fix(git-auto-fetch): background `git-fetch-all` and other fixes (#9468)

plugins/git-auto-fetch/README.md

 # Git auto-fetch
 
-Automatically fetches all changes from all remotes while you are working in git-initialized directory.
+Automatically fetches all changes from all remotes while you are working in a git-initialized directory.
 
-#### Usage
-
-Add `git-auto-fetch` to the plugins array in your zshrc file:
+To use it, add `git-auto-fetch` to the plugins array in your zshrc file:
 
 ```shell
 plugins=(... git-auto-fetch)
 ```
 
-Every time you launch a command in your shell all remotes will be fetched in background.
-By default autofetch will be triggered only if last fetch was done at least 60 seconds ago.
-You can change fetch interval in your .zshrc:
-```
-GIT_AUTO_FETCH_INTERVAL=1200 #in seconds
+## Usage
+
+Every time the command prompt is shown all remotes will be fetched in the background. By default,
+`git-auto-fetch` will be triggered only if the last auto-fetch was done at least 60 seconds ago.
+You can change the fetch interval in your .zshrc:
+
+```sh
+GIT_AUTO_FETCH_INTERVAL=1200 # in seconds
 ```
-Log of `git fetch --all` will be saved into `.git/FETCH_LOG`
 
+A log of `git fetch --all` will be saved in `.git/FETCH_LOG`.
+
+## Toggle auto-fetch per folder
 
-#### Toggle auto fetch per folder
-If you are using mobile connection or for any other reason you can disable git-auto-fetch for any folder:
+If you are using a mobile connection or for any other reason you can disable git-auto-fetch
+for any folder:
 
 ```shell
 $ cd to/your/project
@@ -29,3 +32,19 @@ disabled
 $ git-auto-fetch
 enabled
 ```
+
+## Caveats
+
+Automatically fetching all changes defeats the purpose of `git push --force-with-lease`,
+and makes it behave like `git push --force` in some cases. For example:
+
+Consider that you made some changes and possibly rebased some stuff, which means you'll
+need to use `--force-with-lease` to overwrite the remote history of a branch. Between the
+time when you make the changes (maybe do a `git log`) and the time when you `git push`,
+it's possible that someone else updates the branch you're working on.
+
+If `git-auto-fetch` triggers then, you'll have fetched the remote changes without knowing
+it, and even though you're running the push with `--force-with-lease`, git will overwrite
+the recent changes because you already have them in your local repository. The
+[`git push --force-with-lease` docs](https://git-scm.com/docs/git-push) talk about possible
+solutions to this problem.

plugins/git-auto-fetch/git-auto-fetch.plugin.zsh

-GIT_AUTO_FETCH_INTERVAL=${GIT_AUTO_FETCH_INTERVAL:=60}
+# Default auto-fetch interval: 60 seconds
+: ${GIT_AUTO_FETCH_INTERVAL:=60}
+
+# Necessary for the git-fetch-all function
+zmodload zsh/datetime zsh/stat
 
 function git-fetch-all {
-  (`command git rev-parse --is-inside-work-tree 2>/dev/null` &&
-  dir=`command git rev-parse --git-dir` &&
-  [[ ! -f $dir/NO_AUTO_FETCH ]] &&
-  (( `date +%s` - `date -r $dir/FETCH_LOG +%s 2>/dev/null || echo 0` > $GIT_AUTO_FETCH_INTERVAL )) &&
-  GIT_SSH_COMMAND="command ssh -o BatchMode=yes" \
-    command git fetch --all 2>/dev/null &>! $dir/FETCH_LOG &)
+  (
+    # Get git root directory
+    if ! gitdir="$(command git rev-parse --git-dir 2>/dev/null)"; then
+      return 0
+    fi
+
+    # Do nothing if auto-fetch disabled
+    if [[ -z "$gitdir" || -f "$gitdir/NO_AUTO_FETCH" ]]; then
+      return 0
+    fi
+
+    # Get time (seconds) when auto-fetch was last run
+    lastrun="$(zstat +mtime "$gitdir/FETCH_LOG" 2>/dev/null || echo 0)"
+    # Do nothing if not enough time has passed since last auto-fetch
+    if (( EPOCHSECONDS - lastrun < $GIT_AUTO_FETCH_INTERVAL )); then
+      return 0
+    fi
+
+    # Fetch all remotes (avoid ssh passphrase prompt)
+    GIT_SSH_COMMAND="command ssh -o BatchMode=yes" \
+      command git fetch --all 2>/dev/null &>! "$gitdir/FETCH_LOG"
+  ) &|
 }
 
 function git-auto-fetch {
-  `command git rev-parse --is-inside-work-tree 2>/dev/null` || return
-  guard="`command git rev-parse --git-dir`/NO_AUTO_FETCH"
+  # Do nothing if not in a git repository
+  command git rev-parse --is-inside-work-tree &>/dev/null || return 0
 
-  (rm $guard 2>/dev/null &&
-    echo "${fg_bold[green]}enabled${reset_color}") ||
-  (touch $guard &&
-    echo "${fg_bold[red]}disabled${reset_color}")
+  # Remove or create guard file depending on its existence
+  local guard="$(command git rev-parse --git-dir)/NO_AUTO_FETCH"
+  if [[ -f "$guard" ]]; then
+    command rm "$guard" && echo "${fg_bold[green]}enabled${reset_color}"
+  else
+    command touch "$guard" && echo "${fg_bold[red]}disabled${reset_color}"
+  fi
 }
 
-# Override zle-line-init if it exists
-if (( $+functions[zle-line-init] )); then
-  eval "override-git-auto-fetch-$(declare -f zle-line-init)"
-  
-  function zle-line-init () {
-    git-fetch-all
-    override-git-auto-fetch-zle-line-init
-  }
-else
-  function zle-line-init () {
-    git-fetch-all
-  }
-fi
-  
-zle -N zle-line-init
+# zle-line-init widget (don't redefine if already defined)
+(( ! ${+functions[_git-auto-fetch_zle-line-init]} )) || return 0
+
+case "$widgets[zle-line-init]" in
+  # Simply define the function if zle-line-init doesn't yet exist
+  builtin|"") function _git-auto-fetch_zle-line-init() {
+      git-fetch-all
+    } ;;
+  # Override the current zle-line-init widget, calling the old one
+  user:*) zle -N _git-auto-fetch_orig_zle-line-init "${widgets[zle-line-init]#user:}"
+    function _git-auto-fetch_zle-line-init() {
+      git-fetch-all
+      zle _git-auto-fetch_orig_zle-line-init -- "$@"
+    } ;;
+esac
+
+zle -N zle-line-init _git-auto-fetch_zle-line-init