From f046ff5d0c9ccba5457c71a995905f6e85fa51d4 Mon Sep 17 00:00:00 2001
From: Alex Huszagh <ahuszagh@gmail.com>
Date: Wed, 22 Jun 2022 14:28:53 -0500
Subject: [PATCH] Add a cache dir tag when creating a target directory.

Ensure `cross` is similar in functionality to `cargo`, and this
minimizes network traffic by applications that may copy the project but
do not wish to copy compiled code.

Closes #835.
---
 CHANGELOG.md         |  1 +
 src/docker/shared.rs | 20 +++++++++++++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 67752ebe9..c6c127911 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -48,6 +48,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 
+- #836 - write a `CACHEDIR.TAG` when creating the target directory, similar to `cargo`.
 - #804 - allow usage of env `CARGO_BUILD_TARGET` as an alias for `CROSS_BUILD_TARGET`
 - #792 - fixed container-in-container support when using podman.
 - #781 - ensure `target.$(...)` config options override `build` ones.
diff --git a/src/docker/shared.rs b/src/docker/shared.rs
index 85b462a40..ac3b51a17 100644
--- a/src/docker/shared.rs
+++ b/src/docker/shared.rs
@@ -62,7 +62,7 @@ impl Directories {
         // otherwise `docker` will create them but they will be owned by `root`
         fs::create_dir(&cargo).ok();
         fs::create_dir(&xargo).ok();
-        fs::create_dir(&target).ok();
+        create_target_dir(target)?;
 
         let cargo = mount_finder.find_mount_path(cargo);
         let xargo = mount_finder.find_mount_path(xargo);
@@ -111,6 +111,24 @@ impl Directories {
     }
 }
 
+const CACHEDIR_TAG: &str = "Signature: 8a477f597d28d172789f06886806bc55
+# This file is a cache directory tag created by cross.
+# For information about cache directory tags see https://bford.info/cachedir/";
+
+fn create_target_dir(path: &Path) -> Result<()> {
+    // cargo creates all paths to the target directory, and writes
+    // a cache dir tag only if the path doesn't previously exist.
+    if !path.exists() {
+        fs::create_dir_all(&path)?;
+        fs::OpenOptions::new()
+            .write(true)
+            .create_new(true)
+            .open(&path.join("CACHEDIR.TAG"))?
+            .write_all(CACHEDIR_TAG.as_bytes())?;
+    }
+    Ok(())
+}
+
 pub fn command(engine: &Engine) -> Command {
     Command::new(&engine.path)
 }