Prevent zombie processes (#16314)

Unfortunately go doesn't always ensure that execd processes are completely
waited for. On linux this means that zombie processes can occur.

This PR ensures that these are waited for by using signal notifier in serv and
passing a context elsewhere.

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/markup/external/external.go

@@ -5,6 +5,7 @@
 package external
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -15,6 +16,7 @@ import (
 
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/process"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
 )
@@ -96,7 +98,13 @@ func (p *Renderer) Render(ctx *markup.RenderContext, input io.Reader, output io.
 		args = append(args, f.Name())
 	}
 
-	cmd := exec.Command(commands[0], args...)
+	processCtx, cancel := context.WithCancel(ctx.Ctx)
+	defer cancel()
+
+	pid := process.GetManager().Add(fmt.Sprintf("Render [%s] for %s", commands[0], ctx.URLPrefix), cancel)
+	defer process.GetManager().Remove(pid)
+
+	cmd := exec.CommandContext(processCtx, commands[0], args...)
 	cmd.Env = append(
 		os.Environ(),
 		"GITEA_PREFIX_SRC="+ctx.URLPrefix,