PostIndexChangedHook: skip notification for non-canonical indexes

[tyrielv]

PostIndexChangedHook: skip notification for non-canonical indexes

Problem

Git fires the post-index-change hook for every index write, including temp indexes created via GIT_INDEX_FILE redirect (e.g. read-tree --index-output, git add with a temp index). The GVFS post-index-change.exe hook connects to the mount process via named pipe and waits for acknowledgment — a round-trip that takes ~5 seconds for large repos.

Root Cause

Layer	File	Issue
Git	read-cache.c → do_write_locked_index()	Fires run_hooks_l("post-index-change", ...) unconditionally — no check for temp vs. real index
GVFS	GVFS.PostIndexChangedHook/main.cpp	Always connects to mount pipe — no guard for non-canonical indexes

Why COMMAND_HOOK_LOCK doesn't help

The GVFS command lock (pre/post-command in GVFS.Hooks.exe) is a separate mechanism. post-index-change is fired independently by Git's hook runner. Additionally, hook.c's handle_hook_replacement() explicitly states: "We don't skip post-index-change hooks that exist."

Fix

Added IsNonCanonicalIndex() early-exit check in main.cpp:

1. Read GIT_INDEX_FILE from environment
2. If not set → this is the default index → proceed normally
3. If set → compare against $GIT_DIR/index (normalized separators, case-insensitive)
4. If they differ → exit 0 immediately, no pipe round-trip

Effect: Hook process still spawns (Git-side fires it unconditionally) but exits in <1ms instead of ~5s.

Ideal long-term fix

I think the correct fix would be in Git's do_write_locked_index() (microsoft/git fork, read-cache.c) to skip firing the hook entirely for non-default indexes, avoiding the process spawn altogether. This PR is the GVFS-side mitigation.

Testing

• ✅ Full solution build (including native C++ projects)
• ✅ 793/793 unit tests passed
• Manual verification: set GIT_INDEX_FILE to a temp path → hook exits immediately