Treat empty PANTS_VERSION and PANTS_SHA as if they were unset (#190)

Overriding the pants version via the environment variables is
convenient, and this PR makes it slightly more convenient by having
`PANTS_VERSION= pants ...` treated the same as not setting it.

This means a script can do something like `for PANTS_VERSION in ""
"2.15.0" ...;` to test against multiple versions, including whatever is
natively configured in `pants.toml`, without having to conditionalise
`PANTS_VERSION`'s existence entirely. For a concrete example,
pantsbuild/pants#19161 wants to be able to run a
foreign repo's "native" pants version, but it's not trivial to do, due
to this issue (the `""` part of the pants version matrix is commented
out; obviously I could also have it set the env var conditionally, but
that's less convenient).

Before this change, running `PANTS_VERSION= pants version` next to a
`pants.toml` would prompt for initialising pants:

```
PANTS_VERSION= pants version                                                         
No Pants configuration was found at or above /Users/huon/projects/stellargraph/exoflare.
Would you like to configure /Users/huon/projects/stellargraph/exoflare as a Pants project? (Y/n):
```

After this change, it's the same as not setting `PANTS_VERSION`: use
`pants.toml`'s setting if one exists, or prompt for initialisation, if
there's no `pants.toml`.

Author: huonw

CHANGES.md

@@ -1,5 +1,11 @@
 # Release Notes
 
+## 0.8.1
+
+This release adjusts the `PANTS_SHA` and `PANTS_VERSION` environment variables to be ignored, if
+they're set to an empty string. For instance, `PANTS_SHA= PANTS_VERSION= pants ...` will now behave
+the same as `pants ...`.
+
 ## 0.8.0
 
 This release brings no new features or bug fixes, but it does convert the building of the

Cargo.lock

@@ -6,7 +6,7 @@ members = [
 [package]
 name = "scie-pants"
 description = "Protects your Pants from the elements."
-version = "0.8.0"
+version = "0.8.1"
 edition = "2021"
 authors = [
     "John Sirois <[email protected]>",

Cargo.toml

@@ -100,6 +100,7 @@ pub(crate) fn run_integration_tests(
         test_python_repos_repos(scie_pants_scie);
         test_initialize_new_pants_project(scie_pants_scie);
         test_set_pants_version(scie_pants_scie);
+        test_ignore_empty_pants_version_pants_sha(scie_pants_scie);
 
         let clone_root = create_tempdir()?;
         test_use_in_repo_with_pants_script(scie_pants_scie, &clone_root);
@@ -357,6 +358,35 @@ fn test_set_pants_version(scie_pants_scie: &Path) {
     .unwrap();
 }
 
+fn test_ignore_empty_pants_version_pants_sha(scie_pants_scie: &Path) {
+    integration_test!("Verifying ignoring PANTS_SHA and PANTS_VERSION when set to empty string");
+
+    let tmpdir = create_tempdir().unwrap();
+
+    let pants_release = "2.15.0";
+    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 output = execute(
+        Command::new(scie_pants_scie)
+            .arg("-V")
+            .env("PANTS_SHA", "")
+            .env("PANTS_VERSION", "")
+            .current_dir(&tmpdir)
+            .stdout(Stdio::piped()),
+    );
+    assert_eq!(
+        pants_release,
+        decode_output(output.unwrap().stdout).unwrap().trim()
+    );
+}
+
 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.

package/src/test.rs

@@ -74,12 +74,14 @@ impl Process {
 }
 
 fn env_version(env_var_name: &str) -> Result<Option<String>> {
-    if let Some(raw_version) = env::var_os(env_var_name) {
+    let raw_version = env::var_os(env_var_name).unwrap_or(OsString::new());
+    if raw_version.len() == 0 {
+        // setting PANTS_VERSION= or PANTS_SHA= behaves the same as not setting them
+        Ok(None)
+    } else {
         Ok(Some(raw_version.into_string().map_err(|raw| {
             anyhow!("Failed to interpret {env_var_name} {raw:?} as UTF-8 string.")
         })?))
-    } else {
-        Ok(None)
     }
 }