Make it possible to debug soong_ui.

This works by setting the SOONG_UI_DELVE= environment variable to the
port on which soong_ui should accept a Delve connection on.

This is achieved by reusing the Delve execution logic between soong_ui
and soong_build.

Test: Manual.
Change-Id: Id2c1d4b6faac1a4a3918c91030ce2239f7daf54f

shared/debug.go

@@ -1,8 +1,15 @@
 package shared
 
 import (
+	"fmt"
 	"os"
 	"os/exec"
+	"strings"
+	"syscall"
+)
+
+var (
+	isDebugging bool
 )
 
 // Finds the Delve binary to use. Either uses the SOONG_DELVE_PATH environment
@@ -15,3 +22,48 @@ func ResolveDelveBinary() string {
 
 	return result
 }
+
+// Returns whether the current process is running under Delve due to
+// ReexecWithDelveMaybe().
+func IsDebugging() bool {
+	return isDebugging
+}
+
+// Re-executes the binary in question under the control of Delve when
+// delveListen is not the empty string. delvePath gives the path to the Delve.
+func ReexecWithDelveMaybe(delveListen, delvePath string) {
+	isDebugging = os.Getenv("SOONG_DELVE_REEXECUTED") == "true"
+	if isDebugging || delveListen == "" {
+		return
+	}
+
+	if delvePath == "" {
+		fmt.Fprintln(os.Stderr, "Delve debugging requested but failed to find dlv")
+		os.Exit(1)
+	}
+
+	soongDelveEnv := []string{}
+	for _, env := range os.Environ() {
+		idx := strings.IndexRune(env, '=')
+		if idx != -1 {
+			soongDelveEnv = append(soongDelveEnv, env)
+		}
+	}
+
+	soongDelveEnv = append(soongDelveEnv, "SOONG_DELVE_REEXECUTED=true")
+
+	dlvArgv := []string{
+		delvePath,
+		"--listen=:" + delveListen,
+		"--headless=true",
+		"--api-version=2",
+		"exec",
+		os.Args[0],
+		"--",
+	}
+
+	dlvArgv = append(dlvArgv, os.Args[1:]...)
+	syscall.Exec(delvePath, dlvArgv, soongDelveEnv)
+	fmt.Fprintln(os.Stderr, "exec() failed while trying to reexec with Delve")
+	os.Exit(1)
+}