Re: [PATCH v4 1/1] Make gitdir work with worktrees, respect core.hooksPath, etc

> From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
> 
> Since v2.9.0, Git knows about the config variable core.hookspath that
> allows overriding the path to the directory containing the Git hooks.
> 
> Since v2.10.0, the `--git-path` option respects that config variable,
> too, so we may just as well use that command.
> 
> Other paths inside `.git` are equally subject to differ from
> `<gitdir>/<path>`, i.e. inside worktrees, where _some_ paths live in the
> "gitdir" and some live in the "commondir" (i.e. the "gitdir" of the main
> worktree).
> 
> For Git versions older than v2.5.0 (which was the first version to
> support the `--git-path` option for the `rev-parse` command), we simply
> fall back to the previous code.
> 
> An original patch handled only the hooksPath setting, however, during
> the code submission it was deemed better to fix all call to the `gitdir`
> function.
> 
> To avoid spawning a gazillion `git rev-parse --git-path` instances, we
> cache the returned paths, priming the cache upon startup in a single
> `git rev-parse invocation` with some paths (that have been
> determined via a typical startup and via grepping the source code for
> calls to the `gitdir` function).
> 
> This fixes https://github.com/git-for-windows/git/issues/1755
> 
> Initial-patch-by: Philipp Gortan <philipp@gortan.org>
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>  git-gui.sh | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 58 insertions(+), 4 deletions(-)
> 
> diff --git a/git-gui.sh b/git-gui.sh
> index fd476b6999..c684dc7ae1 100755
> --- a/git-gui.sh
> +++ b/git-gui.sh
> @@ -158,6 +158,7 @@ if {[tk windowingsystem] eq "aqua"} {
>  
>  set _appname {Git Gui}
>  set _gitdir {}
> +array set _gitdir_cache {}
>  set _gitworktree {}
>  set _isbare {}
>  set _gitexec {}
> @@ -197,12 +198,59 @@ proc appname {} {
>  	return $_appname
>  }
>  
> +proc prime_gitdir_cache {} {
> +	global _gitdir _gitdir_cache
> +
> +	set gitdir_cmd [list git rev-parse --git-dir]
> +
> +	# `--git-path` is only supported since Git v2.5.0
> +	if {[package vcompare $::_git_version 2.5.0] >= 0} {
> +		# This list was generated from a typical startup as well as from
> +		# grepping through Git GUI's source code.
> +		set gitdir_keys [list \
> +			CHERRY_PICK_HEAD FETCH_HEAD GITGUI_BCK GITGUI_EDITMSG \
> +			GITGUI_MSG HEAD hooks hooks/prepare-commit-msg \
> +			index.lock info info/exclude logs MERGE_HEAD MERGE_MSG \
> +			MERGE_RR objects "objects/4\[0-1\]/*" \
> +			"objects/4\[0-3\]/*" objects/info \
> +			objects/info/alternates objects/pack packed-refs \
> +			PREPARE_COMMIT_MSG rebase-merge/head-name remotes \
> +			rr-cache rr-cache/MERGE_RR SQUASH_MSG \
> +		]
> +
> +		foreach key $gitdir_keys {
> +			lappend gitdir_cmd --git-path $key
> +		}
> +	}
> +
> +	set i -1
> +	foreach path [split [eval $gitdir_cmd] "\n"] {
> +		if {$i eq -1} {
> +			set _gitdir $path
> +		} else {
> +			set _gitdir_cache([lindex $gitdir_keys $i]) $path
> +		}
> +		incr i
> +	}
> +}
> +
>  proc gitdir {args} {
> -	global _gitdir
> +	global _gitdir _gitdir_cache
> +
>  	if {$args eq {}} {
>  		return $_gitdir
>  	}
> -	return [eval [list file join $_gitdir] $args]
> +
> +	set args [eval [list file join] $args]
> +	if {![info exists _gitdir_cache($args)]} {
> +		if {[package vcompare $::_git_version 2.5.0] >= 0} {
> +			set _gitdir_cache($args) [git rev-parse --git-path $args]
> +		} else {
> +			set _gitdir_cache($args) [file join $_gitdir $args]
> +		}
> +	}
> +
> +	return $_gitdir_cache($args)
>  }
>  
>  proc gitexec {args} {
> @@ -1242,7 +1290,7 @@ if {[catch {
>  	&& [catch {
>  		# beware that from the .git dir this sets _gitdir to .
>  		# and _prefix to the empty string
> -		set _gitdir [git rev-parse --git-dir]
> +		prime_gitdir_cache
>  		set _prefix [git rev-parse --show-prefix]
>  	} err]} {
>  	load_config 1

> @@ -1453,10 +1501,16 @@ proc rescan {after {honor_trustmtime 1}} {
>  	global HEAD PARENT MERGE_HEAD commit_type
>  	global ui_index ui_workdir ui_comm
>  	global rescan_active file_states
> -	global repo_config
> +	global repo_config _gitdir_cache
>  
>  	if {$rescan_active > 0 || ![lock_index read]} return
>  
> +	# Only re-prime gitdir cache on a full rescan
> +	if {$after ne "ui_ready"} {

> +		array unset _gitdir_cache
> +		prime_gitdir_cache
> +	}
> +
>  	repository_state newType newHEAD newMERGE_HEAD
>  	if {[string match amend* $commit_type]
>  		&& $newType eq {normal}