feat: add GitHub OAuth2 provider branch

server/internal/auth/github.go

@@ -2,19 +2,97 @@ package auth
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
+	"net/http"
 
 	"gitea.hostxtra.co.uk/mrhid6/vantage/server/internal/models"
 	"golang.org/x/oauth2"
 )
 
-// githubOAuthConfig and githubIdentity are temporary stubs. Task 7 replaces
-// this file with the real GitHub OAuth2 provider implementation.
+// githubAPIBase is a variable rather than a constant so tests can point it at
+// an httptest server. Nothing in production reassigns it.
+var githubAPIBase = "https://api.github.com"
 
+const (
+	githubAuthURL  = "https://github.com/login/oauth/authorize"
+	githubTokenURL = "https://github.com/login/oauth/access_token"
+)
+
+type githubEmail struct {
+	Email    string `json:"email"`
+	Primary  bool   `json:"primary"`
+	Verified bool   `json:"verified"`
+}
+
+// githubOAuthConfig builds the OAuth2 config for a GitHub provider. GitHub has
+// no discovery document, so the endpoints are constants rather than fetched.
 func githubOAuthConfig(p *models.AuthProvider, secret, redirectURL string) *oauth2.Config {
-	return nil
+	return &oauth2.Config{
+		ClientID:     p.ClientID,
+		ClientSecret: secret,
+		RedirectURL:  redirectURL,
+		Scopes:       p.Scopes,
+		Endpoint: oauth2.Endpoint{
+			AuthURL:  githubAuthURL,
+			TokenURL: githubTokenURL,
+		},
+	}
 }
 
-func githubIdentity(ctx context.Context, cfg *oauth2.Config, token *oauth2.Token) (string, string, error) {
-	return "", "", errors.New("not implemented")
+// selectGitHubEmail requires an address that is both primary and verified.
+//
+// Verified alone is not enough: a non-primary address is one the person happens
+// to have proved, not the one they present as themselves. Primary alone is far
+// worse — an unverified address is not proof of control at all, and accepting
+// one would let anyone with a GitHub account claim any address in the instance.
+func selectGitHubEmail(emails []githubEmail) (string, error) {
+	for _, e := range emails {
+		if e.Primary && e.Verified && e.Email != "" {
+			return e.Email, nil
+		}
+	}
+	return "", errors.New("no primary verified email address on the GitHub account")
+}
+
+func githubGetJSON(ctx context.Context, client *http.Client, url string, out any) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Accept", "application/vnd.github+json")
+	resp, err := client.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return errors.New("github api returned " + resp.Status)
+	}
+	return json.NewDecoder(resp.Body).Decode(out)
+}
+
+// githubIdentity resolves the signed-in GitHub account to an email and a name.
+//
+// The name is best-effort: it is cosmetic, and a failing /user must not fail a
+// sign-in whose identity is already established.
+func githubIdentity(ctx context.Context, cfg *oauth2.Config, token *oauth2.Token) (string, string, error) {
+	client := cfg.Client(ctx, token)
+
+	var emails []githubEmail
+	if err := githubGetJSON(ctx, client, githubAPIBase+"/user/emails", &emails); err != nil {
+		return "", "", err
+	}
+	email, err := selectGitHubEmail(emails)
+	if err != nil {
+		return "", "", err
+	}
+
+	var user struct {
+		Name string `json:"name"`
+	}
+	if err := githubGetJSON(ctx, client, githubAPIBase+"/user", &user); err != nil {
+		return email, "", nil
+	}
+	return email, user.Name, nil
 }