Go: Keep track of all installed toolchains that we know of

Author: mbg

go/extractor/toolchain/toolchain.go

@@ -16,7 +16,15 @@ func IsInstalled() bool {
 	return err == nil
 }
 
+// The default Go version that is available on a system and a set of all versions
+// that we know are installed on the system.
 var goVersion = ""
+var goVersions = map[string]struct{}{}
+
+// Adds an entry to the set of installed Go versions for the normalised `version` number.
+func addGoVersion(version string) {
+	goVersions[semver.Canonical(version)] = struct{}{}
+}
 
 // Returns the current Go version as returned by 'go version', e.g. go1.14.4
 func GetEnvGoVersion() string {
@@ -34,10 +42,57 @@ func GetEnvGoVersion() string {
 		}
 
 		goVersion = parseGoVersion(string(out))
+		addGoVersion(goVersion)
 	}
 	return goVersion
 }
 
+// Determines whether, to our knowledge, `version` is available on the current system.
+func HasGoVersion(version string) bool {
+	_, found := goVersions[semver.Canonical(version)]
+	return found
+}
+
+// Attempts to install the Go toolchain `version`.
+func InstallVersion(workingDir string, version string) bool {
+	// No need to install it if we know that it is already installed.
+	if HasGoVersion(version) {
+		return true
+	}
+
+	// Construct a command to invoke `go version` with `GOTOOLCHAIN=go1.N.0` to give
+	// Go a valid toolchain version to download the toolchain we need; subsequent commands
+	// should then work even with an invalid version that's still in `go.mod`
+	toolchainArg := "GOTOOLCHAIN=go" + semver.Canonical(version)
+	versionCmd := Version()
+	versionCmd.Dir = workingDir
+	versionCmd.Env = append(os.Environ(), toolchainArg)
+
+	log.Printf(
+		"Trying to install Go %s using its canonical representation in `%s`.",
+		version,
+		workingDir,
+	)
+
+	// Run the command. If something goes wrong, report it to the log and signal failure
+	// to the caller.
+	if versionErr := versionCmd.Run(); versionErr != nil {
+		log.Printf(
+			"Failed to invoke `%s go version` in %s: %s\n",
+			toolchainArg,
+			versionCmd.Dir,
+			versionErr.Error(),
+		)
+
+		return false
+	}
+
+	// Add the version to the set of versions that we know are installed and signal
+	// success to the caller.
+	addGoVersion(version)
+	return true
+}
+
 // Returns the current Go version in semver format, e.g. v1.14.4
 func GetEnvGoSemVer() string {
 	goVersion := GetEnvGoVersion()