Enable profiling options when 'pprof' tag is set (#2813)

This commit enables cpu, mem, block, and mutex profling of the FZF
executable. To support flushing the profiles at program exit it adds
util.AtExit to register "at exit" functions and mandates that util.Exit
is used instead of os.Exit to stop the program.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

src/options_pprof.go

@@ -0,0 +1,73 @@
+//go:build pprof
+// +build pprof
+
+package fzf
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/pprof"
+
+	"github.com/junegunn/fzf/src/util"
+)
+
+func (o *Options) initProfiling() error {
+	if o.CPUProfile != "" {
+		f, err := os.Create(o.CPUProfile)
+		if err != nil {
+			return fmt.Errorf("could not create CPU profile: %w", err)
+		}
+
+		if err := pprof.StartCPUProfile(f); err != nil {
+			return fmt.Errorf("could not start CPU profile: %w", err)
+		}
+
+		util.AtExit(func() {
+			pprof.StopCPUProfile()
+			if err := f.Close(); err != nil {
+				fmt.Fprintln(os.Stderr, "Error: closing cpu profile:", err)
+			}
+		})
+	}
+
+	stopProfile := func(name string, f *os.File) {
+		if err := pprof.Lookup(name).WriteTo(f, 0); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: could not write %s profile: %v\n", name, err)
+		}
+		if err := f.Close(); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: closing %s profile: %v\n", name, err)
+		}
+	}
+
+	if o.MEMProfile != "" {
+		f, err := os.Create(o.MEMProfile)
+		if err != nil {
+			return fmt.Errorf("could not create MEM profile: %w", err)
+		}
+		util.AtExit(func() {
+			runtime.GC()
+			stopProfile("allocs", f)
+		})
+	}
+
+	if o.BlockProfile != "" {
+		runtime.SetBlockProfileRate(1)
+		f, err := os.Create(o.BlockProfile)
+		if err != nil {
+			return fmt.Errorf("could not create BLOCK profile: %w", err)
+		}
+		util.AtExit(func() { stopProfile("block", f) })
+	}
+
+	if o.MutexProfile != "" {
+		runtime.SetMutexProfileFraction(1)
+		f, err := os.Create(o.MutexProfile)
+		if err != nil {
+			return fmt.Errorf("could not create MUTEX profile: %w", err)
+		}
+		util.AtExit(func() { stopProfile("mutex", f) })
+	}
+
+	return nil
+}