[PATCH 0/2] Alternate index output file

Linus Torvalds [off-list ref] writes:

(Looking at git-commit.sh, the thing I *really* think we should do is to 
have a "GIT_INDEX_FILE_OUTPUT" environment variable that does locking on 
the input file, but writes the result to another file: rigth now 
git-commit.sh (a) wastes time copying the old index file by hand and (b) 
as a result doesn't even honor any locking on it.

I've done this with an environment variable, and it passes all
the tests, but I think for this application an environment is
really a mistake.  As expected, the change to git-commit.sh
looks like:

        diff --git a/git-commit.sh b/git-commit.sh
        index 292cf96..20c0dc8 100755
        --- a/git-commit.sh
        +++ b/git-commit.sh
        @@ -370,8 +370,8 @@ t,)
                        # the same way.
                        if test -z "$initial_commit"
                        then
        -			cp "$THIS_INDEX" "$TMP_INDEX"
        -			GIT_INDEX_FILE="$TMP_INDEX" git-read-tree -i -m HEAD
        +			_GIT_INDEX_OUTPUT="$TMP_INDEX" \
        +			GIT_INDEX_FILE="$THIS_INDEX" git-read-tree -i -m HEAD
                        else
                                rm -f "$TMP_INDEX"
                        fi || exit

However, I had to say something like this in the documentation:

        diff --git a/Documentation/git.txt b/Documentation/git.txt
        index dceacfa..bc025d6 100644
        --- a/Documentation/git.txt
        +++ b/Documentation/git.txt
        @@ -311,6 +311,15 @@ git so take care if using Cogito etc.
                index file. If not specified, the default of `$GIT_DIR/index`
                is used.

        +'_GIT_INDEX_OUTPUT'::
        +	When this environment is defined, plumbing level
        +	commands that update the index writes the resulting
        +	index to this file, instead of the usual
        +	`GIT_INDEX_FILE` (or its default `$GIT_DIR/index`).
        +	This is solely meant to be used by Porcelain to drive
        +	low-level plumbing.  Defining this in user's environment
        +	is always an error.  Do not use it.
        +
         'GIT_OBJECT_DIRECTORY'::
                If the object storage directory is specified via this
                environment variable then the sha1 directories are created

If a curious user has the environment variable set to something
other than the file GIT_INDEX_FILE points at, almost everything
will break.  This should instead be a command line parameter to
tell these plumbing commands to write the result in the named
file, to prevent stupid mistakes.