Revert "type.go: Deinegrated `ErrUnhandledUnknownTypedTag`"

This reverts commit 4f2f9044e6.

Reintroduces errors from `UndefinedValue()` when the undefined-type is
unhandled.

- We now just return it rather than panic with it so that we can check
  for it directly rather than use `log.Is()`.

- We've updated the checks across the code accordingly.

README.md

@@ -186,9 +186,13 @@ visitor := func(fqIfdPath string, ifdIndex int, tagId uint16, tagType exif.TagTy
     valueString := ""
     if tagType.Type() == exif.TypeUndefined {
         value, err := exif.UndefinedValue(ifdPath, tagId, valueContext, tagType.ByteOrder())
-        log.PanicIf(err)
-
-        valueString = fmt.Sprintf("%v", value)
+        if log.Is(err, exif.ErrUnhandledUnknownTypedTag) {
+            valueString = "!UNDEFINED!"
+        } else if err != nil {
+            panic(err)
+        } else {
+            valueString = fmt.Sprintf("%v", value)
+        }
     } else {
         valueString, err = tagType.ResolveAsString(valueContext, true)
         if err != nil {

exif-read-tool/main.go

@@ -109,7 +109,13 @@ func main() {
 		if tagType.Type() == exif.TypeUndefined {
 			var err error
 			value, err = valueContext.Undefined()
-			log.PanicIf(err)
+			if err != nil {
+				if err == exif.ErrUnhandledUnknownTypedTag {
+					value = nil
+				} else {
+					log.Panic(err)
+				}
+			}
 
 			valueString = fmt.Sprintf("%v", value)
 		} else {

exif_test.go

@@ -82,7 +82,13 @@ func TestVisit(t *testing.T) {
 		valueString := ""
 		if tagType.Type() == TypeUndefined {
 			value, err := valueContext.Undefined()
-			log.PanicIf(err)
+			if err != nil {
+				if err == ErrUnhandledUnknownTypedTag {
+					valueString = "!UNDEFINED!"
+				} else {
+					log.Panic(err)
+				}
+			}
 
 			valueString = fmt.Sprintf("%v", value)
 		} else {

ifd_builder.go

@@ -1119,20 +1119,19 @@ func (ib *IfdBuilder) AddTagsFromExisting(ifd *Ifd, itevr *IfdTagEntryValueResol
 				// It's an undefined-type value. Try to process, or skip if
 				// we don't know how to.
 
-				x, err := valueContext.Undefined()
-				log.PanicIf(err)
+				undefinedInterface, err := valueContext.Undefined()
+				if err != nil {
+					if err == ErrUnhandledUnknownTypedTag {
+						// It's an undefined-type tag that we don't handle. If
+						// we don't know how to handle it, we can't know how
+						// many bytes it is and we must skip it.
+						continue
+					}
 
-				// TODO(dustin): !! Add test for this.
-				_, isUnknownUndefined := x.(TagUnknownType_UnknownValue)
-
-				if isUnknownUndefined == true {
-					// It's an undefined-type tag that we don't handle. If we
-					// don't know how to handle it, we can't know how many bytes
-					// it is and we must skip it.
-					continue
+					log.Panic(err)
 				}
 
-				undefined, ok := x.(UnknownTagValue)
+				undefined, ok := undefinedInterface.(UnknownTagValue)
 				if ok != true {
 					log.Panicf("unexpected value returned from undefined-value processor")
 				}

ifd_enumerate.go

@@ -240,24 +240,31 @@ func (ie *IfdEnumerate) resolveTagValue(ite *IfdTagEntry) (valueBytes []byte, is
 		valueContext := ie.GetValueContext(ite)
 
 		value, err := valueContext.Undefined()
-		log.PanicIf(err)
+		if err != nil {
+			if err == ErrUnhandledUnknownTypedTag {
+				valueBytes = []byte(UnparseableUnknownTagValuePlaceholder)
+				return valueBytes, true, nil
+			}
 
-		switch value.(type) {
-		case []byte:
-			return value.([]byte), false, nil
-		case TagUnknownType_UnknownValue:
-			b := []byte(value.(TagUnknownType_UnknownValue))
-			return b, false, nil
-		case string:
-			return []byte(value.(string)), false, nil
-		case UnknownTagValue:
-			valueBytes, err := value.(UnknownTagValue).ValueBytes()
-			log.PanicIf(err)
+			log.Panic(err)
+		} else {
+			switch value.(type) {
+			case []byte:
+				return value.([]byte), false, nil
+			case TagUnknownType_UnknownValue:
+				b := []byte(value.(TagUnknownType_UnknownValue))
+				return b, false, nil
+			case string:
+				return []byte(value.(string)), false, nil
+			case UnknownTagValue:
+				valueBytes, err := value.(UnknownTagValue).ValueBytes()
+				log.PanicIf(err)
 
-			return valueBytes, false, nil
-		default:
-			// TODO(dustin): !! Finish translating the rest of the types (make reusable and replace into other similar implementations?)
-			log.Panicf("can not produce bytes for unknown-type tag (0x%04x) (1): [%s]", ite.TagId, reflect.TypeOf(value))
+				return valueBytes, false, nil
+			default:
+				// TODO(dustin): !! Finish translating the rest of the types (make reusable and replace into other similar implementations?)
+				log.Panicf("can not produce bytes for unknown-type tag (0x%04x) (1): [%s]", ite.TagId, reflect.TypeOf(value))
+			}
 		}
 	} else {
 		originalType := NewTagType(ite.TagType, ie.byteOrder)
@@ -709,7 +716,13 @@ func (ifd *Ifd) printTagTree(populateValues bool, index, level int, nextLink boo
 				var err error
 
 				value, err = ifd.TagValue(tag)
-				log.PanicIf(err)
+				if err != nil {
+					if err == ErrUnhandledUnknownTypedTag {
+						value = UnparseableUnknownTagValuePlaceholder
+					} else {
+						log.Panic(err)
+					}
+				}
 			}
 
 			fmt.Printf("%s - TAG: %s NAME=[%s] VALUE=[%v]\n", indent, tag, tagName, value)

tags_unknown.go

@@ -11,6 +11,10 @@ import (
 	"github.com/dsoprea/go-logging"
 )
 
+const (
+	UnparseableUnknownTagValuePlaceholder = "!UNKNOWN"
+)
+
 const (
 	TagUnknownType_9298_UserComment_Encoding_ASCII     = iota
 	TagUnknownType_9298_UserComment_Encoding_JIS       = iota
@@ -387,15 +391,7 @@ func UndefinedValue(ifdPath string, tagId uint16, valueContext interface{}, byte
 	//
 	// 0xa40b is device-specific and unhandled.
 
-	// Return encapsulated data rather than an error so that we can at least
-	// print/profile the opaque data.
-
-	// TODO(dustin): This won't ever work. The unit-count isn't necessarily correct. Revert to returning an error.
-	valueContextPtr.SetUnknownValueType(TypeByte)
-
-	valueBytes, err := valueContextPtr.ReadBytes()
-	log.PanicIf(err)
-
-	tutuv := TagUnknownType_UnknownValue(valueBytes)
-	return tutuv, nil
+	// We have no choice but to return the error. We have no way of knowing how
+	// much data there is without already knowing what data-type this tag is.
+	return nil, ErrUnhandledUnknownTypedTag
 }

type.go

@@ -84,6 +84,11 @@ var (
     // ErrWrongType is used when we try to parse anything other than the
     // current type.
     ErrWrongType = errors.New("wrong type, can not parse")
+
+    // ErrUnhandledUnknownTag is used when we try to parse a tag that's
+    // recorded as an "unknown" type but not a documented tag (therefore
+    // leaving us not knowning how to read it).
+    ErrUnhandledUnknownTypedTag = errors.New("not a standard unknown-typed tag")
 )
 
 type Rational struct {

utility.go

@@ -182,10 +182,18 @@ func GetFlatExifData(exifData []byte) (exifTags []ExifTag, err error) {
             }
 
             value, err := ifd.TagValue(ite)
-            log.PanicIf(err)
+            if err != nil {
+                if err == ErrUnhandledUnknownTypedTag {
+                    value = UnparseableUnknownTagValuePlaceholder
+                } else {
+                    log.Panic(err)
+                }
+            }
 
             valueBytes, err := ifd.TagValueBytes(ite)
-            log.PanicIf(err)
+            if err != nil && err != ErrUnhandledUnknownTypedTag {
+                log.Panic(err)
+            }
 
             et := ExifTag{
                 IfdPath:      ifd.IfdPath,