Adjust mix nerves.system.shell for OTP 26

OTP 26 made some big changes around serial interface and it breaks the
`mix nerves.system.shell` task. In the end, the main issue is that we
no longer have an easy way to take over the STDIN of the task to forward
it to a shell running in the system build directory.

Many attempts were made to recapture STDIN, but decided that for now we
need to move on. Instead, this adjusts the task to do _all_ the setup
for the build directory as before and outputs a warning to the user for
the command that can be manually run in order to get the same shell that
was previously started with `mix nerves.system.shell`

For local building, it's simply `cd path/to/build`. The more useful
command is for those using docker which is a fairly involved command.

lib/mix/nerves/shell.ex

@@ -8,6 +8,10 @@ defmodule Mix.Nerves.Shell do
   """
   @spec open(String.t(), [String.t()]) :: no_return()
   def open(command, initial_input \\ []) do
+    # We unregister :user so that the process currently holding fd 0 (stdin)
+    # can't send an error message to the console when we steal it.
+    Process.unregister(:user)
+
     # We need to get raw binary access to the stdout file descriptor
     # so we can directly pass through control characters output by the command
     stdout_port = Port.open({:fd, 0, 1}, [:binary, :eof, :stream, :out])

lib/mix/tasks/nerves.system.shell.ex

@@ -39,11 +39,6 @@ defmodule Mix.Tasks.Nerves.System.Shell do
   @doc false
   @spec run([String.t()]) :: :ok
   def run(_argv) do
-    # We unregister :user so that the process currently holding fd 0 (stdin)
-    # can't send an error message to the console when we steal it.
-    user = Process.whereis(:user)
-    Process.unregister(:user)
-
     # Start disabled so that we can configure the system before building it
     # for the first time.
     try do
@@ -70,11 +65,21 @@ defmodule Mix.Tasks.Nerves.System.Shell do
 
     {build_runner, _opts} = pkg.build_runner
 
-    build_runner.system_shell(pkg)
+    {cmd, initial_input} = build_runner.system_shell(pkg)
+
+    if String.to_integer(System.otp_release()) < 26 do
+      # Legacy shell support
+      Mix.Nerves.Shell.open(cmd, initial_input)
+    else
+      Mix.Nerves.IO.shell_warn("shell start deprecated", """
+      OTP 26 made several changes to the serial interface handling. Unfortunately, this
+      a regression in preventing the Nerves tooling from starting a system sub-shell.
 
-    # Set :user back to the real one
-    Process.register(user, :user)
+      However, the build directory and all needed pieces have been created and can be
+      accessed by manually running the command below:
 
-    :ok
+      #{[IO.ANSI.reset(), cmd]}
+      """)
+    end
   end
 end

lib/nerves/artifact/build_runners/docker.ex

@@ -124,20 +124,17 @@ defmodule Nerves.Artifact.BuildRunners.Docker do
   end
 
   @doc """
-  Connect to a system configuration shell in a Docker container
+  Command and initial input for starting a system shell
   """
-  @spec system_shell(Nerves.Package.t()) :: :ok
+  @spec system_shell(Nerves.Package.t()) :: {cmd :: String.t(), initial_input :: [String.t()]}
   def system_shell(pkg) do
     _ = preflight(pkg)
     {_, image} = config(pkg)
-    platform_config = pkg.config[:platform_config][:defconfig]
-    defconfig = Path.join("/nerves/env/#{pkg.app}", platform_config)
 
-    initial_input = [
-      "echo Updating build directory.",
-      "echo This will take a while if it is the first time...",
-      "/nerves/env/platform/create-build.sh #{defconfig} #{@working_dir} >/dev/null"
-    ]
+    {:ok, pid} = Nerves.Utils.Stream.start_link(file: build_log_path())
+    stream = IO.stream(pid, :line)
+    create_build(pkg, stream)
+    Nerves.Utils.Stream.stop(pid)
 
     mounts = Enum.join(mounts(pkg), " ")
     ssh_agent = Enum.join(ssh_agent(), " ")
@@ -148,7 +145,7 @@ defmodule Nerves.Artifact.BuildRunners.Docker do
 
     set_volume_permissions(pkg)
 
-    Mix.Nerves.Shell.open(cmd, initial_input)
+    {cmd, []}
   end
 
   defp preflight(pkg) do

lib/nerves/artifact/build_runners/local.ex

@@ -6,8 +6,6 @@ defmodule Nerves.Artifact.BuildRunners.Local do
   """
   @behaviour Nerves.Artifact.BuildRunner
 
-  alias Nerves.Artifact
-
   @doc """
   Builds an artifact locally.
 
@@ -50,30 +48,10 @@ defmodule Nerves.Artifact.BuildRunners.Local do
   end
 
   @doc """
-  Connect to a system configuration sub-shell
+  Command and initial input for starting a system shell
   """
-  @spec system_shell(Nerves.Package.t()) :: :ok
+  @spec system_shell(Nerves.Package.t()) :: {cmd :: String.t(), initial_input :: [String.t()]}
   def system_shell(pkg) do
-    dest = Artifact.build_path(pkg)
-
-    error = """
-    Could not find bash or sh executable.
-    Make sure one of them are in your $PATH environment variable
-    """
-
-    shell = System.find_executable("bash") || System.find_executable("sh") || Mix.raise(error)
-
-    script = Path.join(Nerves.Env.package(:nerves_system_br).path, "create-build.sh")
-    platform_config = pkg.config[:platform_config][:defconfig]
-    defconfig = Path.join("#{pkg.path}", platform_config)
-
-    initial_input = [
-      "echo Updating build directory.",
-      "echo This will take a while if it is the first time...",
-      "#{script} #{defconfig} #{dest} >/dev/null",
-      "cd #{dest}"
-    ]
-
-    Mix.Nerves.Shell.open(shell, initial_input)
+    pkg.platform.system_shell(pkg)
   end
 end

lib/nerves/system/br.ex

@@ -62,18 +62,39 @@ defmodule Nerves.System.BR do
     make_archive(type, pkg, toolchain, opts)
   end
 
-  defp make(:linux, pkg, _toolchain, opts) do
-    System.delete_env("BINDIR")
-    dest = Artifact.build_path(pkg)
+  @doc """
+  Command and initial input for starting a system shell
+  """
+  @spec system_shell(Nerves.Package.t()) :: {cmd :: String.t(), initial_input :: [String.t()]}
+  def system_shell(pkg) do
+    {:ok, pid} = Nerves.Utils.Stream.start_link(file: "build.log")
+    stream = IO.stream(pid, :line)
+    create_build(pkg, stream)
+    Nerves.Utils.Stream.stop(pid)
+    {"cd #{Artifact.build_path(pkg)}", []}
+  end
 
+  defp create_build(pkg, stream) do
+    dest = Artifact.build_path(pkg)
     script = Path.join(Nerves.Env.package(:nerves_system_br).path, "create-build.sh")
     platform_config = pkg.config[:platform_config][:defconfig]
     defconfig = Path.join("#{pkg.path}", platform_config)
-    _ = shell(script, [defconfig, dest])
+
+    case shell(script, [defconfig, dest], stream: stream) do
+      {_, 0} -> :ok
+      _ -> Mix.raise("Failed to create build directory")
+    end
+  end
+
+  defp make(:linux, pkg, _toolchain, opts) do
+    System.delete_env("BINDIR")
+    dest = Artifact.build_path(pkg)
 
     {:ok, pid} = Nerves.Utils.Stream.start_link(file: "build.log")
     stream = IO.stream(pid, :line)
 
+    create_build(pkg, stream)
+
     make_args = Keyword.get(opts, :make_args, [])
 
     case shell("make", make_args, cd: dest, stream: stream) do