Let's try a github action to build things (#743)

Summary:
This commit adds a getdeps command that is able to generate
a workflow file for the GitHub Actions CI environment.

The workflow file could be expressed more simply using the matrix
syntax and with three steps (checkout, build, test), but I chose to
break out the steps for each of the dependencies because the UX
while waiting on the build is much nicer that way: the steps show
during and live log tailing for the section of the build that is
underway.  If they were all lumped into a single build step then
the logs from the boost section of the build dominate and make
the github UI work very hard.

Pull Request resolved: https://github.com/facebook/watchman/pull/743

Test Plan:
https://github.com/facebook/watchman/pull/743 successfully
executes the github actions CI flow.

```
$ opensource/fbcode_builder/getdeps.py generate-github-actions --output-file watchman/.github/workflows/main.yml watchman
```

Reviewed By: simpkins

Differential Revision: D17384915

Pulled By: wez

fbshipit-source-id: 9a9e5a3e806c18f6cc38ba1cb7059740cda01ad4

build/fbcode_builder/getdeps.py

@@ -527,6 +527,108 @@ class TestCmd(ProjectCmdBase):
         parser.add_argument("--test-owner", help="Owner for testpilot")
 
 
+@cmd("generate-github-actions", "generate a GitHub actions configuration")
+class GenerateGitHubActionsCmd(ProjectCmdBase):
+    def run_project_cmd(self, args, loader, manifest):
+        platforms = [
+            HostType("linux", "ubuntu", "18"),
+            HostType("darwin", None, None),
+            HostType("windows", None, None),
+        ]
+
+        with open(args.output_file, "w") as out:
+            # Deliberate line break here because the @ and the generated
+            # symbols are meaningful to our internal tooling when they
+            # appear in a single token
+            out.write("# This file was @")
+            out.write("generated by getdeps.py\n")
+            out.write(
+                """
+name: CI
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    branches:
+    - master
+
+jobs:
+"""
+            )
+            for p in platforms:
+                build_opts = setup_build_options(args, p)
+                self.write_job_for_platform(out, args, build_opts)
+
+    def write_job_for_platform(self, out, args, build_opts):
+        ctx_gen = build_opts.get_context_generator()
+        loader = ManifestLoader(build_opts, ctx_gen)
+        manifest = loader.load_manifest(args.project)
+        manifest_ctx = loader.ctx_gen.get_context(manifest.name)
+
+        # Some projects don't do anything "useful" as a leaf project, only
+        # as a dep for a leaf project.  Check for those here; we don't want
+        # to waste the effort scheduling them on CI.
+        # We do this by looking at the builder type in the manifest file
+        # rather than creating a builder and checking its type because we
+        # don't know enough to create the full builder instance here.
+        if manifest.get("build", "builder", ctx=manifest_ctx) == "nop":
+            return None
+
+        if build_opts.is_linux():
+            job_name = "linux"
+            runs_on = "ubuntu-18.04"
+        elif build_opts.is_windows():
+            # We're targeting the windows-2016 image because it has
+            # Visual Studio 2017 installed, and at the time of writing,
+            # the version of boost in the manifests (1.69) is not
+            # buildable with Visual Studio 2019
+            job_name = "windows"
+            runs_on = "windows-2016"
+        else:
+            job_name = "mac"
+            runs_on = "macOS-latest"
+
+        out.write("  %s:\n" % job_name)
+        out.write("    runs-on: %s\n" % runs_on)
+        out.write("    steps:\n")
+        out.write("    - uses: actions/checkout@v1\n")
+
+        projects = loader.manifests_in_dependency_order()
+
+        for m in projects:
+            if m != manifest:
+                out.write("    - name: Fetch %s\n" % m.name)
+                out.write(
+                    "      run: python build/fbcode_builder/getdeps.py fetch "
+                    "--no-tests %s\n" % m.name
+                )
+
+        for m in projects:
+            if m != manifest:
+                out.write("    - name: Build %s\n" % m.name)
+                out.write(
+                    "      run: python build/fbcode_builder/getdeps.py build "
+                    "--no-tests %s\n" % m.name
+                )
+
+        out.write("    - name: Build %s\n" % manifest.name)
+        out.write(
+            "      run: python build/fbcode_builder/getdeps.py build --src-dir=. %s\n"
+            % manifest.name
+        )
+
+        out.write("    - name: Test %s\n" % manifest.name)
+        out.write(
+            "      run: python build/fbcode_builder/getdeps.py test --src-dir=. %s\n"
+            % manifest.name
+        )
+
+    def setup_project_cmd_parser(self, parser):
+        parser.add_argument("--output-file", help="The name of the yaml file")
+
+
 def get_arg_var_name(args):
     for arg in args:
         if arg.startswith("--"):