` optimization logging now reports large (>= 128 byte) copies and includes explanations of escape analysis decisions.

## Linker

This release includes substantial improvements to the Go linker, which reduce linker resource usage (both time and memory) and improve code robustness/maintainability.

For a representative set of large Go programs, linking is 20% faster and requires 30% less memory on average, for `ELF`-based OSes (Linux, FreeBSD, NetBSD, OpenBSD, Dragonfly, and Solaris) running on `amd64` architectures, with more modest improvements for other architecture/OS combinations.

The key contributors to better linker performance are a newly redesigned object file format, and a revamping of internal phases to increase concurrency (for example, applying relocations to symbols in parallel). Object files in Go 1.15 are slightly larger than their 1.14 equivalents.

These changes are part of a multi-release project to modernize the Go linker, meaning that there will be additional linker improvements expected in future releases.

The linker now defaults to internal linking mode for `-buildmode=pie` on `linux/amd64` and `linux/arm64`, so these configurations no longer require a C linker. External linking mode (which was the default in Go 1.14 for `-buildmode=pie`) can still be requested with `-ldflags=-linkmode=external` flag.

## Objdump

The objdump tool now supports disassembling in GNU assembler syntax with the `-gnu` flag.

## Core library

### New embedded tzdata package

Go 1.15 includes a new package, `time/tzdata`, that permits embedding the timezone database into a program. Importing this package (as `import _ "time/tzdata"`) permits the program to find timezone information even if the timezone database is not available on the local system. You can also embed the timezone database by building with `-tags timetzdata`. Either approach increases the size of the program by about 800 KB.

### Cgo

Go 1.15 will translate the C type `EGLConfig` to the Go type `uintptr`. This change is similar to how Go 1.12 and newer treats `EGLDisplay`, Darwin's CoreFoundation and Java's JNI types. See the cgo documentation for more information.

In Go 1.15.3 and later, cgo will not permit Go code to allocate an undefined struct type (a C struct defined as just `struct S;` or similar) on the stack or heap. Go code will only be permitted to use pointers to those types. Allocating an instance of such a struct and passing a pointer, or a full struct value, to C code was always unsafe and unlikely to work correctly; it is now forbidden. The fix is to either rewrite the Go code to use only pointers, or to ensure that the Go code sees the full definition of the struct by including the appropriate C header file.

### X.509 CommonName deprecation

The deprecated, legacy behavior of treating the `CommonName` field on X.509 certificates as a host name when no Subject Alternative Names are present is now disabled by default. It can be temporarily re-enabled by adding the value `x509ignoreCN=0` to the `GODEBUG` environment variable.

Note that if the `CommonName` is an invalid host name, it's always ignored, regardless of `GODEBUG` settings. Invalid names include those with any characters other than letters, digits, hyphens and underscores, and those with empty labels or trailing dots.

### 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.

bufio

  When a `Scanner` is used with an invalid `io.Reader` that incorrectly returns a negative number from `Read`, the `Scanner` will no longer panic, but will instead return the new error `ErrBadReadCount`.

context

  Creating a derived `Context` using a nil parent is now explicitly disallowed. Any attempt to do so with the `WithValue`, `WithDeadline`, or `WithCancel` functions will cause a panic.

crypto

  The `PrivateKey` and `PublicKey` types in the `crypto/rsa`, `crypto/ecdsa`, and `crypto/ed25519` packages now have an `Equal` method to compare keys for equivalence or to make type-safe interfaces for public keys. The method signature is compatible with `go-cmp`'s definition of equality.

  `Hash` now implements `fmt.Stringer`.

crypto/ecdsa

  The new `SignASN1` and `VerifyASN1` functions allow generating and verifying ECDSA signatures in the standard ASN.1 DER encoding.

crypto/elliptic

  The new `MarshalCompressed` and `UnmarshalCompressed` functions allow encoding and decoding NIST elliptic curve points in compressed format.

crypto/rsa

  `VerifyPKCS1v15` now rejects invalid short signatures with missing leading zeroes, according to RFC 8017.

crypto/tls

  The new `Dialer` type and its `DialContext` method permit using a context to both connect and handshake with a TLS server.

  The new `VerifyConnection` callback on the `Config` type allows custom verification logic for every connection. It has access to the `ConnectionState` which includes peer certificates, SCTs, and stapled OCSP responses.

  Auto-generated session ticket keys are now automatically rotated every 24 hours, with a lifetime of 7 days, to limit their impact on forward secrecy.

  Session ticket lifetimes in TLS 1.2 and earlier, where the session keys are reused for resumed connections, are now limited to 7 days, also to limit their impact on forward secrecy.

  The client-side downgrade protection checks specified in RFC 8446 are now enforced. This has the potential to cause connection errors for clients encountering middleboxes that behave like unauthorized downgrade attacks.

  `SignatureScheme`, `CurveID`, and `ClientAuthType` now implement `fmt.Stringer`.

  The `ConnectionState` fields `OCSPResponse` and `SignedCertificateTimestamps` are now repopulated on client-side resumed connections.

  `tls.Conn` now returns an opaque error on permanently broken connections, wrapping the temporary `net.Error`. To access the original `net.Error`, use `errors.As` (or `errors.Unwrap`) instead of a type assertion.

crypto/x509

  If either the name on the certificate or the name being verified (with `VerifyOptions.DNSName` or `VerifyHostname`) are invalid, they will now be compared case-insensitively without further processing (without honoring wildcards or stripping trailing dots). Invalid names include those with any characters other than letters, digits, hyphens and underscores, those with empty labels, and names on certificates with trailing dots.

  The new `CreateRevocationList` function and `RevocationList` type allow creating RFC 5280-compliant X.509 v2 Certificate Revocation Lists.

  `CreateCertificate` now automatically generates the `SubjectKeyId` if the template is a CA and doesn't explicitly specify one.

  `CreateCertificate` now returns an error if the template specifies `MaxPathLen` but is not a CA.

  On Unix systems other than macOS, the `SSL_CERT_DIR` environment variable can now be a colon-separated list.

  On macOS, binaries are now always linked against `Security.framework` to extract the system trust roots, regardless of whether cgo is available. The resulting behavior should be more consistent with the OS verifier.

crypto/x509/pkix

  `Name.String` now prints non-standard attributes from `Names` if `ExtraNames` is nil.

database/sql

  The new `DB.SetConnMaxIdleTime` method allows removing a connection from the connection pool after it has been idle for a period of time, without regard to the total lifespan of the connection. The `DBStats.MaxIdleTimeClosed` field shows the total number of connections closed due to `DB.SetConnMaxIdleTime`.

  The new `Row.Err` getter allows checking for query errors without calling `Row.Scan`.

database/sql/driver

  The new `Validator` interface may be implemented by `Conn` to allow drivers to signal if a connection is valid or if it should be discarded.

debug/pe

  The package now defines the `IMAGE_FILE`, `IMAGE_SUBSYSTEM`, and `IMAGE_DLLCHARACTERISTICS` constants used by the PE file format.

encoding/asn1

  `Marshal` now sorts the components of SET OF according to X.690 DER.

  `Unmarshal` now rejects tags and Object Identifiers which are not minimally encoded according to X.690 DER.

encoding/json

  The package now has an internal limit to the maximum depth of nesting when decoding. This reduces the possibility that a deeply nested input could use large quantities of stack memory, or even cause a "goroutine stack exceeds limit" panic.

flag

  When the `flag` package sees `-h` or `-help`, and those flags are not defined, it now prints a usage message. If the `FlagSet` was created with `ExitOnError`, `FlagSet.Parse` would then exit with a status of 2. In this release, the exit status for `-h` or `-help` has been changed to 0. In particular, this applies to the default handling of command line flags.

fmt

  The printing verbs `%#g` and `%#G` now preserve trailing zeros for floating-point values.

go/format

  The `Source` and `Node` functions now canonicalize number literal prefixes and exponents as part of formatting Go source code. This matches the behavior of the `gofmt` command as it was implemented since Go 1.13.

html/template

  The package now uses Unicode escapes (`\uNNNN`) in all JavaScript and JSON contexts. This fixes escaping errors in `application/ld+json` and `application/json` contexts.

io/ioutil

  `TempDir` and `TempFile` now reject patterns that contain path separators. That is, calls such as `ioutil.TempFile("/tmp",` `"../base*")` will no longer succeed. This prevents unintended directory traversal.

math/big

  The new `Int.FillBytes` method allows serializing to fixed-size pre-allocated byte slices.

math/cmplx

  The functions in this package were updated to conform to the C99 standard (Annex G IEC 60559-compatible complex arithmetic) with respect to handling of special arguments such as infinity, NaN and signed zero.

net

  If an I/O operation exceeds a deadline set by the `Conn.SetDeadline`, `Conn.SetReadDeadline`, or `Conn.SetWriteDeadline` methods, it will now return an error that is or wraps `os.ErrDeadlineExceeded`. This may be used to reliably detect whether an error is due to an exceeded deadline. Earlier releases recommended calling the `Timeout` method on the error, but I/O operations can return errors for which `Timeout` returns `true` although a deadline has not been exceeded.

  The new `Resolver.LookupIP` method supports IP lookups that are both network-specific and accept a context.

net/http

  Parsing is now stricter as a hardening measure against request smuggling attacks: non-ASCII white space is no longer trimmed like SP and HTAB, and support for the "`identity`" `Transfer-Encoding` was dropped.

net/http/httputil

  `ReverseProxy` now supports not modifying the `X-Forwarded-For` header when the incoming `Request.Header` map entry for that field is `nil`.

  When a Switching Protocol (like WebSocket) request handled by `ReverseProxy` is canceled, the backend connection is now correctly closed.

net/http/pprof

  All profile endpoints now support a "`seconds`" parameter. When present, the endpoint profiles for the specified number of seconds and reports the difference. The meaning of the "`seconds`" parameter in the `cpu` profile and the trace endpoints is unchanged.

net/url

  The new `URL` field `RawFragment` and method `EscapedFragment` provide detail about and control over the exact encoding of a particular fragment. These are analogous to `RawPath` and `EscapedPath`.

  The new `URL` method `Redacted` returns the URL in string form with any password replaced with `xxxxx`.

os

  If an I/O operation exceeds a deadline set by the `File.SetDeadline`, `File.SetReadDeadline`, or `File.SetWriteDeadline` methods, it will now return an error that is or wraps `os.ErrDeadlineExceeded`. This may be used to reliably detect whether an error is due to an exceeded deadline. Earlier releases recommended calling the `Timeout` method on the error, but I/O operations can return errors for which `Timeout` returns `true` although a deadline has not been exceeded.

  Packages `os` and `net` now automatically retry system calls that fail with `EINTR`. Previously this led to spurious failures, which became more common in Go 1.14 with the addition of asynchronous preemption. Now this is handled transparently.

  The `os.File` type now supports a `ReadFrom` method. This permits the use of the `copy_file_range` system call on some systems when using `io.Copy` to copy data from one `os.File` to another. A consequence is that `io.CopyBuffer` will not always use the provided buffer when copying to a `os.File`. If a program wants to force the use of the provided buffer, it can be done by writing `io.CopyBuffer(struct{ io.Writer }{dst}, src, buf)`.

plugin

  DWARF generation is now supported (and enabled by default) for `-buildmode=plugin` on macOS.

  Building with `-buildmode=plugin` is now supported on `freebsd/amd64`.

reflect

  Package `reflect` now disallows accessing methods of all non-exported fields, whereas previously it allowed accessing those of non-exported, embedded fields. Code that relies on the previous behavior should be updated to instead access the corresponding promoted method of the enclosing variable.

regexp

  The new `Regexp.SubexpIndex` method returns the index of the first subexpression with the given name within the regular expression.

runtime

  Several functions, including `ReadMemStats` and `GoroutineProfile`, no longer block if a garbage collection is in progress.

runtime/pprof

  The goroutine profile now includes the profile labels associated with each goroutine at the time of profiling. This feature is not yet implemented for the profile reported with `debug=2`.

strconv

  `FormatComplex` and `ParseComplex` are added for working with complex numbers.

  `FormatComplex` converts a complex number into a string of the form (a+bi), where a and b are the real and imaginary parts.

  `ParseComplex` converts a string into a complex number of a specified precision. `ParseComplex` accepts complex numbers in the format `N+Ni`.

sync

  The new method `Map.LoadAndDelete` atomically deletes a key and returns the previous value if present.

  The method `Map.Delete` is more efficient.

syscall

  On Unix systems, functions that use `SysProcAttr` will now reject attempts to set both the `Setctty` and `Foreground` fields, as they both use the `Ctty` field but do so in incompatible ways. We expect that few existing programs set both fields.

  Setting the `Setctty` field now requires that the `Ctty` field be set to a file descriptor number in the child process, as determined by the `ProcAttr.Files` field. Using a child descriptor always worked, but there were certain cases where using a parent file descriptor also happened to work. Some programs that set `Setctty` will need to change the value of `Ctty` to use a child descriptor number.

  It is now possible to call system calls that return floating point values on `windows/amd64`.

testing

  The `testing.T` type now has a `Deadline` method that reports the time at which the test binary will have exceeded its timeout.

  A `TestMain` function is no longer required to call `os.Exit`. If a `TestMain` function returns, the test binary will call `os.Exit` with the value returned by `m.Run`.

  The new methods `T.TempDir` and `B.TempDir` return temporary directories that are automatically cleaned up at the end of the test.

  `go` `test` `-v` now groups output by test name, rather than printing the test name on each line.

text/template

  `JSEscape` now consistently uses Unicode escapes (`\u00XX`), which are compatible with JSON.

time

  The new method `Ticker.Reset` supports changing the duration of a ticker.

  When returning an error, `ParseDuration` now quotes the original value.