How Sign In Prompts Work

This chapter explains how git's and ssh's questions (a username, a password, an SSH passphrase, a new host key) reach a dialog in the app. The user side is in Sign-In Prompts; running git is in Backend.

Why we need it

Writes go through the git CLI (git/cli.rs), with GIT_TERMINAL_PROMPT=0 so git never waits for a terminal the app does not have. A remote that asked for a login therefore failed at once, and the user saw "could not read Username ... terminal prompts disabled" in a toast, with no way to answer. The user asked for a popup that asks what git needs.

How it works

Git and ssh both support an askpass program: when they need an answer and have no terminal, they run the program named in GIT_ASKPASS or SSH_ASKPASS with the prompt as its only argument and read the answer from its stdout. Our own executable is that program.

sequenceDiagram
  participant G as git or ssh
  participant C as nibtree (askpass client)
  participant S as App: askpass.rs server
  participant W as Window: AskpassDialog
  G->>C: run with prompt "Username for 'https://github.com': "
  C->>S: JSON line: token, prompt, repo path (127.0.0.1)
  S->>W: event askpass-request (id, prompt, repoPath)
  W->>S: askpass_respond(id, answer)
  S->>C: JSON line: answer
  S-->>W: event askpass-done (other windows close it)
  C->>G: answer on stdout, exit 0 (exit 1 when cancelled)

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The server (src-tauri/src/askpass.rs)

• install takes the hooks that show a question (commands/askpass.rs): ask sends askpass-request to the windows showing the repository through emit_for_path (every window when none does), and done sends askpass-done to all.
• The first command that may ask starts a listener on a random loopback port with a random token (getrandom). Each connection carries one JSON line; a wrong token gets no answer.
• ask_user registers a channel under a new id, calls ask, and waits up to five minutes. askpass_respond from the page sends the answer (None for Cancel).
• apply(command, repo_path) sets GIT_ASKPASS and SSH_ASKPASS to current_exe(), SSH_ASKPASS_REQUIRE=force (OpenSSH 8.4 and later use askpass without a terminal only then), and GM_ASKPASS_PORT, GM_ASKPASS_TOKEN and GM_ASKPASS_REPO.

apply runs in cli::run_streaming_with_env and cancel::run_streaming, the runners of push, pull, fetch, clone, submodule updates and LFS. Background auto fetch uses cli::run_raw, so it never opens a dialog.

The client

lib.rs calls askpass::client_main before anything else. With the two variables set, the executable connects, sends the prompt, prints the answer with a newline (git and ssh read up to it) and exits, before Tauri starts, so no window or Dock icon appears.

The dialog

askpassPrompt.ts reads the prompt: git's "Username for" and "Password for" (with the host and user), ssh's "Enter passphrase for key", "user@host's password" and the yes/no question about a new host key. Anything else is a generic question, hidden when it mentions a password, token or PIN.

askpass.svelte.ts queues the questions of this window. For a username, AskpassDialog.svelte asks the password too and keeps it in memory for up to a minute: rememberedPassword answers git's following password question for the same host and user without showing a second dialog. The dialog stops key events from reaching window shortcuts and closes on Esc (Cancel).

Where the code lives

File	What it does
src-tauri/src/askpass.rs	Client, loopback server, apply, respond
src-tauri/src/commands/askpass.rs	WindowHooks (events) and askpass_respond
src-tauri/src/lib.rs	client_main at start, install in setup
src-tauri/src/git/cli.rs, git/cancel.rs	apply for commands the user starts
src/lib/askpass/askpassPrompt.ts	Reading prompts, remembering the password
src/lib/askpass/askpass.svelte.ts	The queue of questions
src/lib/askpass/AskpassDialog.svelte	The dialog, mounted once in App.svelte: the UI library's Dialog with urgent, so it shows over any other dialog and window shortcuts never act behind it

Design decisions

Our executable as the askpass program. A separate helper binary would need its own signing and bundling on every platform. The app binary is already there, and leaving before Tauri starts keeps it quick.

Loopback TCP with a token. It works the same on macOS, Windows and Linux. The token lives only in the environment of the git process, so no other program can ask questions or read answers.

Nothing is stored by us. Git passes the login to the user's credential helper (the macOS keychain, Git Credential Manager), which already handles storage well. The app keeps a typed password only until git's next question, at most a minute.

Only commands the user starts. A login dialog that pops up from a background fetch would be confusing, so auto fetch keeps failing quietly.

Tests

• src-tauri/src/askpass.rs: a prompt is answered through the server, a wrong token gets nothing, a cancelled prompt gets None, and git credential fill gets its username and password through the same environment and protocol.
• src/lib/askpass/askpassPrompt.test.ts: every prompt shape, and when a remembered password answers.
• askpass-sign-in in scripts/screenshots.ts runs it all against a local server that always answers 401.

Keeping this page in sync

• Update this page and Sign-In Prompts when the prompts read, the runners that call apply or the dialog change.
• Retake askpass-sign-in.png. See Docs and Screenshots.