Fix build on Plan 9

Plan 9 doesn't have syscall.EAGAIN. Copy some code from Go's internal
locakedfile package to check whether the file is locked by another
process.

fslock.go

@@ -6,7 +6,6 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
-	"syscall"
 
 	util "github.com/ipfs/go-ipfs-util"
 	logging "github.com/ipfs/go-log"
@@ -30,10 +29,7 @@ func Lock(confdir, lockFileName string) (io.Closer, error) {
 	lk, err := lock.Lock(lockFilePath)
 	if err != nil {
 		switch {
-		case err == syscall.EAGAIN:
-			// EAGAIN == someone else has the lock
-			fallthrough
-		case strings.Contains(err.Error(), "resource temporarily unavailable"):
+		case lockedByOthers(err):
 			return lk, &os.PathError{
 				Op:   "lock",
 				Path: lockFilePath,

fslock_plan9.go

@@ -0,0 +1,33 @@
+package fslock
+
+import "strings"
+
+// Opening an exclusive-use file returns an error.
+// The expected error strings are:
+//
+//  - "open/create -- file is locked" (cwfs, kfs)
+//  - "exclusive lock" (fossil)
+//  - "exclusive use file already open" (ramfs)
+//
+// See https://github.com/golang/go/blob/go1.15rc1/src/cmd/go/internal/lockedfile/lockedfile_plan9.go#L16
+var lockedErrStrings = [...]string{
+	"file is locked",
+	"exclusive lock",
+	"exclusive use file already open",
+}
+
+// isLockedPlan9 return whether an os.OpenFile error indicates that
+// a file with the ModeExclusive bit set is already open.
+func isLockedPlan9(s string) bool {
+	for _, frag := range lockedErrStrings {
+		if strings.Contains(s, frag) {
+			return true
+		}
+	}
+	return false
+}
+
+func lockedByOthers(err error) bool {
+	s := err.Error()
+	return strings.Contains(s, "Lock Create of") && isLockedPlan9(s)
+}

fslock_posix.go

@@ -0,0 +1,12 @@
+// +build !plan9
+
+package fslock
+
+import (
+	"strings"
+	"syscall"
+)
+
+func lockedByOthers(err error) bool {
+	return err == syscall.EAGAIN || strings.Contains(err.Error(), "resource temporarily unavailable")
+}

fslock_test.go

@@ -1,9 +1,14 @@
 package fslock_test
 
 import (
+	"bufio"
+	"io/ioutil"
 	"os"
+	"os/exec"
 	"path"
+	"strings"
 	"testing"
+	"time"
 
 	lock "github.com/ipfs/go-fs-lock"
 )
@@ -94,3 +99,63 @@ func TestLockMultiple(t *testing.T) {
 	assertLock(t, confdir, lockFile1, false)
 	assertLock(t, confdir, lockFile2, false)
 }
+
+func TestLockedByOthers(t *testing.T) {
+	const (
+		lockedMsg = "locked\n"
+		lockFile  = "my-test.lock"
+		wantErr   = "someone else has the lock"
+	)
+
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" { // child process
+		confdir := os.Args[3]
+		if _, err := lock.Lock(confdir, lockFile); err != nil {
+			t.Fatalf("child lock: %v", err)
+		}
+		os.Stdout.WriteString(lockedMsg)
+		time.Sleep(10 * time.Minute)
+		return
+	}
+
+	confdir, err := ioutil.TempDir("", "go-fs-lock-test")
+	if err != nil {
+		t.Fatalf("creating temporary directory: %v", err)
+	}
+	defer os.RemoveAll(confdir)
+
+	// Execute a child process that locks the file.
+	cmd := exec.Command(os.Args[0], "-test.run=TestLockedByOthers", "--", confdir)
+	cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
+	cmd.Stderr = os.Stderr
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		t.Fatalf("cmd.StdoutPipe: %v", err)
+	}
+	if err = cmd.Start(); err != nil {
+		t.Fatalf("cmd.Start: %v", err)
+	}
+	defer cmd.Process.Kill()
+
+	// Wait for the child to lock the file.
+	b := bufio.NewReader(stdout)
+	line, err := b.ReadString('\n')
+	if err != nil {
+		t.Fatalf("read from child: %v", err)
+	}
+	if got, want := line, lockedMsg; got != want {
+		t.Fatalf("got %q from child; want %q", got, want)
+	}
+
+	// Parent should not be able to lock the file.
+	_, err = lock.Lock(confdir, lockFile)
+	if err == nil {
+		t.Fatalf("parent successfully acquired the lock")
+	}
+	pe, ok := err.(*os.PathError)
+	if !ok {
+		t.Fatalf("wrong error type %T", err)
+	}
+	if got := pe.Error(); !strings.Contains(got, wantErr) {
+		t.Fatalf("error %q does not contain %q", got, wantErr)
+	}
+}