add MAGEFILE_HASHFAST to avoid rerunning go build

mage/main.go

@@ -110,6 +110,7 @@ type Invocation struct {
 	Args       []string      // args to pass to the compiled binary
 	GoCmd      string        // the go binary command to run
 	CacheDir   string        // the directory where we should store compiled binaries
+	HashFast   bool          // don't rely on GOCACHE, just hash the magefiles
 }
 
 // ParseAndRun parses the command line, and then compiles and runs the mage
@@ -282,7 +283,7 @@ Options:
 	if len(inv.Args) > 0 && cmd != None {
 		return inv, cmd, fmt.Errorf("unexpected arguments to command: %q", inv.Args)
 	}
-
+	inv.HashFast = mg.HashFast()
 	return inv, cmd, err
 }
 
@@ -321,12 +322,16 @@ func Invoke(inv Invocation) int {
 	debug.Println("output exe is ", exePath)
 
 	useCache := false
-	if s, err := internal.OutputDebug(inv.GoCmd, "env", "GOCACHE"); err == nil {
-		// if GOCACHE exists, always rebuild, so we catch transitive
-		// dependencies that have changed.
-		if s != "" {
-			debug.Println("build cache exists, will ignore any compiled binary")
-			useCache = true
+	if inv.HashFast {
+		debug.Println("user has set MAGEFILE_HASHFAST, so we'll ignore GOCACHE")
+	} else {
+		if s, err := internal.OutputDebug(inv.GoCmd, "env", "GOCACHE"); err == nil {
+			// if GOCACHE exists, always rebuild, so we catch transitive
+			// dependencies that have changed.
+			if s != "" {
+				debug.Println("go build cache exists, will ignore any compiled binary")
+				useCache = true
+			}
 		}
 	}
 

mage/main_test.go

@@ -61,7 +61,7 @@ func testmain(m *testing.M) int {
 }
 
 func TestTransitiveDepCache(t *testing.T) {
-	cache, err := internal.OutputDebug("go", "env", "gocache")
+	cache, err := internal.OutputDebug("go", "env", "GOCACHE")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -108,6 +108,61 @@ func TestTransitiveDepCache(t *testing.T) {
 	}
 }
 
+func TestTransitiveHashFast(t *testing.T) {
+	cache, err := internal.OutputDebug("go", "env", "GOCACHE")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cache == "" {
+		t.Skip("skipping hashfast tests on go version without cache")
+	}
+
+	// Test that if we change a transitive dep, that we don't recompile.
+	// We intentionally run the first time without hashfast to ensure that
+	// we recompile the binary with the current code.
+	stdout := &bytes.Buffer{}
+	stderr := &bytes.Buffer{}
+	inv := Invocation{
+		Stderr: stderr,
+		Stdout: stdout,
+		Dir:    "testdata/transitiveDeps",
+		Args:   []string{"Run"},
+	}
+	code := Invoke(inv)
+	if code != 0 {
+		t.Fatalf("got code %v, err: %s", code, stderr)
+	}
+	expected := "woof\n"
+	if actual := stdout.String(); actual != expected {
+		t.Fatalf("expected %q but got %q", expected, actual)
+	}
+
+	// ok, so baseline, the generated and cached binary should do "woof"
+	// now change out the transitive dependency that does the output
+	// so that it produces different output.
+	if err := os.Rename("testdata/transitiveDeps/dep/dog.go", "testdata/transitiveDeps/dep/dog.notgo"); err != nil {
+		t.Fatal(err)
+	}
+	defer os.Rename("testdata/transitiveDeps/dep/dog.notgo", "testdata/transitiveDeps/dep/dog.go")
+	if err := os.Rename("testdata/transitiveDeps/dep/cat.notgo", "testdata/transitiveDeps/dep/cat.go"); err != nil {
+		t.Fatal(err)
+	}
+	defer os.Rename("testdata/transitiveDeps/dep/cat.go", "testdata/transitiveDeps/dep/cat.notgo")
+	stderr.Reset()
+	stdout.Reset()
+	inv.HashFast = true
+	code = Invoke(inv)
+	if code != 0 {
+		t.Fatalf("got code %v, err: %s", code, stderr)
+	}
+	// we should still get woof, even though the dependency was changed to
+	// return "meow", because we're only hashing the top level magefiles, not
+	// dependencies.
+	if actual := stdout.String(); actual != expected {
+		t.Fatalf("expected %q but got %q", expected, actual)
+	}
+}
+
 func TestListMagefilesMain(t *testing.T) {
 	buf := &bytes.Buffer{}
 	files, err := Magefiles("testdata/mixed_main_files", "", "", "go", buf, false)

mage/testdata/transitiveDeps/magefile.go

@@ -2,7 +2,7 @@
 
 package main
 
-import "./dep"
+import "github.com/magefile/mage/mage/testdata/transitiveDeps/dep"
 
 func Run() {
 	dep.Speak()

mg/runtime.go

@@ -27,6 +27,13 @@ const GoCmdEnv = "MAGEFILE_GOCMD"
 // to ignore the default target specified in the magefile.
 const IgnoreDefaultEnv = "MAGEFILE_IGNOREDEFAULT"
 
+// HashFastEnv is the environment variable that indicates the user requested to
+// use a quick hash of magefiles to determine whether or not the magefile binary
+// needs to be rebuilt. This results in faster runtimes, but means that mage
+// will fail to rebuild if a dependency has changed. To force a rebuild, run
+// mage with the -f flag.
+const HashFastEnv = "MAGEFILE_HASHFAST"
+
 // Verbose reports whether a magefile was run with the verbose flag.
 func Verbose() bool {
 	b, _ := strconv.ParseBool(os.Getenv(VerboseEnv))
@@ -48,6 +55,13 @@ func GoCmd() string {
 	return "go"
 }
 
+// HashFast reports whether the user has requested to use the fast hashing
+// mechanism rather than rely on go's rebuilding mechanism.
+func HashFast() bool {
+	b, _ := strconv.ParseBool(os.Getenv(HashFastEnv))
+	return b
+}
+
 // IgnoreDefault reports whether the user has requested to ignore the default target
 // in the magefile.
 func IgnoreDefault() bool {

site/content/environment/_index.en.md

@@ -22,5 +22,13 @@ Sets the binary that mage will use to compile with (default is "go").
 
 ## MAGEFILE_IGNOREDEFAULT
 
-If set to 1 or true, will tell the compiled magefile to ignore the default
-target and print the list of targets when you run `mage`.
+If set to "1" or "true", tells the compiled magefile to ignore the default
+target and print the list of targets when you run `mage`.
+
+## MAGEFILE_HASHFAST
+
+If set to "1" or "true", tells mage to use a quick hash of magefiles to
+determine whether or not the magefile binary needs to be rebuilt. This results
+in faster run times (especially on Windows), but means that mage will fail to
+rebuild if a dependency has changed. To force a rebuild when you know or suspect
+a dependency has changed, run mage with the -f flag.