Bump git-sync image from v3 to v4

[sed-i]

Enhancement Proposal

• To enable ssh with proxy, socat was added to the rock.
• We should probably replace the update-status approach with using git-sync's --period. Basically need to rewrite the charm.
• See migration guide below. The charm is affected by the breaking changes.

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Via Gemini (ref: https://github.com/thockin/git-sync/blob/master/v3-to-v4.md)

Summary of Changes (v3 -> v4)

The transition to v4 is a significant architectural overhaul focused on simplifying the internal sync loop and standardizing configuration.

• New Internal Loop:
  • v3: Distinguished between "initial clone" and "subsequent fetch," leading to complex logic and potential race conditions.
  • v4: Uses a "always fetch" strategy. Every pass is identical, efficient, and idempotent. It fetches the specific SHA needed rather than cloning branches, closing race conditions where references might change during the process.
• Flag Parsing (Breaking Change):
  • v3: Used Go's default flag style, accepting both -flag and --flag.
  • v4: Enforces standard flag syntax. Long flags must use double-dashes (--flag). Short flags use single-dashes (-v). Using -repo (single dash) will now fail.
• Logging:
  • v3: Text-based logs with standard Go flags (e.g., -logtostderr).
  • v4: Structured JSON logs are the default. All old log-formatting flags have been removed.
• Environment Variables:
  • The prefix has changed from GIT_SYNC_ to GITSYNC_. (e.g., GIT_SYNC_REPO is now GITSYNC_REPO).
• git-sync v3 would sync the entire history of the remote repo by default. v4 syncs just one commit by default.

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Migration Guide

1. Update Flag Syntax (Critical)

You must update all command-line arguments in your Deployment/Pod specs. Single-dash long flags are no longer supported.

• Wrong: -repo=...
• Right: --repo=...

2. Replace Renamed & Deprecated Flags

Update your configuration to use the new flag names. While v4 attempts to honor some old flags for backward compatibility, you should migrate to the new ones to ensure future stability.

v3 Flag (Deprecated)	v4 Flag (New)	Notes
--branch, --rev	--ref	Unified flag for branch, tag, or hash (full, not an abbreviated form). Default: HEAD.
--dest	--link	The name of the symlink to create.
--wait	--period	Now requires units (e.g., 10s, 100ms). v3 used float seconds (10.5); v4 uses Duration strings.
--max-sync-failures	--max-failures	Any non-recoverable error in the sync loop counts as a failure.
--timeout	--sync-timeout	Total time allowed for one complete sync. v3 using integer seconds, v4 takes Duration string.
--v	-v or --verbose	Controls log level.
-logtostderr, etc.	Removed	v4 always logs to stderr.
--ssh	Removed	The value of --repo determines when SSH is used.

3. Update Environment Variables

Switch your environment variables to the new GITSYNC_ prefix.

• Old: GIT_SYNC_REPO, GIT_SYNC_DEST, GIT_SYNC_USERNAME
• New: GITSYNC_REPO, GITSYNC_LINK, GITSYNC_USERNAME

Note: v4 generally prefers the new names but may fall back to checking GIT_SYNC_ variables if the new ones are unset.

4. Adjust Configuration Values

• Sync Period: If you used --wait=10 (seconds), you must now use --period=10s.
• Depth: v4 defaults to --depth=1 (shallow clone). If you need the full history (e.g., for running git log in your app), you must explicitly set --depth=0.
• Root Directory: If you are using the official image, the default root has moved from /tmp/git to /git. Ensure your volumeMounts match this new path or strictly define --root.

5. Verify Hook Behavior

Since v4 treats the startup sequence as a "sync" event, any configured webhooks or exec hooks will fire immediately upon container startup, even if the repo hasn't changed since the last run. Ensure your hook handlers are idempotent (safe to run multiple times).

6. Self-monitoring

The logging output for v3 was semi-free-form text. Log output in v4 is structured and rendered as strict JSON.

7. Root dir

git-sync v3 container images defaulted --root to "/tmp/git". In v4, that has moved to "/git". Users who mount a volume and expect to use the default --root must mount it on "/git".