prometheus/vendor/github.com/go-kit/kit/log/log.go
2017-02-07 03:42:02 -08:00

145 lines
5.3 KiB
Go

package log
import "errors"
// Logger is the fundamental interface for all log operations. Log creates a
// log event from keyvals, a variadic sequence of alternating keys and values.
// Implementations must be safe for concurrent use by multiple goroutines. In
// particular, any implementation of Logger that appends to keyvals or
// modifies any of its elements must make a copy first.
type Logger interface {
Log(keyvals ...interface{}) error
}
// ErrMissingValue is appended to keyvals slices with odd length to substitute
// the missing value.
var ErrMissingValue = errors.New("(MISSING)")
// NewContext returns a new Context that logs to logger.
func NewContext(logger Logger) *Context {
if c, ok := logger.(*Context); ok {
return c
}
return &Context{logger: logger}
}
// Context must always have the same number of stack frames between calls to
// its Log method and the eventual binding of Valuers to their value. This
// requirement comes from the functional requirement to allow a context to
// resolve application call site information for a log.Caller stored in the
// context. To do this we must be able to predict the number of logging
// functions on the stack when bindValues is called.
//
// Three implementation details provide the needed stack depth consistency.
// The first two of these details also result in better amortized performance,
// and thus make sense even without the requirements regarding stack depth.
// The third detail, however, is subtle and tied to the implementation of the
// Go compiler.
//
// 1. NewContext avoids introducing an additional layer when asked to
// wrap another Context.
// 2. With avoids introducing an additional layer by returning a newly
// constructed Context with a merged keyvals rather than simply
// wrapping the existing Context.
// 3. All of Context's methods take pointer receivers even though they
// do not mutate the Context.
//
// Before explaining the last detail, first some background. The Go compiler
// generates wrapper methods to implement the auto dereferencing behavior when
// calling a value method through a pointer variable. These wrapper methods
// are also used when calling a value method through an interface variable
// because interfaces store a pointer to the underlying concrete value.
// Calling a pointer receiver through an interface does not require generating
// an additional function.
//
// If Context had value methods then calling Context.Log through a variable
// with type Logger would have an extra stack frame compared to calling
// Context.Log through a variable with type Context. Using pointer receivers
// avoids this problem.
// A Context wraps a Logger and holds keyvals that it includes in all log
// events. When logging, a Context replaces all value elements (odd indexes)
// containing a Valuer with their generated value for each call to its Log
// method.
type Context struct {
logger Logger
keyvals []interface{}
hasValuer bool
}
// Log replaces all value elements (odd indexes) containing a Valuer in the
// stored context with their generated value, appends keyvals, and passes the
// result to the wrapped Logger.
func (l *Context) Log(keyvals ...interface{}) error {
kvs := append(l.keyvals, keyvals...)
if len(kvs)%2 != 0 {
kvs = append(kvs, ErrMissingValue)
}
if l.hasValuer {
// If no keyvals were appended above then we must copy l.keyvals so
// that future log events will reevaluate the stored Valuers.
if len(keyvals) == 0 {
kvs = append([]interface{}{}, l.keyvals...)
}
bindValues(kvs[:len(l.keyvals)])
}
return l.logger.Log(kvs...)
}
// With returns a new Context with keyvals appended to those of the receiver.
func (l *Context) With(keyvals ...interface{}) *Context {
if len(keyvals) == 0 {
return l
}
kvs := append(l.keyvals, keyvals...)
if len(kvs)%2 != 0 {
kvs = append(kvs, ErrMissingValue)
}
return &Context{
logger: l.logger,
// Limiting the capacity of the stored keyvals ensures that a new
// backing array is created if the slice must grow in Log or With.
// Using the extra capacity without copying risks a data race that
// would violate the Logger interface contract.
keyvals: kvs[:len(kvs):len(kvs)],
hasValuer: l.hasValuer || containsValuer(keyvals),
}
}
// WithPrefix returns a new Context with keyvals prepended to those of the
// receiver.
func (l *Context) WithPrefix(keyvals ...interface{}) *Context {
if len(keyvals) == 0 {
return l
}
// Limiting the capacity of the stored keyvals ensures that a new
// backing array is created if the slice must grow in Log or With.
// Using the extra capacity without copying risks a data race that
// would violate the Logger interface contract.
n := len(l.keyvals) + len(keyvals)
if len(keyvals)%2 != 0 {
n++
}
kvs := make([]interface{}, 0, n)
kvs = append(kvs, keyvals...)
if len(kvs)%2 != 0 {
kvs = append(kvs, ErrMissingValue)
}
kvs = append(kvs, l.keyvals...)
return &Context{
logger: l.logger,
keyvals: kvs,
hasValuer: l.hasValuer || containsValuer(keyvals),
}
}
// LoggerFunc is an adapter to allow use of ordinary functions as Loggers. If
// f is a function with the appropriate signature, LoggerFunc(f) is a Logger
// object that calls f.
type LoggerFunc func(...interface{}) error
// Log implements Logger by calling f(keyvals...).
func (f LoggerFunc) Log(keyvals ...interface{}) error {
return f(keyvals...)
}