[ABLD-8] Proof of concept of creating install scripts with bazel

BUILD.bazel

@@ -49,9 +49,34 @@ config_setting(
     ],
 )
 
-# The location base dir to install packages in while building.
+# The location that the bulk of the agent gets installed to. Like most *nix tools, this
+# is in /opt. That will hold shared libraries, and python packages.  It is not the
+# complete installation, becuase some things go to /etc or /usr.
+#
+# Migration to bazel warning:
+# The goal we are aiming for is that output base is always passed to build
+# scripts and we install artifacts to output_base + install_dir. Unfortunately,
+# many existing scripts only use install_dir, without regard to a a base. They
+# put etc files under opt/datadog, then move them to /etc in the final part of the
+# build.
+# TODO: We need to learn how people expect to use install dir to create custom
+# builds for local testing that do not wipe out their local installation.
 string_flag(
     name = "install_dir",
-    build_setting_default = "/tmp/opt/datadog-agent",
+    build_setting_default = "/opt/datadog-agent",
     make_variable = "INSTALL_DIR",
 )
+
+# This seems to be the base of where to write config files during
+# the build. That is, if we were creating /etc/foo, it would be
+# the path prepended to /etc to get a safe place to write.
+# We can set this from the omnibus variable OUTPUT_CONFIG_DIR.
+# But, it seems to be empty in CI runs, so we end up writing to /.
+# The default is /tmp so we don't shoot our feet off in local testing.
+# If we end up keeping this flag, let's give it a better name and
+# git it a better value.
+string_flag(
+    name = "output_config_dir",
+    build_setting_default = "/tmp",
+    make_variable = "OUTPUT_CONFIG_DIR",
+)

bazel/rules/dd_agent_expand_template.bzl

@@ -0,0 +1,79 @@
+"""dd_agent_expand_template. Expand a template, splicing in agent specific flags."""
+
+load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
+
+# The place we will install to if we run bazel pkg_install without a destdir
+# We use /tmp for lack of a better safe space.
+DEFAULT_OUTPUT_CONFIG_DIR = "/tmp"
+
+# The location where the product should be installed on a user system.
+DEFAULT_PRODUCT_DIR = "/opt/datadog-agent"
+
+def _dd_agent_expand_template_impl(ctx):
+    # Set up a fallback default.
+    # TODO: should this be different for windows? Or should we have different variables for windows?
+    subs = {}
+    subs["{output_config_dir}"] = DEFAULT_OUTPUT_CONFIG_DIR
+    if ctx.attr._output_config_dir:
+        if BuildSettingInfo in ctx.attr._output_config_dir:
+            output_config_dir = ctx.attr._output_config_dir[BuildSettingInfo].value.rstrip("/")
+            subs["{output_config_dir}"] = output_config_dir
+    subs["{install_dir}"] = DEFAULT_PRODUCT_DIR
+    if ctx.attr._install_dir:
+        if BuildSettingInfo in ctx.attr._install_dir:
+            install_dir = ctx.attr._install_dir[BuildSettingInfo].value
+            subs["{install_dir}"] = install_dir
+
+    # TODO: decide if we should default etc to the output base or relative to install_dir.
+    # There are use cases for either. For now, we are relative to the output base.
+    subs["{etc_dir}"] = subs["{output_config_dir}"] + "/etc"
+
+    # Now let local substitutions override the flags.
+    subs.update(ctx.attr.substitutions)
+
+    # let's add a little flavor. We don't need this today but it will be
+    # needed if we expand things like the file name of an artifact and we want
+    # to show compliation mode. This mostly shows the technique.
+    subs["{TARGET_CPU}"] = ctx.var["TARGET_CPU"]
+    subs["{COMPILATION_MODE}"] = ctx.var["COMPILATION_MODE"]
+
+    ctx.actions.expand_template(
+        template = ctx.file.template,
+        output = ctx.outputs.out,
+        substitutions = subs,
+    )
+
+dd_agent_expand_template = rule(
+    implementation = _dd_agent_expand_template_impl,
+    doc = """Template expansion
+
+This performs a simple search over the template file for the keys in
+substitutions, and replaces them with the corresponding values.  There are
+some default substitutions which are computed from the build environment.
+
+Default substitutions:
+  {install_dir}:  The value of the flag //:install_dir
+  {output_config_dir}:  The value of the flag //:output_config_dir
+  {etc_dir}: The output_config_dir + "/etc"
+  {TARGET_CPU}: Target CPU arch
+  {COMPILATION_MODE}: usually fastbuild, opt, or release.
+
+There is no special syntax for the keys from substitutions. To avoid conflicts, you would need to
+explicitly add delimiters to the key strings, for example "{KEY}" or "@KEY@".""",
+    attrs = {
+        "template": attr.label(
+            mandatory = True,
+            allow_single_file = True,
+            doc = "The template file to expand.",
+        ),
+        "substitutions": attr.string_dict(
+            doc = "A dictionary mapping strings to their substitutions. These take precedence over flags.",
+        ),
+        "out": attr.output(
+            mandatory = True,
+            doc = "The destination of the expanded file.",
+        ),
+        "_install_dir": attr.label(default = "@@//:install_dir"),
+        "_output_config_dir": attr.label(default = "@@//:output_config_dir"),
+    },
+)

omnibus/config/software/init-scripts-agent.rb

@@ -3,20 +3,23 @@
 description "Generate and configure init scripts packaging"
 
 always_build true
+skip_transitive_dependency_licensing true
 
 build do
+  destdir = ENV["OMNIBUS_BASE_DIR"] || "/"
   output_config_dir = ENV["OUTPUT_CONFIG_DIR"] || ""
   if linux_target?
     etc_dir = "#{output_config_dir}/etc/datadog-agent"
     mkdir "/etc/init"
     if debian_target?
+      # building into / is not acceptable. We'll continue to to that for now,
+      # but the replacement has to build to a build output tree.
+      command_on_repo_root "bazelisk run --//:output_config_dir='#{output_config_dir}' --//:install_dir=#{install_dir} -- //packages/debian/etc:install --verbose --destdir=#{destdir}"
+      command_on_repo_root "cp bazel-bin/packages/debian/etc/etc/init/datadog-agent.conf /etc/init"
+
       # sysvinit support for debian only for now
       mkdir "/etc/init.d"
 
-      erb source: "upstart_debian.conf.erb",
-          dest: "/etc/init/datadog-agent.conf",
-          mode: 0644,
-          vars: { install_dir: install_dir, etc_dir: etc_dir }
       erb source: "upstart_debian.process.conf.erb",
           dest: "/etc/init/datadog-agent-process.conf",
           mode: 0644,
@@ -58,6 +61,7 @@
           mode: 0755,
           vars: { install_dir: install_dir, etc_dir: etc_dir }
 
+      project.extra_package_file '/etc/init/datadog-agent.conf'
       project.extra_package_file '/etc/init.d/datadog-agent'
       project.extra_package_file '/etc/init.d/datadog-agent-process'
       project.extra_package_file '/etc/init.d/datadog-agent-trace'

packages/debian/etc/BUILD.bazel

@@ -0,0 +1,30 @@
+"""Rules to build /etc for Debian distributions."""
+
+load("@rules_pkg//pkg:install.bzl", "pkg_install")
+load("@rules_pkg//pkg:mappings.bzl", "pkg_files", "strip_prefix")
+load("//bazel/rules:dd_agent_expand_template.bzl", "dd_agent_expand_template")
+
+package(default_visibility = ["//packages:__subpackages__"])
+
+dd_agent_expand_template(
+    name = "gen_etc",
+    out = "etc/init/datadog-agent.conf",
+    template = "upstart_debian.conf.in",
+)
+
+# TODO: Have dd_agent_expand_template emit a pkg provider, so we
+# don't need this intermediary any more.
+pkg_files(
+    name = "all_files",
+    srcs = [
+        ":gen_etc",
+    ],
+    strip_prefix = strip_prefix.from_pkg(),
+)
+
+pkg_install(
+    name = "install",
+    srcs = [
+        ":all_files",
+    ],
+)

packages/debian/etc/upstart_debian.conf.in

@@ -0,0 +1,24 @@
+description "Datadog Agent"
+
+start on started networking
+stop on runlevel [!2345]
+
+respawn
+respawn limit 4 25
+normal exit 0
+
+# Logging to console from the agent is disabled since the agent already logs using file or
+# syslog depending on its configuration. We make upstart log what the process still outputs in order
+# to log panics/crashes to /var/log/upstart/datadog-agent.log
+console log
+env DD_LOG_TO_CONSOLE=false
+
+setuid dd-agent
+
+script
+  exec {install_dir}/bin/agent/agent run -p {install_dir}/run/agent.pid
+end script
+
+post-stop script
+  rm -f {install_dir}/run/agent.pid
+end script