Improve oauth2 client "preferred username field" logic and the error handling (#30622)

Follow #30454
And fix #24957

When using "preferred_username", if no such field,
`extractUserNameFromOAuth2` (old `getUserName`) shouldn't return an
error. All other USERNAME options do not return such error.

And fine tune some logic and error messages, make code more stable and
more friendly to end users.

models/unittest/testdb.go

@@ -16,6 +16,7 @@ import (
 	"code.gitea.io/gitea/models/system"
 	"code.gitea.io/gitea/modules/auth/password/hash"
 	"code.gitea.io/gitea/modules/base"
+	"code.gitea.io/gitea/modules/cache"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/setting/config"
@@ -106,6 +107,7 @@ func MainTest(m *testing.M, testOpts ...*TestOptions) {
 		fatalTestError("Error creating test engine: %v\n", err)
 	}
 
+	setting.IsInTesting = true
 	setting.AppURL = "https://try.gitea.io/"
 	setting.RunUser = "runuser"
 	setting.SSH.User = "sshuser"
@@ -148,6 +150,9 @@ func MainTest(m *testing.M, testOpts ...*TestOptions) {
 
 	config.SetDynGetter(system.NewDatabaseDynKeyGetter())
 
+	if err = cache.Init(); err != nil {
+		fatalTestError("cache.Init: %v\n", err)
+	}
 	if err = storage.Init(); err != nil {
 		fatalTestError("storage.Init: %v\n", err)
 	}

models/user/user.go

@@ -501,19 +501,19 @@ func GetUserSalt() (string, error) {
 // Note: The set of characters here can safely expand without a breaking change,
 // but characters removed from this set can cause user account linking to break
 var (
-	customCharsReplacement    = strings.NewReplacer("Æ", "AE")
-	removeCharsRE             = regexp.MustCompile(`['´\x60]`)
-	removeDiacriticsTransform = transform.Chain(norm.NFD, runes.Remove(runes.In(unicode.Mn)), norm.NFC)
-	replaceCharsHyphenRE      = regexp.MustCompile(`[\s~+]`)
+	customCharsReplacement = strings.NewReplacer("Æ", "AE")
+	removeCharsRE          = regexp.MustCompile("['`´]")
+	transformDiacritics    = transform.Chain(norm.NFD, runes.Remove(runes.In(unicode.Mn)), norm.NFC)
+	replaceCharsHyphenRE   = regexp.MustCompile(`[\s~+]`)
 )
 
-// normalizeUserName returns a string with single-quotes and diacritics
-// removed, and any other non-supported username characters replaced with
-// a `-` character
+// NormalizeUserName only takes the name part if it is an email address, transforms it diacritics to ASCII characters.
+// It returns a string with the single-quotes removed, and any other non-supported username characters are replaced with a `-` character
 func NormalizeUserName(s string) (string, error) {
-	strDiacriticsRemoved, n, err := transform.String(removeDiacriticsTransform, customCharsReplacement.Replace(s))
+	s, _, _ = strings.Cut(s, "@")
+	strDiacriticsRemoved, n, err := transform.String(transformDiacritics, customCharsReplacement.Replace(s))
 	if err != nil {
-		return "", fmt.Errorf("Failed to normalize character `%v` in provided username `%v`", s[n], s)
+		return "", fmt.Errorf("failed to normalize the string of provided username %q at position %d", s, n)
 	}
 	return replaceCharsHyphenRE.ReplaceAllLiteralString(removeCharsRE.ReplaceAllLiteralString(strDiacriticsRemoved, ""), "-"), nil
 }

models/user/user_test.go

@@ -506,15 +506,16 @@ func Test_NormalizeUserFromEmail(t *testing.T) {
 		Expected          string
 		IsNormalizedValid bool
 	}{
-		{"test", "test", true},
+		{"name@example.com", "name", true},
+		{"test'`´name", "testname", true},
 		{"Sinéad.O'Connor", "Sinead.OConnor", true},
 		{"Æsir", "AEsir", true},
-		// \u00e9\u0065\u0301
-		{"éé", "ee", true},
+		{"éé", "ee", true}, // \u00e9\u0065\u0301
 		{"Awareness Hub", "Awareness-Hub", true},
 		{"double__underscore", "double__underscore", false}, // We should consider squashing double non-alpha characters
 		{".bad.", ".bad.", false},
 		{"new😀user", "new😀user", false}, // No plans to support
+		{`"quoted"`, `"quoted"`, false}, // No plans to support
 	}
 	for _, testCase := range testCases {
 		normalizedName, err := user_model.NormalizeUserName(testCase.Input)