os: find Hostname using Uname to fix Android

It's also fewer system calls. Fall back to longer read
only if it seems like the Uname result is truncated.

Fixes #24701

Change-Id: Ib6550acede8dddaf184e8fa9de36377e17bbddab
Reviewed-on: https://go-review.googlesource.com/110295Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/os_test.go

@@ -1544,7 +1544,16 @@ func TestHostname(t *testing.T) {
 	// On Plan 9 it can be taken from #c/sysname as Hostname() does.
 	switch runtime.GOOS {
 	case "android", "plan9":
-		t.Skipf("%s doesn't have /bin/hostname", runtime.GOOS)
+		// No /bin/hostname to verify against, but at least
+		// verify we get something back from Hostname.
+		hostname, err := Hostname()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if hostname == "" {
+			t.Fatal("Hostname returned empty string and no error")
+		}
+		return
 	case "windows":
 		testWindowsHostname(t)
 		return

src/os/sys_linux.go

@@ -6,15 +6,43 @@
 
 package os
 
+import (
+	"runtime"
+	"syscall"
+)
+
 func hostname() (name string, err error) {
+	// Try uname first, as it's only one system call and reading
+	// from /proc is not allowed on Android.
+	var un syscall.Utsname
+	err = syscall.Uname(&un)
+
+	var buf [512]byte // Enough for a DNS name.
+	for i, b := range un.Nodename[:] {
+		buf[i] = uint8(b)
+		if b == 0 {
+			name = string(buf[:i])
+		}
+	}
+	// If we got a name and it's not potentially truncated
+	// (Nodename is 65 bytes), return it.
+	if err == nil && len(name) > 0 && len(name) < 64 {
+		return name, nil
+	}
+	if runtime.GOOS == "android" {
+		if name != "" {
+			return name, nil
+		}
+		return "localhost", nil
+	}
+
 	f, err := Open("/proc/sys/kernel/hostname")
 	if err != nil {
 		return "", err
 	}
 	defer f.Close()
 
-	var buf [512]byte // Enough for a DNS name.
-	n, err := f.Read(buf[0:])
+	n, err := f.Read(buf[:])
 	if err != nil {
 		return "", err
 	}
@@ -22,5 +50,5 @@ func hostname() (name string, err error) {
 	if n > 0 && buf[n-1] == '\n' {
 		n--
 	}
-	return string(buf[0:n]), nil
+	return string(buf[:n]), nil
 }