docstore/mongodocstore: Update Mongo dialer when MONGO_SERVER_URL rotates

[concaf]

Prior to this commit, the dialer for MongoDB was generated once from MONGO_SERVER_URL environment variable but was never updated even when the environment variable was updated in subsequent calls. While this works fine when MONGO_SERVER_URL is not expected to update, but as MONGO_SERVER_URL also contains the credentials to connect to MongoDB, it's a fairly common use case to rotate these credentials (and hence the environment variable) at regular intervals.

This commit fixes that and updates the dialer when MONGO_SERVER_URL is updated.

This PR blocks tektoncd/chains#1089

[vangent]

This is not thread-safe (OpenCollectionURL can be called concurrently on the same o).

You can protect it with a sync.Mutex.

[concaf]

makes sense, updated 👍🏼

[vangent]

This doesn't work. If MONGO_SERVER_URL isn't set, then both of these will be the empty string and we'll fall through without returning an error.

How about something like if o.opener == nil || serverURL != o.openerServerURL ... ?

[concaf]

good catch 👍🏼

i've updated this to check for an empty MONGO_SERVER_URL outside the if condition, so it should fail each time the env var is empty

	if currentEnv == "" {
		o.err = errors.New("MONGO_SERVER_URL environment variable is not set")
		return nil, fmt.Errorf("open collection %s: %v", u, o.err)
	}

	// If MONGO_SERVER_URL has been updated, then update o.opener as well
	if currentEnv != o.mongoServerURL {
		client, err := Dial(ctx, currentEnv)
...
...

wdyt?

[concaf]

@vangent thanks for the quick review 🚀 🙂 - i've updated the PR and also added some tests.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.20%. Comparing base (3f641fd) to head (024ea8c).

Files	Patch %	Lines
docstore/mongodocstore/urls.go	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3429      +/-   ##
==========================================
+ Coverage   73.17%   73.20%   +0.03%     
==========================================
  Files         113      113              
  Lines       14872    14873       +1     
==========================================
+ Hits        10882    10888       +6     
+ Misses       3216     3213       -3     
+ Partials      774      772       -2     

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