Make profiling easier (#3634)

Add a `-profile` command line flag, and some documentation about how to
collect and view profile data.

CONTRIBUTING.md

@@ -154,31 +154,7 @@ If you want to trigger a debug session from VSCode, you can use the following sn
 
 ## Profiling
 
-If you want to investigate what's contributing to CPU usage you can add the following to the top of the `main()` function in `main.go`
-
-```go
-import "runtime/pprof"
-
-func main() {
-	f, err := os.Create("cpu.prof")
-	if err != nil {
-		log.Fatal("could not create CPU profile: ", err)
-	}
-	defer f.Close()
-	if err := pprof.StartCPUProfile(f); err != nil {
-		log.Fatal("could not start CPU profile: ", err)
-	}
-	defer pprof.StopCPUProfile()
-	...
-```
-
-Then run lazygit, and afterwards, from your terminal, run:
-
-```sh
-go tool pprof --web cpu.prof
-```
-
-That should open an application which allows you to view the breakdown of CPU usage.
+If you want to investigate what's contributing to CPU or memory usage, see [this separate document](docs/dev/Profiling.md).
 
 ## Testing
 

docs/dev/Profiling.md

@@ -0,0 +1,69 @@
+# Profiling Lazygit
+
+If you want to investigate what's contributing to CPU or memory usage, start
+lazygit with the `-profile` command line flag. This tells it to start an
+integrated web server that listens for profiling requests.
+
+## Save profile data
+
+### CPU
+
+While lazygit is running with the `-profile` flag, perform a CPU profile and
+save it to a file by running this command in another terminal window:
+
+```sh
+curl -o cpu.out http://127.0.0.1:6060/debug/pprof/profile
+```
+
+By default, it profiles for 30 seconds. To change the duration, use
+
+```sh
+curl -o cpu.out 'http://127.0.0.1:6060/debug/pprof/profile?seconds=60'
+```
+
+### Memory
+
+To save a heap profile (containing information about all memory allocated so
+far since startup), use
+
+```sh
+curl -o mem.out http://127.0.0.1:6060/debug/pprof/heap
+```
+
+Sometimes it can be useful to get a delta log, i.e. to see how memory usage
+developed from one point in time to another. For that, use
+
+```sh
+curl -o mem.out 'http://127.0.0.1:6060/debug/pprof/heap?seconds=20'
+```
+
+This will log the memory usage difference between now and 20 seconds later, so
+it gives you 20 seconds to perform the action in lazygit that you are interested
+in measuring.
+
+## View profile data
+
+To display the profile data, you can either use speedscope.app, or the pprof
+tool that comes with go. I prefer the former because it has a nicer UI and is a
+little more powerful; however, I have seen cases where it wasn't able to load a
+profile for some reason, in which case it's good to have the pprof tool as a
+fallback.
+
+### Speedscope.app
+
+Go to https://www.speedscope.app/ in your browser, and drag the saved profile
+onto the browser window. Refer to [the
+documentation](https://github.com/jlfwong/speedscope?tab=readme-ov-file#usage)
+for how to navigate the data.
+
+### Pprof tool
+
+To view a profile that you saved as `cpu.out`, use
+
+```sh
+go tool pprof -http=:8080 cpu.out
+```
+
+By default this shows the graph view, which I don't find very useful myself.
+Choose "Flame Graph" from the View menu to show a much more useful
+representation of the data.

docs/dev/README.md

@@ -5,3 +5,4 @@
 * [Integration Tests](../../pkg/integration/README.md)
 * [Demo Recordings](./Demo_Recordings.md)
 * [Find base commit for fixup design](Find_Base_Commit_For_Fixup_Design.md)
+* [Profiling](Profiling.md)

pkg/app/entry_point.go

@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"fmt"
 	"log"
+	"net/http"
+	_ "net/http/pprof"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -30,6 +32,7 @@ type cliArgs struct {
 	PrintVersionInfo   bool
 	Debug              bool
 	TailLogs           bool
+	Profile            bool
 	PrintDefaultConfig bool
 	PrintConfigDir     bool
 	UseConfigDir       string
@@ -145,6 +148,14 @@ func Start(buildInfo *BuildInfo, integrationTest integrationTypes.IntegrationTes
 		return
 	}
 
+	if cliArgs.Profile {
+		go func() {
+			if err := http.ListenAndServe("localhost:6060", nil); err != nil {
+				log.Fatal(err)
+			}
+		}()
+	}
+
 	parsedGitArg := parseGitArg(cliArgs.GitArg)
 
 	Run(appConfig, common, appTypes.NewStartArgs(cliArgs.FilterPath, parsedGitArg, integrationTest))
@@ -171,6 +182,9 @@ func parseCliArgsAndEnvVars() *cliArgs {
 	tailLogs := false
 	flaggy.Bool(&tailLogs, "l", "logs", "Tail lazygit logs (intended to be used when `lazygit --debug` is called in a separate terminal tab)")
 
+	profile := false
+	flaggy.Bool(&profile, "", "profile", "Start the profiler and serve it on http port 6060. See CONTRIBUTING.md for more info.")
+
 	printDefaultConfig := false
 	flaggy.Bool(&printDefaultConfig, "c", "config", "Print the default config")
 
@@ -202,6 +216,7 @@ func parseCliArgsAndEnvVars() *cliArgs {
 		PrintVersionInfo:   printVersionInfo,
 		Debug:              debug,
 		TailLogs:           tailLogs,
+		Profile:            profile,
 		PrintDefaultConfig: printDefaultConfig,
 		PrintConfigDir:     printConfigDir,
 		UseConfigDir:       useConfigDir,