docker info: skip API connection if possible

[thaJeztah]

The docker info output contains both "local" and "remote" (daemon-side) information.
The API endpoint to collect daemon information (/info) is known to be "heavy",
and (depending on what information is needed) not needed.

This patch checks if the template (--format) used requires information from the
daemon, and if not, omits making an API request.

This will improve performance if (for example), the current "context" is requested
from docker info or if only plugin information is requested.

Before:

time docker info --format '{{range .ClientInfo.Plugins}}Plugin: {{.Name}}, {{end}}'
Plugin: buildx, Plugin: compose, Plugin: scan,
________________________________________________________
Executed in 301.91 millis fish external
usr time 168.64 millis 82.00 micros 168.56 millis
sys time 113.72 millis 811.00 micros 112.91 millis
time docker info --format '{{json .ClientInfo.Plugins}}'
time docker info --format '{{.ClientInfo.Context}}'
default
________________________________________________________
Executed in 334.38 millis fish external
usr time 177.23 millis 93.00 micros 177.13 millis
sys time 124.90 millis 927.00 micros 123.97 millis
docker context use remote-ssh-daemon
time docker info --format '{{.ClientInfo.Context}}'
remote-ssh-daemon
________________________________________________________
Executed in 1.22 secs fish external
usr time 116.93 millis 110.00 micros 116.82 millis
sys time 144.36 millis 887.00 micros 143.47 millis

And daemon logs:

Jul 06 12:42:12 remote-ssh-daemon dockerd[14377]: time="2021-07-06T12:42:12.139529947Z" level=debug msg="Calling HEAD /_ping"
Jul 06 12:42:12 remote-ssh-daemon dockerd[14377]: time="2021-07-06T12:42:12.140772052Z" level=debug msg="Calling HEAD /_ping"
Jul 06 12:42:12 remote-ssh-daemon dockerd[14377]: time="2021-07-06T12:42:12.163832016Z" level=debug msg="Calling GET /v1.41/info"

After:

time ./build/docker info --format '{{range .ClientInfo.Plugins}}Plugin: {{.Name}}, {{end}}'
Plugin: buildx, Plugin: compose, Plugin: scan,
________________________________________________________
Executed in 139.84 millis fish external
usr time 76.53 millis 62.00 micros 76.46 millis
sys time 69.25 millis 723.00 micros 68.53 millis
time ./build/docker info --format '{{.ClientInfo.Context}}'
default
________________________________________________________
Executed in 136.94 millis fish external
usr time 74.61 millis 74.00 micros 74.54 millis
sys time 65.77 millis 858.00 micros 64.91 millis
docker context use remote-ssh-daemon
time ./build/docker info --format '{{.ClientInfo.Context}}'
remote-ssh-daemon
________________________________________________________
Executed in 1.02 secs fish external
usr time 74.25 millis 76.00 micros 74.17 millis
sys time 65.09 millis 643.00 micros 64.44 millis

And daemon logs:

Jul 06 12:42:55 remote-ssh-daemon dockerd[14377]: time="2021-07-06T12:42:55.313654687Z" level=debug msg="Calling HEAD /_ping"
Jul 06 12:42:55 remote-ssh-daemon dockerd[14377]: time="2021-07-06T12:42:55.314811624Z" level=debug msg="Calling HEAD /_ping"

- Description for the changelog

Improve performance of `docker info` if a custom `--format` is used that only uses local information. Previously, the `docker info` command would unconditionally make an API call to request daemon system information. With this change, the CLI only uses the daemon API if it detects that information from the daemon is needed.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Merging #3179 (d738e7c) into master (d7a311b) will increase coverage by 0.01%.
The diff coverage is 57.89%.

@@ Coverage Diff @@## master #3179 +/- ##
==========================================
+ Coverage 57.06% 57.08% +0.01% 
==========================================
Files 299 299 Lines 18742 18756 +14 ==========================================
+ Hits 10696 10707 +11 - Misses 7176 7178 +2 - Partials 870 871 +1 

[thaJeztah]

@silvin-lubecki@mat007@StefanScherer ptal

[StefanScherer]

Looks good and makes sense.

I wonder if we could avoid any connection to docker daemon in such cases? 😃
Then listing the plugins would be totally independent from a running engine or not.

[thaJeztah]

Yes, looking at that; the /_ping call is done elsewhere (during initialization); I have a branch that removes it, but ISTR we removed it before, and there was some reason it had to be done there.

Won't do that as part of this PR (for that reason)

[StefanScherer]

LGTM

[silvin-lubecki]

LGTM