os: depend on Readlink only when necessary

Currently Readlink gets linked into the binary even when Executable is
not needed.

This reduces a simple "os.Stdout.Write([]byte("hello"))" by ~10KiB.

Previously the executable path was read during init time, because
deleting the executable would make "Readlink" return "(deleted)" suffix.
There's probably a slight chance that the init time reading would return
it anyways.

Updates #6853

Change-Id: Ic76190c5b64d9320ceb489cd6a553108614653d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/311790
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>

1 files changed, 13 insertions, 8 deletions

diff --git a/src/os/executable_procfs.go b/src/os/executable_procfs.go
index 9c64a0d474..76ba0e6d08 100644
--- a/src/os/executable_procfs.go
+++ b/src/os/executable_procfs.go
@@ -12,10 +12,7 @@ import (
 	"runtime"
 )
 
-// We query the executable path at init time to avoid the problem of
-// readlink returns a path appended with " (deleted)" when the original
-// binary gets deleted.
-var executablePath, executablePathErr = func() (string, error) {
+func executable() (string, error) {
 	var procfn string
 	switch runtime.GOOS {
 	default:
@@ -25,9 +22,17 @@ var executablePath, executablePathErr = func() (string, error) {
 	case "netbsd":
 		procfn = "/proc/curproc/exe"
 	}
-	return Readlink(procfn)
-}()
+	path, err := Readlink(procfn)
 
-func executable() (string, error) {
-	return executablePath, executablePathErr
+	// When the executable has been deleted then Readlink returns a
+	// path appended with " (deleted)".
+	return stringsTrimSuffix(path, " (deleted)"), err
+}
+
+// stringsTrimSuffix is the same as strings.TrimSuffix.
+func stringsTrimSuffix(s, suffix string) string {
+	if len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix {
+		return s[:len(s)-len(suffix)]
+	}
+	return s
 }