Read go version from go.mod

src/installer.ts

@@ -266,3 +266,12 @@ export function makeSemver(version: string): string {
 
   return `${verPart}${prereleasePart}`;
 }
+
+export function parseGoVersionFile(contents: string, isMod: boolean): string {
+  if (!isMod) {
+    return contents.trim();
+  }
+
+  const match = contents.match(/^go (\d+(\.\d+)*)/m);
+  return match ? match[1] : '';
+}

src/main.ts

@@ -12,14 +12,7 @@ export async function run() {
     // versionSpec is optional.  If supplied, install / use from the tool cache
     // If not supplied then problem matchers will still be setup.  Useful for self-hosted.
     //
-    const versionFilePath = core.getInput('go-version-from-file');
-    const versionSpecFromFile =
-      versionFilePath &&
-      fs
-        .readFileSync(versionFilePath)
-        .toString()
-        .trim();
-    let versionSpec = core.getInput('go-version') || versionSpecFromFile;
+    const versionSpec = resolveVersionInput();
 
     // stable will be true unless false is the exact input
     // since getting unstable versions should be explicit
@@ -97,3 +90,21 @@ function isGhes(): boolean {
   );
   return ghUrl.hostname.toUpperCase() !== 'GITHUB.COM';
 }
+
+function resolveVersionInput(): string {
+  let version = core.getInput('go-version');
+  const versionFilePath = core.getInput('go-version-file');
+
+  if (version) {
+    return version;
+  }
+
+  if (versionFilePath) {
+    version = installer.parseGoVersionFile(
+      fs.readFileSync(versionFilePath).toString(),
+      path.basename(versionFilePath) === 'go.mod'
+    );
+  }
+
+  return version;
+}