Disable file locking when DEPNOLOCK set (#1206)

* Disable file locking when DEPNOLOCK set

* Add DisableLocking bool members to Ctx and gps.SourceManagerConfig structs.
  This effectively communicates DEPNOLOCK from the shell, to Ctx, to
  SourceManager.

  The member is named DisableLocking to make its zero-value useful.

* Add locker interface which implements TryLock(), Unlock(), and GetOwner()
  which lockfile.Lockfile alredy adheres to.  This interface replaces the new
  type for the lf member of the SourceMgr struct.

* Add a FalseLocker type which adheres to the Locker interface which does
  nothing.

* Conditionally set the lf member of SourceMgr to either an instance of
  lockfile.Lockfile or FalseLocker depending on the value of
  SourceManagerConfig.DisableLocking.

Signed-off-by: Ayan George <[email protected]>

* Revert stray edit.

Signed-off-by: Ayan George <[email protected]>

* Improve comment for DisableLocking

Signed-off-by: Ayan George <[email protected]>

* Fix comment type-os

* Fix comment type-os

Signed-off-by: Ayan George <[email protected]>

* Fix yet more type-os.

Signed-off-by: Ayan George <[email protected]>

cmd/dep/main.go

@@ -147,9 +147,10 @@ func (c *Config) Run() (exitCode int) {
 
 			// Set up dep context.
 			ctx := &dep.Ctx{
-				Out:     outLogger,
-				Err:     errLogger,
-				Verbose: *verbose,
+				Out:            outLogger,
+				Err:            errLogger,
+				Verbose:        *verbose,
+				DisableLocking: getEnv(c.Env, "DEPNOLOCK") != "",
 			}
 
 			GOPATHS := filepath.SplitList(getEnv(c.Env, "GOPATH"))

context.go

@@ -34,11 +34,12 @@ import (
 //	}
 //
 type Ctx struct {
-	WorkingDir string      // Where to execute.
-	GOPATH     string      // Selected Go path, containing WorkingDir.
-	GOPATHs    []string    // Other Go paths.
-	Out, Err   *log.Logger // Required loggers.
-	Verbose    bool        // Enables more verbose logging.
+	WorkingDir     string      // Where to execute.
+	GOPATH         string      // Selected Go path, containing WorkingDir.
+	GOPATHs        []string    // Other Go paths.
+	Out, Err       *log.Logger // Required loggers.
+	Verbose        bool        // Enables more verbose logging.
+	DisableLocking bool        // When set, no lock file will be created to protect against simultaneous dep processes.
 }
 
 // SetPaths sets the WorkingDir and GOPATHs fields. If GOPATHs is empty, then
@@ -87,8 +88,9 @@ func defaultGOPATH() string {
 // initialized to log to the receiver's logger.
 func (c *Ctx) SourceManager() (*gps.SourceMgr, error) {
 	return gps.NewSourceManager(gps.SourceManagerConfig{
-		Cachedir: filepath.Join(c.GOPATH, "pkg", "dep"),
-		Logger:   c.Out,
+		Cachedir:       filepath.Join(c.GOPATH, "pkg", "dep"),
+		Logger:         c.Out,
+		DisableLocking: c.DisableLocking,
 	})
 }
 

internal/gps/source_manager.go

@@ -28,6 +28,41 @@ import (
 // Used to compute a friendly filepath from a URL-shaped input.
 var sanitizer = strings.NewReplacer("-", "--", ":", "-", "/", "-", "+", "-")
 
+// A locker is responsible for preventing multiple instances of dep from
+// interfering with one-another.
+//
+// Currently, anything that can either TryLock(), Unlock(), or GetOwner()
+// satifies that need.
+type locker interface {
+	TryLock() error
+	Unlock() error
+	GetOwner() (*os.Process, error)
+}
+
+// A falselocker adheres to the locker interface and its purpose is to quietly
+// fail to lock when the DEPNOLOCK environment variable is set.
+//
+// This allows dep to run on systems where file locking doesn't work --
+// particularly those that use union mount type filesystems that don't
+// implement hard links or fnctl() style locking.
+type falseLocker struct{}
+
+// Always returns an error to indicate there's no current ower PID for our
+// lock.
+func (fl falseLocker) GetOwner() (*os.Process, error) {
+	return nil, fmt.Errorf("falseLocker always fails")
+}
+
+// Does nothing and returns a nil error so caller beleives locking succeeded.
+func (fl falseLocker) TryLock() error {
+	return nil
+}
+
+// Does nothing and returns a nil error so caller beleives unlocking succeeded.
+func (fl falseLocker) Unlock() error {
+	return nil
+}
+
 // A SourceManager is responsible for retrieving, managing, and interrogating
 // source repositories. Its primary purpose is to serve the needs of a Solver,
 // but it is handy for other purposes, as well.
@@ -121,7 +156,7 @@ func (p ProjectAnalyzerInfo) String() string {
 // tools; control via dependency injection is intended to be sufficient.
 type SourceMgr struct {
 	cachedir    string                // path to root of cache dir
-	lf          *lockfile.Lockfile    // handle for the sm lock file on disk
+	lf          locker                // handle for the sm lock file on disk
 	suprvsr     *supervisor           // subsystem that supervises running calls/io
 	cancelAll   context.CancelFunc    // cancel func to kill all running work
 	deduceCoord *deductionCoordinator // subsystem that manages import path deduction
@@ -142,8 +177,9 @@ var _ SourceManager = &SourceMgr{}
 
 // SourceManagerConfig holds configuration information for creating SourceMgrs.
 type SourceManagerConfig struct {
-	Cachedir string      // Where to store local instances of upstream sources.
-	Logger   *log.Logger // Optional info/warn logger. Discards if nil.
+	Cachedir       string      // Where to store local instances of upstream sources.
+	Logger         *log.Logger // Optional info/warn logger. Discards if nil.
+	DisableLocking bool        // True if the SourceManager should NOT use a lock file to protect the Cachedir from multiple processes.
 }
 
 // NewSourceManager produces an instance of gps's built-in SourceManager.
@@ -175,7 +211,14 @@ func NewSourceManager(c SourceManagerConfig) (*SourceMgr, error) {
 	// we can spin on.
 
 	glpath := filepath.Join(c.Cachedir, "sm.lock")
-	lockfile, err := lockfile.New(glpath)
+
+	lockfile, err := func() (locker, error) {
+		if c.DisableLocking {
+			return falseLocker{}, nil
+		}
+		return lockfile.New(glpath)
+	}()
+
 	if err != nil {
 		return nil, CouldNotCreateLockError{
 			Path: glpath,
@@ -238,7 +281,7 @@ func NewSourceManager(c SourceManagerConfig) (*SourceMgr, error) {
 
 	sm := &SourceMgr{
 		cachedir:    c.Cachedir,
-		lf:          &lockfile,
+		lf:          lockfile,
 		suprvsr:     superv,
 		cancelAll:   cf,
 		deduceCoord: deducer,