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