Fix unwanted rebuilds in cargo xtask diff-walk

Rustup/cargo don't handle nested invocations well. Ideally, `cargo run`
would behave the same as `cargo build && <run the program>`, but in
practice it does not, because rustup/cargo set env vars which affect
nested invocations of rustup/cargo.

Recently (possibly in 1.85, but not sure) this resulted in `cargo xtask
diff-walk` always building twice: once for `cargo xtask`, then a second
time for the release build of `mount_and_walk`. Running the same command
again would not use the cache, but rebuild everything. Fix by clearing
all env vars, except for the PATH. The PATH needs to be preserved in
order to find which `cargo` to execute.

Author: nicholasbishop

xtask/src/diff_walk.rs

@@ -10,6 +10,7 @@ use crate::{capture_cmd, run_cmd, sudo};
 use anyhow::{Result, bail};
 use ext4_view::{Ext4, Ext4Error};
 use sha2::{Digest, Sha256};
+use std::env;
 use std::fs::File;
 use std::io::Read;
 use std::os::unix::ffi::OsStrExt;
@@ -155,14 +156,30 @@ fn is_compressed(path: &Path) -> Result<bool> {
 /// requires elevated permissions.
 pub fn diff_walk(orig_path: &Path) -> Result<()> {
     // Build `mount_and_walk` in release mode.
-    run_cmd(Command::new("cargo").args([
-        "build",
-        "--release",
-        "--package",
-        "xtask",
-        "--bin",
-        "mount_and_walk",
-    ]))?;
+    let path = env::var("PATH")?;
+    run_cmd(
+        Command::new("cargo")
+            .args([
+                "build",
+                "--release",
+                "--package",
+                "xtask",
+                "--bin",
+                "mount_and_walk",
+            ])
+            // Clear the env except for the PATH var. This avoids
+            // unwanted rebuilds. The details are complicated, but
+            // basically rustup/cargo are not designed to allow nested
+            // invocations. Clearing env vars tricks rustup/cargo into
+            // behaving correctly.
+            //
+            // Some past issues that may be relevant (but probably don't
+            // tell the whole story):
+            // * https://github.com/rust-lang/rustup/issues/3036
+            // * https://github.com/rust-lang/cargo/issues/15099
+            .env_clear()
+            .env("PATH", path),
+    )?;
 
     // If the input file is compressed, decompress it and write to a
     // temporary file.