fix(login): avoid SSH passphrase prompt with ssh-agent logins (#1102)

Fixes #866

## Problem

`tea` still prompts for an SSH key passphrase for logins created with
`--ssh-agent-key` or `--ssh-agent-principal`. During `tea login add`, tea
auto-discovers a matching private key in `~/.ssh` even when the login is
configured to use the running ssh-agent. That on-disk key is stored in
`ssh_key`, so later `Login.Client()` asks for its passphrase and tells the SDK
to load the file from disk instead of signing through the agent.

## Changes

- Stop auto-discovering a private key when the login uses the ssh-agent.
- Add `Login.SSHKeyPath()` and use it for HTTPSign and git auth, so agent
  logins pass an empty key path and the SDK talks to `ssh-agent`.
- Prefer SSH remotes for agent logins, matching key-file logins.
- Add unit tests for the new key-path and auto-discovery behavior.

## Testing

- `go test ./modules/config ./modules/task ./modules/git`
- `go build ./...`

---------

Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/tea/pulls/1102
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/task/login_create.go

@@ -153,7 +153,7 @@ func CreateLogin(ctx stdctx.Context, name, token, user, passwd, otp, scopes, ssh
 	// so we just use the host
 	login.SSHHost = serverURL.Host
 
-	if len(sshKey) == 0 {
+	if shouldFindSSHKey(sshKey, sshAgent) {
 		login.SSHKey, err = findSSHKey(ctx, client)
 		if err != nil {
 			fmt.Printf("Warning: problem while finding a SSH key: %s\n", err)
@@ -176,6 +176,10 @@ func CreateLogin(ctx stdctx.Context, name, token, user, passwd, otp, scopes, ssh
 	return nil
 }
 
+func shouldFindSSHKey(sshKey string, sshAgent bool) bool {
+	return sshKey == "" && !sshAgent
+}
+
 func shouldCheckTokenUniqueness(token string, sshAgent bool, sshKey, sshCertPrincipal, sshKeyFingerprint string) bool {
 	if sshAgent || sshKey != "" || sshCertPrincipal != "" || sshKeyFingerprint != "" {
 		return false

modules/task/login_create_test.go

@@ -55,3 +55,35 @@ func TestShouldCheckTokenUniqueness(t *testing.T) {
 		})
 	}
 }
+
+func TestShouldFindSSHKey(t *testing.T) {
+	tests := []struct {
+		name     string
+		sshKey   string
+		sshAgent bool
+		want     bool
+	}{
+		{
+			name:   "explicit key",
+			sshKey: "~/.ssh/id_ed25519",
+			want:   false,
+		},
+		{
+			name:     "ssh agent",
+			sshAgent: true,
+			want:     false,
+		},
+		{
+			name: "no key configured",
+			want: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := shouldFindSSHKey(tt.sshKey, tt.sshAgent); got != tt.want {
+				t.Fatalf("expected %v, got %v", tt.want, got)
+			}
+		})
+	}
+}

modules/task/pull_checkout.go

@@ -61,10 +61,9 @@ func remoteURLForPR(login *config.Login, pr *gitea.PullRequest) string {
 	if isRemoteDeleted(pr) {
 		repo = pr.Base.Repository
 	}
-	if len(login.SSHKey) != 0 {
-		// login.SSHKey is nonempty, if user specified a key manually or we automatically
-		// found a matching private key on this machine during login creation.
-		// this means, we are very likely to have a working ssh setup.
+	if login.SSHKeyPath() != "" || login.SSHAgent {
+		// Use SSH when a key file is configured, or when the login authenticates
+		// through a running ssh-agent. In both cases we have a working SSH setup.
 		return repo.SSHURL
 	}
 	return repo.CloneURL
@@ -84,7 +83,7 @@ func doPRFetch(
 	if err != nil {
 		return "", err
 	}
-	auth, err := local_git.GetAuthForURL(url, login.GetAccessToken(), login.SSHKey, callback)
+	auth, err := local_git.GetAuthForURL(url, login.GetAccessToken(), login.SSHKeyPath(), callback)
 	if err != nil {
 		return "", err
 	}

modules/task/pull_clean.go

@@ -95,7 +95,7 @@ call me again with the --ignore-sha flag`, remoteBranch)
 		if urlErr != nil {
 			return urlErr
 		}
-		auth, authErr := local_git.GetAuthForURL(url, login.GetAccessToken(), login.SSHKey, callback)
+		auth, authErr := local_git.GetAuthForURL(url, login.GetAccessToken(), login.SSHKeyPath(), callback)
 		if authErr != nil {
 			return authErr
 		}

modules/task/pull_create.go

@@ -210,7 +210,7 @@ func CreateAgitFlowPull(requestCtx stdctx.Context, ctx *context.TeaContext, remo
 		return err
 	}
 
-	auth, err := local_git.GetAuthForURL(url, ctx.Login.GetAccessToken(), ctx.Login.SSHKey, callback)
+	auth, err := local_git.GetAuthForURL(url, ctx.Login.GetAccessToken(), ctx.Login.SSHKeyPath(), callback)
 	if err != nil {
 		return err
 	}

modules/task/repo_clone.go

@@ -33,7 +33,7 @@ func RepoClone(
 		return nil, err
 	}
 
-	auth, err := local_git.GetAuthForURL(originURL, login.GetAccessToken(), login.SSHKey, callback)
+	auth, err := local_git.GetAuthForURL(originURL, login.GetAccessToken(), login.SSHKeyPath(), callback)
 	if err != nil {
 		return nil, err
 	}
@@ -68,7 +68,7 @@ func RepoClone(
 
 func cloneURL(repo *gitea.Repository, login *config.Login) (*url.URL, error) {
 	urlStr := repo.CloneURL
-	if login.SSHKey != "" {
+	if login.SSHKeyPath() != "" || login.SSHAgent {
 		urlStr = repo.SSHURL
 	}
 	return local_git.ParseURL(urlStr)