ruleguard: add profiling labels support (#338)

When ruleguard is compiled with `--tags pproflabels`, profiling
will record per rules group labels, like `rules:groupname`.

This makes it easies to find the rules that may need to be
optimized or re-written in some other way.

Build tags protection is used to avoid the overhead in
the normal mode, when labels are not needed.

```
(pprof) tags
 rules: Total 2.3s
        410.0ms (18.14%): dupArg
        350.0ms (15.49%): offBy1
        250.0ms (11.06%): badLock
        160.0ms ( 7.08%): argOrder
        140.0ms ( 6.19%): equalFold
        120.0ms ( 5.31%): redundantSprint
        110.0ms ( 4.87%): wrapperFunc
         90.0ms ( 3.98%): ioutilDeprecated
         80.0ms ( 3.54%): preferFprint
         70.0ms ( 3.10%): assignOp
         60.0ms ( 2.65%): badCall
         60.0ms ( 2.65%): externalErrorReassign
         60.0ms ( 2.65%): stringXbytes
         50.0ms ( 2.21%): valSwap
         40.0ms ( 1.77%): stringConcatSimplify
         40.0ms ( 1.77%): yodaStyleExpr
         20.0ms ( 0.88%): badSorting
         20.0ms ( 0.88%): flagDeref
         20.0ms ( 0.88%): httpNoBody
         20.0ms ( 0.88%): preferStringWriter
         20.0ms ( 0.88%): timeExprSimplify
         10.0ms ( 0.44%): dynamicFmtString
         10.0ms ( 0.44%): emptyStringTest
         10.0ms ( 0.44%): preferWriteByte
         10.0ms ( 0.44%): regexpMust
         10.0ms ( 0.44%): returnAfterHttpError
         10.0ms ( 0.44%): sloppyLen
         10.0ms ( 0.44%): sprintfQuotedString
```

Author: quasilyte

ruleguard/profiling/no_labels.go

@@ -0,0 +1,16 @@
+//go:build !pproflabels
+// +build !pproflabels
+
+package profiling
+
+import (
+	"context"
+)
+
+const LabelsEnabled = false
+
+func EnterWithLabels(origContext context.Context, name string) {
+}
+
+func Leave(origContext context.Context) {
+}

ruleguard/profiling/with_labels.go

@@ -0,0 +1,21 @@
+//go:build pproflabels
+// +build pproflabels
+
+package profiling
+
+import (
+	"context"
+	"runtime/pprof"
+)
+
+const LabelsEnabled = true
+
+func EnterWithLabels(origContext context.Context, name string) {
+	labels := pprof.Labels("rules", name)
+	ctx := pprof.WithLabels(origContext, labels)
+	pprof.SetGoroutineLabels(ctx)
+}
+
+func Leave(origContext context.Context) {
+	pprof.SetGoroutineLabels(origContext)
+}

ruleguard/runner.go

@@ -2,6 +2,7 @@ package ruleguard
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"go/ast"
 	"go/build"
@@ -13,13 +14,16 @@ import (
 	"strings"
 
 	"github.com/quasilyte/go-ruleguard/ruleguard/goutil"
+	"github.com/quasilyte/go-ruleguard/ruleguard/profiling"
 	"github.com/quasilyte/gogrep"
 	"github.com/quasilyte/gogrep/nodetag"
 )
 
 type rulesRunner struct {
 	state *engineState
 
+	bgContext context.Context
+
 	ctx   *RunContext
 	rules *goRuleSet
 
@@ -60,6 +64,7 @@ func newRulesRunner(ctx *RunContext, buildContext *build.Context, state *engineS
 	gogrepState := gogrep.NewMatcherState()
 	gogrepState.Types = ctx.Types
 	rr := &rulesRunner{
+		bgContext:   context.Background(),
 		ctx:         ctx,
 		importer:    importer,
 		rules:       rules,
@@ -209,12 +214,25 @@ func (rr *rulesRunner) runCommentRules(comment *ast.Comment) {
 }
 
 func (rr *rulesRunner) runRules(n ast.Node) {
+	// profiling.LabelsEnabled is constant, so labels-related
+	// code should be a no-op inside normal build.
+	// To enable labels, use "-tags pproflabels" build tag.
+
 	tag := nodetag.FromNode(n)
 	for _, rule := range rr.rules.universal.rulesByTag[tag] {
+		if profiling.LabelsEnabled {
+			profiling.EnterWithLabels(rr.bgContext, rule.group.Name)
+		}
+
 		matched := false
 		rule.pat.MatchNode(&rr.gogrepState, n, func(m gogrep.MatchData) {
 			matched = rr.handleMatch(rule, m)
 		})
+
+		if profiling.LabelsEnabled {
+			profiling.Leave(rr.bgContext)
+		}
+
 		if matched {
 			break
 		}