Preparation for auto-downloading micromamba

We will attempt to download micromamba if we can't find a working one in
$PATH. This establishes some of the groundwork for doing that, including
the initial check to see if we already have a working `micromamba`
command in $PATH, but it does not do attempt the download, yet.

Author: relrod

src/env.rs

@@ -96,7 +96,7 @@ pub fn run(config: Config, subcommand: Subcommand) -> ExitCode {
                 error!("No environment name could be determined. You can specify one with --name");
                 return ExitCode::FAILURE;
             };
-            let mut cmd = micromamba(
+            let Some(mut cmd) = micromamba(
                 &config,
                 vec![
                     "env",
@@ -107,7 +107,10 @@ pub fn run(config: Config, subcommand: Subcommand) -> ExitCode {
                     &env_name,
                     "--yes",
                 ],
-            );
+            ) else {
+                // micromamba() will already log an error, so just exit
+                return ExitCode::FAILURE;
+            };
             if config.noop_mode {
                 info!("Would run: {:?}", cmd);
                 ExitCode::SUCCESS

src/micromamba.rs

@@ -1,9 +1,8 @@
 //! This module deals with `micromamba` - obtaining it, calling it, etc.
 
 use crate::csmrc::Config;
-use log::debug;
+use log::{debug, error};
 use std::collections::HashMap;
-use std::io::{Error, ErrorKind};
 use std::path::PathBuf;
 use std::process::Command;
 
@@ -12,6 +11,7 @@ pub enum MicromambaLocation {
     InPath,
 
     /// A working `micromamba` exists at the given path.
+    #[allow(dead_code)]
     Path(PathBuf),
 }
 
@@ -24,17 +24,27 @@ pub fn micromamba_at(location: MicromambaLocation, config: &Config, args: Vec<&s
         env_vars.insert("MAMBA_ROOT_PREFIX", mamba_root_prefix.to_string());
     }
 
-    let mut cmd = Command::new("micromamba");
+    let binary_path = match location {
+        MicromambaLocation::InPath => "micromamba",
+        MicromambaLocation::Path(ref path) => &path.to_string_lossy(),
+    };
+    let mut cmd = Command::new(binary_path);
     cmd.args(args);
     cmd.envs(env_vars);
     debug!("About to run: {:?}", cmd);
     cmd
 }
 
-pub fn micromamba(config: &Config, args: Vec<&str>) -> Result<Command, Error> {
+pub fn micromamba(config: &Config, args: Vec<&str>) -> Option<Command> {
     match micromamba_path(config) {
-        None => Err(Error::from(ErrorKind::NotFound)),
-        Some(location) => Ok(micromamba_at(location, config, args))
+        None => {
+            // TODO: Make this error nicer
+            error!(
+                "Could not find a micromamba executable to run. Please download it manually, make sure it's executable, and put it somewhere in $PATH."
+            );
+            None
+        }
+        Some(location) => Some(micromamba_at(location, config, args)),
     }
 }
 
@@ -47,6 +57,9 @@ pub fn micromamba(config: &Config, args: Vec<&str>) -> Result<Command, Error> {
 ///    cache directory. (We cannot rely on this - it could be that the user's
 ///    cache directory is mounted noexec or similar).
 ///
+/// Note that this function tries to call `micromamba --version`, even in no-op
+/// mode.
+///
 /// Alternative approaches that we do not take here currently:
 /// - On Linux, we *could* in theory use memfd_create + fexecve to embed the app
 ///   and run it from memory. This won't work on Windows.
@@ -56,9 +69,13 @@ pub fn micromamba(config: &Config, args: Vec<&str>) -> Result<Command, Error> {
 ///   than downloading it. But this inflates our binary size.
 pub fn micromamba_path(config: &Config) -> Option<MicromambaLocation> {
     // Assume it's in $PATH already and see what happens.
-    if let Ok(output) = micromamba_at(MicromambaLocation::InPath, config, vec!["--version"]).output() {
+    let cmd = micromamba_at(MicromambaLocation::InPath, config, vec!["--version"]).output();
+    if let Ok(output) = cmd {
         // The full path doesn't matter, it's in $PATH
-        debug!("micromamba reported version {}", String::from_utf8_lossy(&output.stdout));
+        debug!(
+            "micromamba reported version {}",
+            String::from_utf8_lossy(&output.stdout)
+        );
         return Some(MicromambaLocation::InPath);
     }
     None