main: Set HOME very early on if unset

This works around bootc-image-builder not setting HOME, in combination
with `podman` bombing if it's unset.

Needed for bound images as part of bootc-image-builder.

Signed-off-by: Colin Walters <walters@verbum.org>

cli/src/main.rs

@@ -1,6 +1,8 @@
 use anyhow::Result;
 
-async fn run() -> Result<()> {
+/// The code called after we've done process global init and created
+/// an async runtime.
+async fn async_main() -> Result<()> {
     // Don't include timestamps and such because they're not really useful and
     // too verbose, and plus several log targets such as journald will already
     // include timestamps.
@@ -15,12 +17,33 @@ async fn run() -> Result<()> {
         .with_writer(std::io::stderr)
         .init();
     tracing::trace!("starting");
+    // As you can see, the role of this file is mostly to just be a shim
+    // to call into the code that lives in the internal shared library.
     bootc_lib::cli::run_from_iter(std::env::args()).await
 }
 
-#[tokio::main(flavor = "current_thread")]
-async fn main() {
-    if let Err(e) = run().await {
+/// Perform process global initialization, then create an async runtime
+/// and do the rest of the work there.
+fn run() -> Result<()> {
+    // Initialize global state before we've possibly created other threads, etc.
+    bootc_lib::cli::global_init()?;
+    // We only use the "current thread" runtime because we don't perform
+    // a lot of CPU heavy work in async tasks. Where we do work on the CPU,
+    // or we do want explicit concurrency, we typically use
+    // tokio::task::spawn_blocking to create a new OS thread explicitly.
+    let runtime = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .expect("Failed to build tokio runtime");
+    // And invoke the async_main
+    runtime.block_on(async move { async_main().await })
+}
+
+fn main() {
+    // In order to print the error in a custom format (with :#) our
+    // main simply invokes a run() where all the work is done.
+    // This code just captures any errors.
+    if let Err(e) = run() {
         tracing::error!("{:#}", e);
         std::process::exit(1);
     }

lib/src/cli.rs

@@ -711,6 +711,21 @@ async fn usroverlay() -> Result<()> {
         .into());
 }
 
+/// Perform process global initialization. This should be called as early as possible
+/// in the standard `main` function.
+pub fn global_init() -> Result<()> {
+    let am_root = rustix::process::getuid().is_root();
+    // Work around bootc-image-builder not setting HOME, in combination with podman (really c/common)
+    // bombing out if it is unset.
+    if std::env::var_os("HOME").is_none() && am_root {
+        // Setting the environment is thread-unsafe, but we ask calling code
+        // to invoke this as early as possible. (In practice, that's just the cli's `main.rs`)
+        // xref https://internals.rust-lang.org/t/synchronized-ffi-access-to-posix-environment-variable-functions/15475
+        std::env::set_var("HOME", "/root");
+    }
+    Ok(())
+}
+
 /// Parse the provided arguments and execute.
 /// Calls [`structopt::clap::Error::exit`] on failure, printing the error message and aborting the program.
 pub async fn run_from_iter<I>(args: I) -> Result<()>