Go 1.15 Release Notes
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.
Standard 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.