136 lines
4.8 KiB
Go
136 lines
4.8 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 or retains 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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// With returns a new contextual logger with keyvals prepended to those passed
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// to calls to Log. If logger is also a contextual logger created by With or
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// WithPrefix, keyvals is appended to the existing context.
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//
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// The returned Logger replaces all value elements (odd indexes) containing a
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// Valuer with their generated value for each call to its Log method.
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func With(logger Logger, keyvals ...interface{}) Logger {
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if len(keyvals) == 0 {
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return logger
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}
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l := newContext(logger)
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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 contextual logger with keyvals prepended to those
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// passed to calls to Log. If logger is also a contextual logger created by
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// With or WithPrefix, keyvals is prepended to the existing context.
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//
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// The returned Logger replaces all value elements (odd indexes) containing a
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// Valuer with their generated value for each call to its Log method.
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func WithPrefix(logger Logger, keyvals ...interface{}) Logger {
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if len(keyvals) == 0 {
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return logger
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}
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l := newContext(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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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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// context is the Logger implementation returned by With and WithPrefix. It
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// wraps a Logger and holds keyvals that it includes in all log events. Its
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// Log method calls bindValues to generate values for each Valuer in the
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// context keyvals.
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//
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// A 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 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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// Two implementation details provide the needed stack depth consistency.
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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 and WithPrefix avoid introducing an additional layer by
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// returning a newly constructed context with a merged keyvals rather
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// than simply wrapping the existing context.
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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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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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// 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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// 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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