Add support for docker in docker

When using docker-in-docker, for example in dev or CI containers, one
normally gives access to the host docker environment to the docker
container. If the container spins up another child container, and wishes
to share a directory, one needs to compute the pass the right paths from
view of the host system.

If the environment variable CROSS_DOCKER_IN_DOCKER=true, we call `docker
inspect $HOSTNAME`, to learn about details of the current container.
The `Mounts` field contains information about all mounts from the host
into the current container. The `GraphDriver` field gives us information
about the storage driver and the location in the host system of the
current containers root directory. Based on these information we compute
a table of all container mounts (`source->destination`).

When starting a child container we adapt all paths to be mounted having
a prefix in the table, to start with source. For example when mounting a
dev containers `/usr/local/cargo` directory, we will actually mount
`/var/lib/docker/overlay2/<parent container id>/merged/usr/local/cargo`.

If the project itself is mounted into the parent container, we will not
use the overlay directory, but find the project path on the host.

In order to access the parent containers rust setup, the child container
mounts the parents overlayfs. The parent must not be stopped before the
child container, as the overlayfs can not be unmounted correctly by
docker if the child container still accesses it.

Cargo.toml

@@ -21,6 +21,7 @@ semver = "0.9"
 toml = "0.5"
 which = { version = "3.1.0", default_features = false }
 shell-escape = "0.1.4"
+serde_json = "1.0.48"
 
 [target.'cfg(not(windows))'.dependencies]
 nix = "0.15"

README.md

@@ -123,6 +123,49 @@ RUN dpkg --add-architecture arm64 && \
 $ docker build -t my/image:tag path/to/where/the/Dockerfile/resides
 ```
 
+### Docker in Docker
+
+When running `cross` from inside a docker container, `cross` needs access to
+the hosts docker daemon itself. This is normally achieved by mounting the
+docker daemons socket `/var/run/docker.sock`. For example:
+
+```
+$ docker run -v /var/run/docker.sock:/var/run/docker.sock -v .:/project \
+  -w /project my/development-image:tag cross build --target mips64-unknown-linux-gnuabi64
+```
+
+The image running `cross` requires the rust development tools to be installed.
+
+With this setup `cross` must find and mount the correct host paths into the
+container used for cross compilation. This includes the original project directory as
+well as the root path of the parent container to give access to the rust build
+tools.
+
+To inform `cross` that it is running inside a container set `CROSS_DOCKER_IN_DOCKER=true`.
+
+A development or CI container can be created like this:
+
+```
+FROM rust:1
+
+# set CROSS_DOCKER_IN_DOCKER to inform `cross` that it is executed from within a container
+ENV CROSS_DOCKER_IN_DOCKER=true
+
+# install `cross`
+RUN cargo install cross
+
+...
+
+```
+
+**Limitations**: Finding the mount point for the containers root directory is
+currently only available for the overlayfs2 storage driver. In order to access
+the parent containers rust setup, the child container mounts the parents
+overlayfs. The parent must not be stopped before the child container, as the
+overlayfs can not be unmounted correctly by Docker if the child container still
+accesses it.
+
+
 ### Passing environment variables into the build environment
 
 By default, `cross` does not pass any environment variables into the build

azure-pipelines.yml

@@ -79,6 +79,9 @@ jobs:
       - bash: echo "##vso[task.setvariable variable=TAG]${BUILD_SOURCEBRANCH##refs/tags/}"
         displayName: Set TAG Variable
         condition: startsWith(variables['Build.SourceBranch'], 'refs/tags/')
+      - bash: cargo test 
+        displayName: Run unit tests
+        timeoutInMinutes: 5
       - bash: ./build-docker-image.sh "${TARGET}"
         displayName: Build Docker Image
         timeoutInMinutes: 360

src/cli.rs

@@ -1,14 +1,16 @@
+use std::str::FromStr;
 use std::{env, path::PathBuf};
 
-use crate::Target;
 use crate::cargo::Subcommand;
 use crate::rustc::TargetList;
+use crate::Target;
 
 pub struct Args {
     pub all: Vec<String>,
     pub subcommand: Option<Subcommand>,
     pub target: Option<Target>,
     pub target_dir: Option<PathBuf>,
+    pub docker_in_docker: bool,
 }
 
 pub fn parse(target_list: &TargetList) -> Args {
@@ -27,7 +29,10 @@ pub fn parse(target_list: &TargetList) -> Args {
                     all.push(t);
                 }
             } else if arg.starts_with("--target=") {
-                target = arg.splitn(2, '=').nth(1).map(|s| Target::from(&*s, target_list));
+                target = arg
+                    .splitn(2, '=')
+                    .nth(1)
+                    .map(|s| Target::from(&*s, target_list));
                 all.push(arg);
             } else if arg == "--target-dir" {
                 all.push(arg);
@@ -41,19 +46,24 @@ pub fn parse(target_list: &TargetList) -> Args {
                     all.push(format!("--target-dir=/target"));
                 }
             } else {
-              if !arg.starts_with('-') && sc.is_none() {
-                  sc = Some(Subcommand::from(arg.as_ref()));
-              }
+                if !arg.starts_with('-') && sc.is_none() {
+                    sc = Some(Subcommand::from(arg.as_ref()));
+                }
 
-              all.push(arg.to_string());
+                all.push(arg.to_string());
             }
         }
     }
 
+    let docker_in_docker = env::var("CROSS_DOCKER_IN_DOCKER")
+        .map(|s| bool::from_str(&s).unwrap_or_default())
+        .unwrap_or_default();
+
     Args {
         all,
         subcommand: sc,
         target,
         target_dir,
+        docker_in_docker,
     }
 }