Fix:interation:Pinboard Do not override existing bookmarks

The issue:
When saving an entry that is already bookmarked on pinboard,
miniflux was basically overriding all existing data on piboard
with this action. This removed any `extended` content or worse
changed the `private` settings to no private making any previously
private bookmark publicly available.

The fix:
Now, upon saving an entry as bookmark I first fetch it. If it
already exists, I do the proper modifications (adding tag and any state)
miniflux would have normally done, then do the add again. This way, no
data is lost in the process. Pinboard has a pretty stable API, I don't think
any new fields will be added soon.

I manually tested the integration by hitting the save button in the following situations:
 - Entry url does not exist on pinboard
  - Bookmark is properly added on pinboard with tags and `to read` acroding to miniflux settings
 - Entry url already exist on piboard
  - Existing data stays unchanged
  - Tags in miniflux settings are properly added to the bookmark
  - Set `to read` is properly set to `yes` when the option is checked on miniflux. Nothing is changed otherwise

internal/integration/pinboard/pinboard.go

@@ -4,14 +4,18 @@
 package pinboard // import "miniflux.app/v2/internal/integration/pinboard"
 
 import (
+	"encoding/xml"
+	"errors"
 	"fmt"
+	"miniflux.app/v2/internal/version"
 	"net/http"
 	"net/url"
 	"time"
-
-	"miniflux.app/v2/internal/version"
 )
 
+var errPostNotFound = fmt.Errorf("pinboard: post not found")
+var errMissingCredentials = fmt.Errorf("pinboard: missing auth token")
+
 const defaultClientTimeout = 10 * time.Second
 
 type Client struct {
@@ -24,20 +28,27 @@ func NewClient(authToken string) *Client {
 
 func (c *Client) CreateBookmark(entryURL, entryTitle, pinboardTags string, markAsUnread bool) error {
 	if c.authToken == "" {
-		return fmt.Errorf("pinboard: missing auth token")
+		return errMissingCredentials
+	}
+
+	// We check if the url is already bookmarked to avoid overriding existing data.
+	post, err := c.getBookmark(entryURL)
+
+	if err != nil && errors.Is(err, errPostNotFound) {
+		post = NewPost(entryURL, entryTitle)
+	} else if err != nil {
+		// In case of any other error, we return immediately to avoid overriding existing data.
+		return err
 	}
 
-	toRead := "no"
+	post.addTag(pinboardTags)
 	if markAsUnread {
-		toRead = "yes"
+		post.SetToread()
 	}
 
 	values := url.Values{}
 	values.Add("auth_token", c.authToken)
-	values.Add("url", entryURL)
-	values.Add("description", entryTitle)
-	values.Add("tags", pinboardTags)
-	values.Add("toread", toRead)
+	post.AddValues(values)
 
 	apiEndpoint := "https://api.pinboard.in/v1/posts/add?" + values.Encode()
 	request, err := http.NewRequest(http.MethodGet, apiEndpoint, nil)
@@ -61,3 +72,45 @@ func (c *Client) CreateBookmark(entryURL, entryTitle, pinboardTags string, markA
 
 	return nil
 }
+
+// getBookmark fetches a bookmark from Pinboard. https://www.pinboard.in/api/#posts_get
+func (c *Client) getBookmark(entryURL string) (*Post, error) {
+	if c.authToken == "" {
+		return nil, errMissingCredentials
+	}
+
+	values := url.Values{}
+	values.Add("auth_token", c.authToken)
+	values.Add("url", entryURL)
+
+	apiEndpoint := "https://api.pinboard.in/v1/posts/get?" + values.Encode()
+	request, err := http.NewRequest(http.MethodGet, apiEndpoint, nil)
+	if err != nil {
+		return nil, fmt.Errorf("pinboard: unable to create request: %v", err)
+	}
+
+	request.Header.Set("User-Agent", "Miniflux/"+version.Version)
+
+	httpClient := &http.Client{Timeout: defaultClientTimeout}
+	response, err := httpClient.Do(request)
+	if err != nil {
+		return nil, fmt.Errorf("pinboard: unable fetch bookmark: %v", err)
+	}
+	defer response.Body.Close()
+
+	if response.StatusCode >= 400 {
+		return nil, fmt.Errorf("pinboard: unable to fetch bookmark, status=%d", response.StatusCode)
+	}
+
+	var results posts
+	err = xml.NewDecoder(response.Body).Decode(&results)
+	if err != nil {
+		return nil, fmt.Errorf("pinboard: unable to decode XML: %v", err)
+	}
+
+	if len(results.Posts) == 0 {
+		return nil, errPostNotFound
+	}
+
+	return &results.Posts[0], nil
+}

internal/integration/pinboard/post.go

@@ -0,0 +1,59 @@
+package pinboard
+
+import (
+	"encoding/xml"
+	"net/url"
+	"strings"
+	"time"
+)
+
+// Post a Pinboard bookmark.  "inspiration" from https://github.com/drags/pinboard/blob/master/posts.go#L32-L42
+type Post struct {
+	XMLName     xml.Name  `xml:"post"`
+	Url         string    `xml:"href,attr"`
+	Description string    `xml:"description,attr"`
+	Tags        string    `xml:"tag,attr"`
+	Extended    string    `xml:"extended,attr"`
+	Date        time.Time `xml:"time,attr"`
+	Shared      string    `xml:"shared,attr"`
+	Toread      string    `xml:"toread,attr"`
+}
+
+// Posts A result of a Pinboard API call
+type posts struct {
+	XMLName xml.Name `xml:"posts"`
+	Posts   []Post   `xml:"post"`
+}
+
+func NewPost(url string, description string) *Post {
+	return &Post{
+		Url:         url,
+		Description: description,
+		Date:        time.Now(),
+		Toread:      "no",
+	}
+}
+
+func (p *Post) addTag(tag string) {
+	if !strings.Contains(p.Tags, tag) {
+		p.Tags += " " + tag
+	}
+}
+
+func (p *Post) SetToread() {
+	p.Toread = "yes"
+}
+
+func (p *Post) AddValues(values url.Values) {
+	values.Add("url", p.Url)
+	values.Add("description", p.Description)
+	values.Add("tags", p.Tags)
+	if p.Toread != "" {
+		values.Add("toread", p.Toread)
+	}
+	if p.Shared != "" {
+		values.Add("shared", p.Shared)
+	}
+	values.Add("dt", p.Date.Format(time.RFC3339))
+	values.Add("extended", p.Extended)
+}