fix(avatar): use sha256 and inline the federated avatar lookup (#38843)

- Hash emails with sha256. Gravatar moved to sha256, and both it and
libravatar.org serve the same image for either hash.
- Drop `strk.kbt.io/projects/go/libravatar` for a 46 line inline SRV
lookup. It could not bound or cancel its DNS query and panicked on an
unexpected resolver error. The replacement carries the request context
and a 3s timeout.
- Fix federated avatars querying DNS for every avatar on every render.
`loadAvatarSetting` compared a cache field that was never assigned, so
each call rebuilt the resolver and dropped its cache. That cache is
gone, both settings are read where they are used.
- Migration 348 recreates `email_hash` with a 64 char hash column and a
`hash_type` column, so a later algorithm change can tell old rows apart.
The MD5 rows are unreachable and their `UNIQUE` email index would reject
the SHA256 replacements.
- Fix a re-saved avatar form replacing an uploaded avatar with a random
one.
- Remove the `duoshuo` `GRAVATAR_SOURCE` alias, that service shut down
in 2017.
- Remove dead i18n key.

Fixes: https://github.com/go-gitea/gitea/issues/34284
Fixes: https://github.com/go-gitea/gitea/issues/28110
Docs: https://gitea.com/gitea/docs/pulls/499
Signed-off-by: silverwind <me@silverwind.io>

models/user/avatar.go

@@ -16,6 +16,7 @@ import (
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/setting"
 	"gitea.dev/modules/storage"
+	"gitea.dev/modules/util"
 )
 
 // CustomAvatarRelativePath returns user custom avatar relative path.
@@ -25,21 +26,13 @@ func (u *User) CustomAvatarRelativePath() string {
 
 // GenerateRandomAvatar generates a random avatar for user.
 func GenerateRandomAvatar(ctx context.Context, u *User) error {
-	seed := u.Email
-	if len(seed) == 0 {
-		seed = u.Name
-	}
+	seed := []byte(util.IfZero(u.Email, u.Name))
+	u.Avatar = avatar.HashAvatar(u.ID, seed)
 
-	img := avatar.RandomImageDefaultSize([]byte(seed))
-
-	u.Avatar = avatars.HashEmail(seed)
-
-	_, err := storage.Avatars.Stat(u.CustomAvatarRelativePath())
-	if err != nil {
-		// If unable to Stat the avatar file (usually it means non-existing), then try to save a new one
-		// Don't share the images so that we can delete them easily
+	// a failed Stat usually means the file is not there yet
+	if _, err := storage.Avatars.Stat(u.CustomAvatarRelativePath()); err != nil {
 		if err := storage.SaveFrom(storage.Avatars, u.CustomAvatarRelativePath(), func(w io.Writer) error {
-			return png.Encode(w, img)
+			return png.Encode(w, avatar.RandomImageDefaultSize(seed))
 		}); err != nil {
 			return fmt.Errorf("failed to save avatar %s: %w", u.CustomAvatarRelativePath(), err)
 		}