builder: use build ID as cache key

Instead of storing an increasing version number in relevant packages
(compiler.Version, interp.Version, cgo.Version, ...), read the build ID
from the currently running executable. This has several benefits:

  * All changes relevant to the compiled packages are caught.
  * No need to bump the version for each change to these packages.
    This avoids merge conflicts.
  * During development, `go install` is enough. No need to run
    `tinygo clean` all the time.

Of course, the drawback is that it might be updated a bit more often
than necessary but I think the overall benefit is big.

Regular release users shouldn't see any difference. Because the tinygo
binary stays the same, the cache works well.

builder/build.go

@@ -24,7 +24,6 @@ import (
 	"strings"
 
 	"github.com/gofrs/flock"
-	"github.com/tinygo-org/tinygo/cgo"
 	"github.com/tinygo-org/tinygo/compileopts"
 	"github.com/tinygo-org/tinygo/compiler"
 	"github.com/tinygo-org/tinygo/goenv"
@@ -64,9 +63,8 @@ type BuildResult struct {
 // implementation of an imported package changes.
 type packageAction struct {
 	ImportPath       string
-	CGoVersion       int // cgo.Version
-	CompilerVersion  int // compiler.Version
-	InterpVersion    int // interp.Version
+	CompilerBuildID  string
+	TinyGoVersion    string
 	LLVMVersion      string
 	Config           *compiler.Config
 	CFlags           []string
@@ -84,6 +82,13 @@ type packageAction struct {
 // The error value may be of type *MultiError. Callers will likely want to check
 // for this case and print such errors individually.
 func Build(pkgName, outpath string, config *compileopts.Config, action func(BuildResult) error) error {
+	// Read the build ID of the tinygo binary.
+	// Used as a cache key for package builds.
+	compilerBuildID, err := ReadBuildID()
+	if err != nil {
+		return err
+	}
+
 	// Create a temporary directory for intermediary files.
 	dir, err := ioutil.TempDir("", "tinygo")
 	if err != nil {
@@ -192,9 +197,8 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 		// the parameters for the build.
 		actionID := packageAction{
 			ImportPath:       pkg.ImportPath,
-			CGoVersion:       cgo.Version,
-			CompilerVersion:  compiler.Version,
-			InterpVersion:    interp.Version,
+			CompilerBuildID:  string(compilerBuildID),
+			TinyGoVersion:    goenv.Version,
 			LLVMVersion:      llvm.Version,
 			Config:           compilerConfig,
 			CFlags:           pkg.CFlags,