main: fix -m mode stale binary bug (#84)

In 6038506 we incorrectly optimised away an install step when -m is
supplied. We must always install if we are in -m mode, because the main
module is responsible for resolving versions. We therefore utilise the
install step to effectively ensure the target binary is up to date.

This logic would change if were to adopt #81 because we would then only
install non-versioned module packages (i.e. non-main module,
non-directory replaced), or in the case a versioned target does not
exist in the gobin cache.

main.go

@@ -369,10 +369,27 @@ func mainerr() error {
 			// safe as it could/should be, but people shouldn't be messing with
 			// the cache anyway. The target in the cache is already hash by
 			// build tags so we should never have "overlapping" gobin runs.
-			if _, err := os.Stat(target); err != nil {
-				if !os.IsNotExist(err) {
-					return fmt.Errorf("failed to read %v: %v", target, err)
+			//
+			// Always install if we are in -m mode, because the main module is
+			// responsible for resolving versions. We therefore utilise the
+			// install step to effectively ensure the target binary is up to date.
+			// This logic would change if were to adopt
+			// https://github.com/myitcv/gobin/issues/81 because we would then
+			// only install non-versioned module packages (i.e. non-main module,
+			// non-directory replaced), or in the case a versioned target does not
+			// exist in the gobin cache.
+			var install bool
+			if *fMainMod {
+				install = true
+			} else {
+				if _, err := os.Stat(target); err != nil {
+					if !os.IsNotExist(err) {
+						return fmt.Errorf("failed to read %v: %v", target, err)
+					}
+					install = true
 				}
+			}
+			if install {
 				// optimistically remove our target in case we are installing over self
 				// TODO work out what to do for Windows
 				if mp.ImportPath == "github.com/myitcv/gobin" {

testdata/main-module-change.txt

@@ -0,0 +1,36 @@
+# Specifically test the logic that ensures -m packages are always
+# installed. Per the code comment, we always install as a means of
+# establishing whether the target is current. If we instead chose
+# to perform these checks ourself, we'd do just the same amount of
+# work.
+
+# Before
+cp msg.go.before msg.go
+gobin -m -run .
+stdout ^before$
+! stderr .+
+
+# After
+cp msg.go.after msg.go
+gobin -m -run .
+stdout ^after$
+! stderr .+
+
+-- go.mod --
+module mod.com
+-- main.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println(msg)
+}
+-- msg.go.before --
+package main
+
+var msg = "before"
+-- msg.go.after --
+package main
+
+var msg = "after"