Create vmtest.log lazily

It is super annoying to have an empty .vmtest.log dropped anywhere the
program has been run. Provide some logic that makes it so that the log
file is created lazily and only when data is actually written to it.

Signed-off-by: Daniel Müller <deso@posteo.net>

src/main.rs

@@ -1,9 +1,10 @@
-use std::env;
+use std::cell::OnceCell;
 use std::env::consts::ARCH;
-use std::fs;
+use std::fs::{self, File};
 use std::io::{stdout, IsTerminal as _};
 use std::path::{Path, PathBuf};
 use std::process::exit;
+use std::{env, io};
 
 use anyhow::{Context, Result};
 use clap::Parser;
@@ -43,6 +44,38 @@ struct Args {
     command: Vec<String>,
 }
 
+/// A type representing a log that creates the associated file lazily
+/// upon first write.
+#[derive(Default)]
+struct DeferredLog {
+    file: OnceCell<File>,
+}
+
+impl DeferredLog {
+    fn file(&mut self) -> &File {
+        self.file.get_or_init(|| {
+            fs::OpenOptions::new()
+                .create(true)
+                .append(true)
+                .open(".vmtest.log")
+                // The `log` infrastructure would just swallow errors on
+                // the regular write path and so we panic here to convey
+                // any issues to users.
+                .unwrap_or_else(|err| panic!("failed to create .vmtest.log: {err}"))
+        })
+    }
+}
+
+impl io::Write for DeferredLog {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.file().write(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.file().flush()
+    }
+}
+
 /// Initialize logging
 ///
 /// This will send logs to a file named `.vmtest.log` if user is running
@@ -52,14 +85,7 @@ struct Args {
 fn init_logging() -> Result<()> {
     let target = match stdout().is_terminal() {
         false => LogTarget::Stderr,
-        true => {
-            let file = fs::OpenOptions::new()
-                .create(true)
-                .append(true)
-                .open(".vmtest.log")
-                .context("Failed to open log file")?;
-            LogTarget::Pipe(Box::new(file))
-        }
+        true => LogTarget::Pipe(Box::new(DeferredLog::default())),
     };
 
     Builder::from_default_env()