Async git prompt (#288)

prompt/async_git_prompt/README.md

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+This module exports commands for creating a nushell prompt that computes git status (staged and
+unstaged changes) asynchronously. This can be useful in large git repos when it is slow to obtain
+this information synchronously.
+
+To use this module:
+
+0. Place the file `async-git-prompt.nu` in the `$nu.config-path` directory (this is the directory
+   containing `init.nu` and `env.nu`).
+
+1. Use the command `async-git-prompt-string` in your own `PROMPT_COMMAND` (the file prompt.nu
+   contains an example of doing this.)
+   At this point, your prompt will be computing the information synchronously, because the cache
+   file does not yet exist.
+
+2. In a repo where git is slow, run the command `async-git-prompt-refresh-cache`.
+   Now, your prompt will be fast, but it also won't update automatically. You could investigate a good
+   way to invalidate the cache automatically, but the manual alternative is:
+
+3. Whenever you think your prompt might be stale, re-run the command `async-git-prompt-refresh-cache`.
+   Your prompt will update on one of the next times that you hit <enter>.
+
+4. It will probably be convenient to alias this, e.g.
+
+   ```nu
+   alias r = async-git-prompt-refresh-cache
+   ```
+
+5. To go back to synchronous mode, run `async-git-prompt-delete-cache`.

prompt/async_git_prompt/async-git-prompt.nu

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+# This module exports commands for creating a nushell prompt that computes git status (staged and
+# unstaged changes) asynchronously. This can be useful in large git repos when it is slow to obtain
+# this information synchronously.
+
+# See README.md for usage.
+
+def unstaged-symbol [] { 'અ' }
+def staged-symbol [] { 'જ' }
+def in-progress-symbol [] { '…' }
+def cached-result-symbol [] { $"·" } #〈
+def cache-file [] { '.nu-async-git-prompt-cache'}
+
+def do-async [commands: string] {
+    bash -c $"nu -c '($commands)' &"
+}
+
+export def async-git-prompt-string [] {
+    let cache_path = (cache-path)
+    if ($cache_path | empty?) {
+        ""
+    } else if ($cache_path | path exists) {
+        $"(cached-result-symbol)(open $cache_path | str trim)"
+    } else {
+        async-git-prompt-compute-sync
+    }
+}
+
+export def async-git-prompt-compute-sync [] {
+    let unstaged = {
+        let symbol = if ((git diff --quiet | complete).exit_code == 1) {
+            (unstaged-symbol)
+        } else {
+            ''
+        }
+        { unstaged: $symbol}
+    }
+    let staged = {
+        let symbol = if ((git diff --cached --quiet | complete).exit_code == 1) {
+            (staged-symbol)
+        } else {
+            ''
+        }
+        { staged: $symbol}
+    }
+    # Execute the two slow git commands in parallel and merge the results into a single record
+    let symbols = ([ $unstaged $staged ] | par-each { |it| do $it } | reduce {|a b| $a | merge {$b}})
+
+    $"($symbols | get 'unstaged') ($symbols | get 'staged')" | str trim
+}
+
+export def async-git-prompt-refresh-cache [] {
+    let cache_path = (cache-path)
+    if ($cache_path != null) {
+        echo (in-progress-symbol) | save $cache_path
+        do-async $"use ($nu.config-path | path expand | path dirname)/async-git-prompt.nu *; async-git-prompt-compute-sync | save ($cache_path)"
+    }
+}
+
+export def async-git-prompt-delete-cache [] {
+    let cache_path = (cache-path)
+    if ($cache_path != null) {
+        rm -f $cache_path
+    }
+}
+
+def cache-path [] {
+    let dir = if ('.git' | path exists) {
+        '.'
+    } else {
+        do -i { git rev-parse --show-toplevel | str trim -r }
+    }
+    if ($dir | empty?) {
+        null
+    } else {
+        $dir | path join (cache-file)
+    }
+}

prompt/async_git_prompt/prompt.nu

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+# This file contains an example nushell prompt, making use of the async-git-prompt module.
+use async-git-prompt.nu *
+
+def prompt-concat [parts: table] {
+    $parts
+    | where (not ($it.text | empty?))
+    | each { |it| $"($it.color)($it.text)" }
+    | str collect ' '
+}
+
+def prompt-git-branch [] {
+    do -i { git rev-parse --abbrev-ref HEAD | str trim -r}
+}
+
+def prompt-create-left-prompt [] {
+    let pwd = ($env.PWD | str replace $env.HOME '~')
+    prompt-concat [
+        {text: $pwd, color: (ansi green_bold)}
+        {text: (prompt-git-branch), color: (ansi blue_bold)}
+        {text: (async-git-prompt-string), color: (ansi green_bold)}
+    ]
+}
+
+def prompt-create-right-prompt [] {
+    $nothing
+}
+
+let-env PROMPT_COMMAND = { prompt-create-left-prompt }
+let-env PROMPT_COMMAND_RIGHT = { prompt-create-right-prompt }
+let-env PROMPT_INDICATOR = { $" (ansi green_bold)〉" }