MM-37573: Ignore nil fields in attachments (#18052)

We extend the same logic done in https://github.com/mattermost/mattermost-server/pull/16556
to Field.

Also handled the case elsewhere in the code with a light grep
for extra safety.

https://mattermost.atlassian.net/browse/MM-37573

```release-note
NONE
```

app/post_metadata.go

@@ -283,6 +283,9 @@ func getEmojiNamesForPost(post *model.Post, reactions []*model.Reaction) []strin
 		}
 
 		for _, field := range attachment.Fields {
+			if field == nil {
+				continue
+			}
 			if value, ok := field.Value.(string); ok {
 				names = append(names, getEmojiNamesForString(value)...)
 			}
@@ -361,6 +364,9 @@ func (a *App) getImagesInMessageAttachments(post *model.Post) []string {
 		images = append(images, imagesInPretext...)
 
 		for _, field := range attachment.Fields {
+			if field == nil {
+				continue
+			}
 			if value, ok := field.Value.(string); ok {
 				_, imagesInFieldValue := a.getFirstLinkAndImages(value)
 				images = append(images, imagesInFieldValue...)

app/slack.go

@@ -72,7 +72,7 @@ func (a *App) ProcessSlackAttachments(attachments []*model.SlackAttachment) []*m
 		attachment.Title = a.ProcessSlackText(attachment.Title)
 
 		for _, field := range attachment.Fields {
-			if field.Value != nil {
+			if field != nil && field.Value != nil {
 				// Ensure the value is set to a string if it is set
 				field.Value = a.ProcessSlackText(fmt.Sprintf("%v", field.Value))
 			}

model/post.go

@@ -605,6 +605,7 @@ func (o *Post) Attachments() []*SlackAttachment {
 			if enc, err := json.Marshal(attachment); err == nil {
 				var decoded SlackAttachment
 				if json.Unmarshal(enc, &decoded) == nil {
+					// Ignoring nil actions
 					i := 0
 					for _, action := range decoded.Actions {
 						if action != nil {
@@ -613,6 +614,16 @@ func (o *Post) Attachments() []*SlackAttachment {
 						}
 					}
 					decoded.Actions = decoded.Actions[:i]
+
+					// Ignoring nil fields
+					i = 0
+					for _, field := range decoded.Fields {
+						if field != nil {
+							decoded.Fields[i] = field
+							i++
+						}
+					}
+					decoded.Fields = decoded.Fields[:i]
 					ret = append(ret, &decoded)
 				}
 			}

model/post_test.go

@@ -929,4 +929,20 @@ func TestPostAttachments(t *testing.T) {
 		require.Equal(t, attachments[0].Actions[0].Id, "test1")
 		require.Equal(t, attachments[0].Actions[1].Id, "test2")
 	})
+
+	t.Run("nil fields", func(t *testing.T) {
+		p.Props["attachments"] = []interface{}{
+			map[string]interface{}{"fields": []interface{}{
+				map[string]interface{}{"value": ":emoji1:"},
+				nil,
+				map[string]interface{}{"value": ":emoji2:"},
+			},
+			},
+		}
+
+		attachments := p.Attachments()
+		require.Len(t, attachments[0].Fields, 2)
+		assert.Equal(t, attachments[0].Fields[0].Value, ":emoji1:")
+		assert.Equal(t, attachments[0].Fields[1].Value, ":emoji2:")
+	})
 }

model/slack_attachment.go

@@ -179,6 +179,9 @@ func ParseSlackAttachment(post *Post, attachments []*SlackAttachment) {
 		attachment.Pretext = ParseSlackLinksToMarkdown(attachment.Pretext)
 
 		for _, field := range attachment.Fields {
+			if field == nil {
+				continue
+			}
 			if value, ok := field.Value.(string); ok {
 				field.Value = ParseSlackLinksToMarkdown(value)
 			}