add in-built logging support for a better dev experience

vendor/github.com/kr/logfmt/decode.go

@@ -0,0 +1,184 @@
+// Package implements the decoding of logfmt key-value pairs.
+//
+// Example logfmt message:
+//
+//	foo=bar a=14 baz="hello kitty" cool%story=bro f %^asdf
+//
+// Example result in JSON:
+//
+//	{ "foo": "bar", "a": 14, "baz": "hello kitty", "cool%story": "bro", "f": true, "%^asdf": true }
+//
+// EBNFish:
+//
+// 	ident_byte = any byte greater than ' ', excluding '=' and '"'
+// 	string_byte = any byte excluding '"' and '\'
+// 	garbage = !ident_byte
+// 	ident = ident_byte, { ident byte }
+// 	key = ident
+// 	value = ident | '"', { string_byte | '\', '"' }, '"'
+// 	pair = key, '=', value | key, '=' | key
+// 	message = { garbage, pair }, garbage
+package logfmt
+
+import (
+	"reflect"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// Handler is the interface implemented by objects that accept logfmt
+// key-value pairs. HandleLogfmt must copy the logfmt data if it
+// wishes to retain the data after returning.
+type Handler interface {
+	HandleLogfmt(key, val []byte) error
+}
+
+// The HandlerFunc type is an adapter to allow the use of ordinary functions as
+// logfmt handlers. If f is a function with the appropriate signature,
+// HandlerFunc(f) is a Handler object that calls f.
+type HandlerFunc func(key, val []byte) error
+
+func (f HandlerFunc) HandleLogfmt(key, val []byte) error {
+	return f(key, val)
+}
+
+// Unmarshal parses the logfmt encoding data and stores the result in the value
+// pointed to by v. If v is an Handler, HandleLogfmt will be called for each
+// key-value pair.
+//
+// If v is not a Handler, it will pass v to NewStructHandler and use the
+// returned StructHandler for decoding.
+func Unmarshal(data []byte, v interface{}) (err error) {
+	h, ok := v.(Handler)
+	if !ok {
+		h, err = NewStructHandler(v)
+		if err != nil {
+			return err
+		}
+	}
+	return gotoScanner(data, h)
+}
+
+// StructHandler unmarshals logfmt into a struct. It matches incoming keys to
+// the the struct's fields (either the struct field name or its tag, preferring
+// an exact match but also accepting a case-insensitive match.
+//
+// Field types supported by StructHandler are:
+//
+// 	all numeric types (e.g. float32, int, etc.)
+// 	[]byte
+// 	string
+// 	bool - true if key is present, false otherwise (the value is ignored).
+//	time.Duration - uses time.ParseDuration
+//
+// If a field is a pointer to an above type, and a matching key is not present
+// in the logfmt data, the pointer will be untouched.
+//
+// If v is not a pointer to an Handler or struct, Unmarshal will return an
+// error.
+type StructHandler struct {
+	rv reflect.Value
+}
+
+func NewStructHandler(v interface{}) (Handler, error) {
+	rv := reflect.ValueOf(v)
+	if rv.Kind() != reflect.Ptr || rv.IsNil() {
+		return nil, &InvalidUnmarshalError{reflect.TypeOf(v)}
+	}
+	return &StructHandler{rv: rv}, nil
+}
+
+func (h *StructHandler) HandleLogfmt(key, val []byte) error {
+	el := h.rv.Elem()
+	skey := string(key)
+	for i := 0; i < el.NumField(); i++ {
+		fv := el.Field(i)
+		ft := el.Type().Field(i)
+		switch {
+		case ft.Name == skey:
+		case ft.Tag.Get("logfmt") == skey:
+		case strings.EqualFold(ft.Name, skey):
+		default:
+			continue
+		}
+		if fv.Kind() == reflect.Ptr {
+			if fv.IsNil() {
+				t := fv.Type().Elem()
+				v := reflect.New(t)
+				fv.Set(v)
+				fv = v
+			}
+			fv = fv.Elem()
+		}
+		switch fv.Interface().(type) {
+		case time.Duration:
+			d, err := time.ParseDuration(string(val))
+			if err != nil {
+				return &UnmarshalTypeError{string(val), fv.Type()}
+			}
+			fv.Set(reflect.ValueOf(d))
+		case string:
+			fv.SetString(string(val))
+		case []byte:
+			b := make([]byte, len(val))
+			copy(b, val)
+			fv.SetBytes(b)
+		case bool:
+			fv.SetBool(true)
+		default:
+			switch {
+			case reflect.Int <= fv.Kind() && fv.Kind() <= reflect.Int64:
+				v, err := strconv.ParseInt(string(val), 10, 64)
+				if err != nil {
+					return err
+				}
+				fv.SetInt(v)
+			case reflect.Uint32 <= fv.Kind() && fv.Kind() <= reflect.Uint64:
+				v, err := strconv.ParseUint(string(val), 10, 64)
+				if err != nil {
+					return err
+				}
+				fv.SetUint(v)
+			case reflect.Float32 <= fv.Kind() && fv.Kind() <= reflect.Float64:
+				v, err := strconv.ParseFloat(string(val), 10)
+				if err != nil {
+					return err
+				}
+				fv.SetFloat(v)
+			default:
+				return &UnmarshalTypeError{string(val), fv.Type()}
+			}
+		}
+
+	}
+	return nil
+}
+
+// An InvalidUnmarshalError describes an invalid argument passed to Unmarshal.
+// (The argument to Unmarshal must be a non-nil pointer.)
+type InvalidUnmarshalError struct {
+	Type reflect.Type
+}
+
+func (e *InvalidUnmarshalError) Error() string {
+	if e.Type == nil {
+		return "logfmt: Unmarshal(nil)"
+	}
+
+	if e.Type.Kind() != reflect.Ptr {
+		return "logfmt: Unmarshal(non-pointer " + e.Type.String() + ")"
+	}
+	return "logfmt: Unmarshal(nil " + e.Type.String() + ")"
+}
+
+// An UnmarshalTypeError describes a logfmt value that was
+// not appropriate for a value of a specific Go type.
+type UnmarshalTypeError struct {
+	Value string       // the logfmt value
+	Type  reflect.Type // type of Go value it could not be assigned to
+}
+
+func (e *UnmarshalTypeError) Error() string {
+	return "logfmt: cannot unmarshal " + e.Value + " into Go value of type " + e.Type.String()
+}