ci: add skip directives for long tests (#3151)

This PR introduces many CI optimizations:

1. The `[ci-skip]` PR body directive to skip the Postgres and end to end tests
2. Improved caching that cuts the Go test matrix in half
3. Increasing Go test parallelism for ~20% gains
4. Enable caching in webpack (4x frontend build)

.github/workflows/prcontext/README.md

@@ -0,0 +1,7 @@
+# prcontext
+
+`prcontext` is a simple Go program that extracts CI directives from PRs for a
+more efficient merge cycle.
+
+Right now it only supports the `[ci-skip [job ...]]` directive. Since skips are
+only possible within PRs, the full suite will still run on merge.

.github/workflows/prcontext/main.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/coder/flog"
+)
+
+// githubContext is structured as documented here:
+// https://docs.github.com/en/actions/learn-github-actions/contexts#github-context.
+type githubContext struct {
+	EventName string `json:"event_name"`
+	Event     struct {
+		PullRequest struct {
+			Body string `json:"body"`
+		} `json:"pull_request"`
+	} `json:"event"`
+}
+
+func main() {
+	var c githubContext
+	err := json.Unmarshal([]byte(os.Getenv("GITHUB_CONTEXT")), &c)
+	if err != nil {
+		flog.Fatal("decode stdin: %+v", err)
+	}
+	flog.Info("detected event %q", c.EventName)
+	if c.EventName != "pull_request" {
+		flog.Info("aborting since not Pull Request")
+		return
+	}
+
+	_, _ = fmt.Printf("::group::{PR Body}\n%s\n::endgroup::\n", c.Event.PullRequest.Body)
+
+	skips := parseBody(c.Event.PullRequest.Body)
+	_, _ = fmt.Printf("::echo::on\n::set-output name=skips::[%s]\n", strings.Join(skips, " "))
+}

.github/workflows/prcontext/parse.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"regexp"
+	"strings"
+)
+
+const ciSkipPrefix = "ci-skip"
+
+var skipDirective = regexp.MustCompile(`\[` + ciSkipPrefix + ` ([\w-\/ ]+)]`)
+
+func parseBody(body string) (skips []string) {
+	matches := skipDirective.FindAllStringSubmatch(body, -1)
+	// flog.Info("matches: %+v", matches)
+
+	var skipMatches []string
+	for i := range matches {
+		for j := range matches[i] {
+			v := matches[i][j]
+			// flog.Info("%q", v)
+			if !strings.Contains(v, ciSkipPrefix) {
+				skipMatches = append(skipMatches, strings.Split(v, " ")...)
+			}
+		}
+	}
+
+	return skipMatches
+}

.github/workflows/prcontext/parse_test.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"reflect"
+	"testing"
+)
+
+func Test_parseBody_basic(t *testing.T) {
+	parseBody(`
+This is a test PR.	
+
+[ci-skip postgres windows]
+	`)
+}
+
+func Test_parseBody(t *testing.T) {
+	type args struct {
+		body string
+	}
+	tests := []struct {
+		name      string
+		args      args
+		wantSkips []string
+	}{
+		{"no directive", args{"test pr 123\n\n"}, nil},
+		{"single dir single skip", args{"test pr [ci-skip dog] 123\n\n"}, []string{"dog"}},
+		{"double dir double skip", args{"test pr [ci-skip dog] [ci-skip cat] 123\n\n"}, []string{"dog", "cat"}},
+		{"single dir double skip", args{"test pr [ci-skip test/go/postgres cat] 123\n\n"}, []string{"test/go/postgres", "cat"}},
+		{"confuse", args{"ci ci [ci-skip] dog [ci-skip test/go/postgres test/e2e/ubuntu-latest] 123\n\n"}, []string{"test/go/postgres", "test/e2e/ubuntu-latest"}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if gotSkips := parseBody(tt.args.body); !reflect.DeepEqual(gotSkips, tt.wantSkips) {
+				t.Errorf("parseBody() = %v, want %v", gotSkips, tt.wantSkips)
+			}
+		})
+	}
+}