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/110295
Reviewed-by: Ian Lance Taylor <iant@golang.org>

diff --git a/src/os/os_test.go b/src/os/os_test.go
index 388673938962e566bc5eebbef3071aaee53d03ad..af3a2fbee65a1302637e5887b996948794efa204 100644 (file)
--- a/src/os/os_test.go
+++ b/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

diff --git a/src/os/sys_linux.go b/src/os/sys_linux.go
index 76cdf5043239963d6a01430929edaf04af6e777a..3fbe5e9f82cf678626d8e73ac463895977cfaf1a 100644 (file)
--- a/src/os/sys_linux.go
+++ b/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
 }