Enforce requirement to be inside of a go src directory.

A lot of time is usually spent tracking down bug reports where users are
using godep outside of their $GOPATH. This should help with that, at least
until there it time to properly test godep use outside of a $GOPATH and
fix the issues.

diff.go

@@ -16,7 +16,8 @@ Shows the difference, in a unified diff format, between the
 current set of dependencies and those generated on a
 previous 'go save' execution.
 `,
-	Run: runDiff,
+	Run:          runDiff,
+	OnlyInGOPATH: true,
 }
 
 func runDiff(cmd *Command, args []string) {

get.go

@@ -21,7 +21,8 @@ If -t is given, dependencies of test files are also downloaded and installed.
 
 For more about specifying packages, see 'go help packages'.
 `,
-	Run: runGet,
+	Run:          runGet,
+	OnlyInGOPATH: true,
 }
 
 var getT bool

go.go

@@ -21,7 +21,8 @@ Any go tool command can run this way, but "godep go get"
 is unnecessary and has been disabled. Instead, use
 "godep go install".
 `,
-	Run: runGo,
+	Run:          runGo,
+	OnlyInGOPATH: true,
 }
 
 // Find the godep GOPATH for this file tree and run the go tool.

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"go/build"
 	"io"
 	"log"
 	"os"
@@ -43,6 +44,9 @@ type Command struct {
 
 	// Flag is a set of flags specific to this command.
 	Flag flag.FlagSet
+
+	// OnlyInGOPATH limits this command to being run only while inside of a GOPATH
+	OnlyInGOPATH bool
 }
 
 // UsageExit prints usage information and exits.
@@ -110,14 +114,17 @@ func main() {
 		log.Fatal(err)
 	}
 
-	VendorExperiment = determineVendor(majorGoVersion)
-
-	// sep is the signature set of path elements that
-	// precede the original path of an imported package.
-	sep = defaultSep(VendorExperiment)
-
 	for _, cmd := range commands {
 		if cmd.Name == args[0] {
+			if cmd.OnlyInGOPATH {
+				checkInGOPATH()
+			}
+
+			VendorExperiment = determineVendor(majorGoVersion)
+			// sep is the signature set of path elements that
+			// precede the original path of an imported package.
+			sep = defaultSep(VendorExperiment)
+
 			cmd.Flag.BoolVar(&verbose, "v", false, "enable verbose output")
 			cmd.Flag.BoolVar(&debug, "d", false, "enable debug output")
 			cmd.Flag.StringVar(&cpuprofile, "cpuprofile", "", "Write cpu profile to this file")
@@ -147,6 +154,33 @@ func main() {
 	os.Exit(2)
 }
 
+func subPath(sub, path string) bool {
+	ls := strings.ToLower(sub)
+	lp := strings.ToLower(path)
+	if ls == lp {
+		return false
+	}
+	return strings.HasPrefix(ls, lp)
+}
+
+func checkInGOPATH() {
+	pwd, err := os.Getwd()
+	if err != nil {
+		log.Fatal("Unable to determine current working directory", err)
+	}
+	dirs := build.Default.SrcDirs()
+	for _, p := range dirs {
+		if ok := subPath(pwd, p); ok {
+			return
+		}
+	}
+
+	log.Println("godep should only be used inside a valid go package directory.")
+	log.Println("You are probably outside of your $GOPATH.")
+	log.Printf("\tCurrent Directory: %s\n", pwd)
+	log.Fatalf("\t$GOPATH: %s\n", os.Getenv("GOPATH"))
+}
+
 var usageTemplate = `
 Godep is a tool for managing Go package dependencies.
 

match_test.go

@@ -30,3 +30,34 @@ func TestMatchPattern(t *testing.T) {
 		}
 	}
 }
+
+func TestSubPath(t *testing.T) {
+	cases := []struct {
+		sub  string
+		dir  string
+		want bool
+	}{
+		//Basic
+		{`/Users/emuller/go/src/github.com/tools/godep`, `/Users/emuller/go`, true},
+		//Case insensitive filesystem used in dir
+		{`/Users/emuller/go/src/github.com/tools/godep`, `/Users/Emuller/go`, true},
+		{`/Users/emuller/go/Src/github.com/tools/godep`, `/Users/Emuller/go`, true},
+		//spaces
+		{`/Users/e muller/go/Src/github.com/tools/godep`, `/Users/E muller/go`, true},
+		// ()
+		{`/Users/e muller/(Personal)/go/Src/github.com/tools/godep`, `/Users/E muller/(Personal)/go`, true},
+		//Not even close, but same length
+		{`/foo`, `/bar`, false},
+		// Same, so not sub path (same path)
+		{`/foo`, `/foo`, false},
+		// Windows with different cases
+		{`c:\foo\bar`, `C:\foo`, true},
+	}
+
+	for _, test := range cases {
+		ok := subPath(test.sub, test.dir)
+		if ok != test.want {
+			t.Errorf("subdir(%s,%s) = %v want %v", test.sub, test.dir, ok, test.want)
+		}
+	}
+}

path.go

@@ -17,7 +17,8 @@ the environment.
 
 For more about how GOPATH works, see 'go help gopath'.
 `,
-	Run: runPath,
+	Run:          runPath,
+	OnlyInGOPATH: true,
 }
 
 // Print the gopath that points to

restore.go

@@ -17,7 +17,8 @@ var cmdRestore = &Command{
 Restore checks out the Godeps-specified version of each package in GOPATH.
 
 `,
-	Run: runRestore,
+	Run:          runRestore,
+	OnlyInGOPATH: true,
 }
 
 // Three phases:

save.go

@@ -58,7 +58,8 @@ also saved.
 
 For more about specifying packages, see 'go help packages'.
 `,
-	Run: runSave,
+	Run:          runSave,
+	OnlyInGOPATH: true,
 }
 
 var (

update.go

@@ -18,15 +18,17 @@ var cmdUpdate = &Command{
 	Long: `
 Update changes the named dependency packages to use the
 revision of each currently installed in GOPATH. New code will
-be copied into the Godeps workspace or vendor folder and the 
+be copied into the Godeps workspace or vendor folder and the
 new revision will be written to the manifest.
 
-If -goversion is specified, update the recorded go version. 
+If -goversion is specified, update the recorded go version.
 
 For more about specifying packages, see 'go help packages'.
 `,
-	Run: runUpdate,
+	Run:          runUpdate,
+	OnlyInGOPATH: true,
 }
+
 var (
 	updateGoVer bool
 )

version.go

@@ -5,7 +5,7 @@ import (
 	"runtime"
 )
 
-const version = 58
+const version = 59
 
 var cmdVersion = &Command{
 	Name:  "version",