docker info: skip API connection if possible

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"

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/system/info.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"io/ioutil"
+	"regexp"
 	"sort"
 	"strings"
 
@@ -64,13 +66,6 @@ func NewInfoCommand(dockerCli command.Cli) *cobra.Command {
 func runInfo(cmd *cobra.Command, dockerCli command.Cli, opts *infoOptions) error {
 	var info info
 
-	ctx := context.Background()
-	if dinfo, err := dockerCli.Client().Info(ctx); err == nil {
-		info.Info = &dinfo
-	} else {
-		info.ServerErrors = append(info.ServerErrors, err.Error())
-	}
-
 	info.ClientInfo = &clientInfo{
 		Context: dockerCli.CurrentContext(),
 		Debug:   debug.IsEnabled(),
@@ -81,12 +76,60 @@ func runInfo(cmd *cobra.Command, dockerCli command.Cli, opts *infoOptions) error
 		info.ClientErrors = append(info.ClientErrors, err.Error())
 	}
 
+	if needsServerInfo(opts.format, info) {
+		ctx := context.Background()
+		if dinfo, err := dockerCli.Client().Info(ctx); err == nil {
+			info.Info = &dinfo
+		} else {
+			info.ServerErrors = append(info.ServerErrors, err.Error())
+		}
+	}
+
 	if opts.format == "" {
 		return prettyPrintInfo(dockerCli, info)
 	}
 	return formatInfo(dockerCli, info, opts.format)
 }
 
+// placeHolders does a rudimentary match for possible placeholders in a
+// template, matching a '.', followed by an letter (a-z/A-Z).
+var placeHolders = regexp.MustCompile(`\.[a-zA-Z]`)
+
+// needsServerInfo detects if the given template uses any server information.
+// If only client-side information is used in the template, we can skip
+// connecting to the daemon. This allows (e.g.) to only get cli-plugin
+// information, without also making a (potentially expensive) API call.
+func needsServerInfo(template string, info info) bool {
+	if len(template) == 0 || placeHolders.FindString(template) == "" {
+		// The template is empty, or does not contain formatting fields
+		// (e.g. `table` or `raw` or `{{ json .}}`). Assume we need server-side
+		// information to render it.
+		return true
+	}
+
+	// A template is provided and has at least one field set.
+	tmpl, err := templates.NewParse("", template)
+	if err != nil {
+		// ignore parsing errors here, and let regular code handle them
+		return true
+	}
+
+	type sparseInfo struct {
+		ClientInfo   *clientInfo `json:",omitempty"`
+		ClientErrors []string    `json:",omitempty"`
+	}
+
+	// This constructs an "info" object that only has the client-side fields.
+	err = tmpl.Execute(ioutil.Discard, sparseInfo{
+		ClientInfo:   info.ClientInfo,
+		ClientErrors: info.ClientErrors,
+	})
+	// If executing the template failed, it means the template needs
+	// server-side information as well. If it succeeded without server-side
+	// information, we don't need to make API calls to collect that information.
+	return err != nil
+}
+
 func prettyPrintInfo(dockerCli command.Cli, info info) error {
 	fmt.Fprintln(dockerCli.Out(), "Client:")
 	if info.ClientInfo != nil {

cli/command/system/info_test.go

@@ -420,3 +420,55 @@ func TestFormatInfo(t *testing.T) {
 		})
 	}
 }
+
+func TestNeedsServerInfo(t *testing.T) {
+	tests := []struct {
+		doc      string
+		template string
+		expected bool
+	}{
+		{
+			doc:      "no template",
+			template: "",
+			expected: true,
+		},
+		{
+			doc:      "JSON",
+			template: "json",
+			expected: true,
+		},
+		{
+			doc:      "JSON (all fields)",
+			template: "{{json .}}",
+			expected: true,
+		},
+		{
+			doc:      "JSON (Server ID)",
+			template: "{{json .ID}}",
+			expected: true,
+		},
+		{
+			doc:      "ClientInfo",
+			template: "{{json .ClientInfo}}",
+			expected: false,
+		},
+		{
+			doc:      "JSON ClientInfo",
+			template: "{{json .ClientInfo}}",
+			expected: false,
+		},
+		{
+			doc:      "JSON (Active context)",
+			template: "{{json .ClientInfo.Context}}",
+			expected: false,
+		},
+	}
+
+	inf := info{ClientInfo: &clientInfo{}}
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.doc, func(t *testing.T) {
+			assert.Equal(t, needsServerInfo(tc.template, inf), tc.expected)
+		})
+	}
+}