feat(rootless): help detection with execv check "is current builder rootless?" (#1380)

I recently encountered #1098 and propose an iteration on the fix in #890

This patch adds a last minute check that looks up the current builder
endpoint. This may be seen as a costly operation however, as mentioned
in #889

Here's the output of `docker builder inspect` on my rootless install:
```
Name:          rootless
Driver:        docker
Last Activity: 2023-12-03 02:04:14 +0000 UTC

Nodes:
Name:      rootless
Endpoint:  rootless  # <= THIS HERE
Status:    running
Buildkit:  v0.11.7+d3e6c1360f6e
Platforms: linux/amd64, linux/amd64/v2, linux/amd64/v3, linux/amd64/v4, linux/386
Labels:
 org.mobyproject.buildkit.worker.moby.host-gateway-ip: 172.17.0.1
```

Author: Emilgardis

src/docker/engine.rs

@@ -66,6 +66,7 @@ pub struct Engine {
     pub arch: Option<Architecture>,
     pub os: Option<ContainerOs>,
     pub is_remote: bool,
+    pub is_rootless: bool,
 }
 
 impl Engine {
@@ -94,6 +95,7 @@ impl Engine {
             None => Self::in_docker(msg_info)?,
         };
         let (kind, arch, os) = get_engine_info(&path, msg_info)?;
+        let is_rootless = is_rootless(kind).unwrap_or_else(|| is_docker_rootless(&path, msg_info));
         let is_remote = is_remote.unwrap_or_else(Self::is_remote);
         Ok(Engine {
             path,
@@ -102,6 +104,7 @@ impl Engine {
             arch,
             os,
             is_remote,
+            is_rootless,
         })
     }
 
@@ -143,6 +146,69 @@ impl Engine {
     }
 }
 
+fn is_rootless(kind: EngineType) -> Option<bool> {
+    env::var("CROSS_ROOTLESS_CONTAINER_ENGINE")
+        .ok()
+        .and_then(|s| match s.as_ref() {
+            "auto" => None,
+            b => Some(bool_from_envvar(b)),
+        })
+        .or_else(|| (!kind.is_docker()).then_some(true))
+}
+
+#[must_use]
+fn is_docker_rootless(ce: &Path, msg_info: &mut MessageInfo) -> bool {
+    let mut cmd = Command::new(ce);
+    cmd.args(["info", "-f", "{{.SecurityOptions}}"])
+        .run_and_get_output(msg_info)
+        .ok()
+        .and_then(|cmd| cmd.stdout().ok())
+        .map(|out| {
+            out.to_lowercase()
+                .replace(['[', ' ', ']'], ",")
+                .contains(",name=rootless,")
+        })
+        .unwrap_or_default()
+}
+
+#[test]
+fn various_is_rootless_configs() {
+    let var = "CROSS_ROOTLESS_CONTAINER_ENGINE";
+    let old = env::var(var);
+    env::remove_var(var);
+
+    assert!(!is_rootless(EngineType::Docker).unwrap_or(false));
+    assert!(is_rootless(EngineType::Docker).unwrap_or(true));
+
+    assert_eq!(is_rootless(EngineType::Docker), None);
+    assert_eq!(is_rootless(EngineType::Podman), Some(true));
+    assert_eq!(is_rootless(EngineType::PodmanRemote), Some(true));
+    assert_eq!(is_rootless(EngineType::Other), Some(true));
+
+    env::set_var(var, "0");
+    assert_eq!(is_rootless(EngineType::Docker), Some(false));
+    assert_eq!(is_rootless(EngineType::Podman), Some(false));
+    assert_eq!(is_rootless(EngineType::PodmanRemote), Some(false));
+    assert_eq!(is_rootless(EngineType::Other), Some(false));
+
+    env::set_var(var, "1");
+    assert_eq!(is_rootless(EngineType::Docker), Some(true));
+    assert_eq!(is_rootless(EngineType::Podman), Some(true));
+    assert_eq!(is_rootless(EngineType::PodmanRemote), Some(true));
+    assert_eq!(is_rootless(EngineType::Other), Some(true));
+
+    env::set_var(var, "auto");
+    assert_eq!(is_rootless(EngineType::Docker), None);
+    assert_eq!(is_rootless(EngineType::Podman), Some(true));
+    assert_eq!(is_rootless(EngineType::PodmanRemote), Some(true));
+    assert_eq!(is_rootless(EngineType::Other), Some(true));
+
+    match old {
+        Ok(v) => env::set_var(var, v),
+        Err(_) => env::remove_var(var),
+    }
+}
+
 // determine if the container engine is docker. this fixes issues with
 // any aliases (#530), and doesn't fail if an executable suffix exists.
 fn get_engine_info(

src/docker/local.rs

@@ -73,7 +73,7 @@ pub(crate) fn run(
     docker
         .add_seccomp(engine.kind, &options.target, &paths.metadata)
         .wrap_err("when copying seccomp profile")?;
-    docker.add_user_id(engine.kind);
+    docker.add_user_id(engine.is_rootless);
 
     docker
         .args([

src/docker/remote.rs

@@ -972,7 +972,7 @@ symlink_recurse \"${{prefix}}\"
 
     // 6. execute our cargo command inside the container
     let mut docker = engine.subcommand("exec");
-    docker.add_user_id(engine.kind);
+    docker.add_user_id(engine.is_rootless);
     docker.add_envvars(&options, toolchain_dirs, msg_info)?;
     docker.add_cwd(&paths)?;
     docker.arg(&container_id);

src/docker/shared.rs

@@ -10,7 +10,7 @@ use super::image::PossibleImage;
 use super::Image;
 use super::PROVIDED_IMAGES;
 use crate::cargo::CargoMetadata;
-use crate::config::{bool_from_envvar, Config};
+use crate::config::Config;
 use crate::errors::*;
 use crate::extensions::{CommandExt, SafeCommand};
 use crate::file::{self, write_file, PathExt, ToUtf8};
@@ -947,7 +947,7 @@ pub(crate) trait DockerCommandExt {
     ) -> Result<()>;
     fn add_cwd(&mut self, paths: &DockerPaths) -> Result<()>;
     fn add_build_command(&mut self, dirs: &ToolchainDirectories, cmd: &SafeCommand) -> &mut Self;
-    fn add_user_id(&mut self, engine_type: EngineType);
+    fn add_user_id(&mut self, is_rootless: bool);
     fn add_userns(&mut self);
     fn add_seccomp(
         &mut self,
@@ -1094,17 +1094,10 @@ impl DockerCommandExt for Command {
         self.args(["sh", "-c", &build_command])
     }
 
-    fn add_user_id(&mut self, engine_type: EngineType) {
+    fn add_user_id(&mut self, is_rootless: bool) {
         // by default, docker runs as root so we need to specify the user
         // so the resulting file permissions are for the current user.
         // since we can have rootless docker, we provide an override.
-        let is_rootless = env::var("CROSS_ROOTLESS_CONTAINER_ENGINE")
-            .ok()
-            .and_then(|s| match s.as_ref() {
-                "auto" => None,
-                b => Some(bool_from_envvar(b)),
-            })
-            .unwrap_or_else(|| engine_type != EngineType::Docker);
         if !is_rootless {
             self.args(["--user", &format!("{}:{}", user_id(), group_id(),)]);
         }
@@ -1519,45 +1512,17 @@ mod tests {
 
     #[test]
     fn test_docker_user_id() {
-        let var = "CROSS_ROOTLESS_CONTAINER_ENGINE";
-        let old = env::var(var);
-        env::remove_var(var);
-
         let rootful = format!("\"engine\" \"--user\" \"{}:{}\"", id::user(), id::group());
         let rootless = "\"engine\"".to_owned();
 
-        let test = |engine, expected| {
+        let test = |noroot, expected| {
             let mut cmd = Command::new("engine");
-            cmd.add_user_id(engine);
+            cmd.add_user_id(noroot);
             assert_eq!(expected, &format!("{cmd:?}"));
         };
-        test(EngineType::Docker, &rootful);
-        test(EngineType::Podman, &rootless);
-        test(EngineType::PodmanRemote, &rootless);
-        test(EngineType::Other, &rootless);
-
-        env::set_var(var, "0");
-        test(EngineType::Docker, &rootful);
-        test(EngineType::Podman, &rootful);
-        test(EngineType::PodmanRemote, &rootful);
-        test(EngineType::Other, &rootful);
-
-        env::set_var(var, "1");
-        test(EngineType::Docker, &rootless);
-        test(EngineType::Podman, &rootless);
-        test(EngineType::PodmanRemote, &rootless);
-        test(EngineType::Other, &rootless);
-
-        env::set_var(var, "auto");
-        test(EngineType::Docker, &rootful);
-        test(EngineType::Podman, &rootless);
-        test(EngineType::PodmanRemote, &rootless);
-        test(EngineType::Other, &rootless);
 
-        match old {
-            Ok(v) => env::set_var(var, v),
-            Err(_) => env::remove_var(var),
-        }
+        test(false, &rootful);
+        test(true, &rootless);
     }
 
     #[test]