Add validation for pants_version format (#338)

Implemented a check to ensure the 'pants_version' follows the
'major.minor.micro' format. This update raises a clear error message if
the version string does not meet the expected format, enhancing user
guidance and preventing configuration errors.

Fix #337

package/src/test.rs

@@ -129,6 +129,7 @@ pub(crate) fn run_integration_tests(
         test_ignore_empty_pants_version_pants_sha(scie_pants_scie);
 
         test_pants_from_pex_version(scie_pants_scie);
+        test_pants_from_bad_pex_version(scie_pants_scie);
 
         let clone_root = create_tempdir()?;
         test_use_in_repo_with_pants_script(scie_pants_scie, &clone_root);
@@ -460,6 +461,36 @@ fn test_pants_from_pex_version(scie_pants_scie: &Path) {
     );
 }
 
+fn test_pants_from_bad_pex_version(scie_pants_scie: &Path) {
+    integration_test!(
+        "Verify the output of scie-pants is user-friendly if they provide an invalid pants version"
+    );
+
+    let tmpdir = create_tempdir().unwrap();
+
+    let pants_release = "2.18";
+    let pants_toml_content = format!(
+        r#"
+        [GLOBAL]
+        pants_version = "{pants_release}"
+        "#
+    );
+    let pants_toml = tmpdir.path().join("pants.toml");
+    write_file(&pants_toml, false, pants_toml_content).unwrap();
+
+    let err = execute(
+        Command::new(scie_pants_scie)
+            .arg("-V")
+            .current_dir(&tmpdir)
+            .stderr(Stdio::piped()),
+    )
+    .unwrap_err();
+
+    let error_text = err.to_string();
+    assert!(error_text.contains("Wasn't able to fetch the Pants PEX at"));
+    assert!(error_text.contains("Pants version format not recognized. Please add `.<patch_version>` to the end of the version. For example: `2.18` -> `2.18.0`"));
+}
+
 fn test_use_in_repo_with_pants_script(scie_pants_scie: &Path, clone_root: &TempDir) {
     integration_test!("Verify scie-pants can be used as `pants` in a repo with the `pants` script");
     // This verifies a fix for https://github.com/pantsbuild/scie-pants/issues/28.

tools/src/scie_pants/install_pants.py

@@ -105,8 +105,14 @@ def install_pants_from_pex(
         try:
             ptex.fetch_to_fp(pex_url, pants_pex.file)
         except subprocess.CalledProcessError as e:
+            #  if there's only one dot in version, specifically suggest adding the `.patch`)
+            suggestion = (
+                "Pants version format not recognized. Please add `.<patch_version>` to the end of the version. For example: `2.18` -> `2.18.0`.\n\n"
+                if version.base_version.count(".") < 2
+                else ""
+            )
             fatal(
-                f"Wasn't able to fetch the Pants PEX at {pex_url}.\n\n"
+                f"Wasn't able to fetch the Pants PEX at {pex_url}.\n\n{suggestion}"
                 "Check to see if the URL is reachable (i.e. GitHub isn't down) and if"
                 f" {pex_name} asset exists within the release."
                 " If the asset doesn't exist it may be that this platform isn't yet supported."