Ensure go_binary.embed libraries' importpath is main (#2135)

Go 1.13 is stricter on the package path passed to go tool compile -p.
This means that go_library passed in go_binary.embed fails to declare a
main.main function, as they now retain their original import path. Fix
that by adding a is_main boolean to the GoLibrary provider, which will
tell wether a library is supposed to be a main package; that is,
compiled from within a go_binary.

Fixes #2133

go/private/actions/archive.bzl

@@ -85,6 +85,8 @@ def emit_archive(go, source = None):
         cxxopts = [f for fs in source.cxxopts for f in fs.split(" ")]
         clinkopts = [f for fs in source.clinkopts for f in fs.split(" ")]
 
+        importpath, _ = effective_importpath_pkgpath(source.library)
+        importmap = "main" if source.library.is_main else source.library.importmap
         if source.cgo and not go.mode.pure:
             cgo = cgo_configure(
                 go,
@@ -103,8 +105,8 @@ def emit_archive(go, source = None):
                 go,
                 sources = split.go + split.c + split.asm + split.cxx + split.objc + split.headers,
                 cover = source.cover,
-                importpath = effective_importpath_pkgpath(source.library)[0],
-                importmap = source.library.importmap,
+                importpath = importpath,
+                importmap = importmap,
                 archives = direct,
                 out_lib = out_lib,
                 out_export = out_export,
@@ -127,8 +129,8 @@ def emit_archive(go, source = None):
                 go,
                 sources = split.go + split.c + split.asm + split.cxx + split.objc + split.headers,
                 cover = source.cover,
-                importpath = effective_importpath_pkgpath(source.library)[0],
-                importmap = source.library.importmap,
+                importpath = importpath,
+                importmap = importmap,
                 archives = direct,
                 out_lib = out_lib,
                 out_export = out_export,
@@ -152,7 +154,7 @@ def emit_archive(go, source = None):
             go.compile(
                 go,
                 sources = split.go,
-                importpath = source.library.importmap,
+                importpath = importmap,
                 archives = direct,
                 out_lib = out_lib,
                 out_export = out_export,
@@ -169,7 +171,7 @@ def emit_archive(go, source = None):
             go.compile(
                 go,
                 sources = split.go + split.asm + split.headers,
-                importpath = source.library.importmap,
+                importpath = importmap,
                 archives = direct,
                 out_lib = partial_lib,
                 out_export = out_export,

go/private/context.bzl

@@ -145,7 +145,7 @@ def _tool_args(go):
     args.set_param_file_format("multiline")
     return args
 
-def _new_library(go, name = None, importpath = None, resolver = None, importable = True, testfilter = None, **kwargs):
+def _new_library(go, name = None, importpath = None, resolver = None, importable = True, testfilter = None, is_main = False, **kwargs):
     if not importpath:
         importpath = go.importpath
         importmap = go.importmap
@@ -164,6 +164,7 @@ def _new_library(go, name = None, importpath = None, resolver = None, importable
         pathtype = pathtype,
         resolve = resolver,
         testfilter = testfilter,
+        is_main = is_main,
         **kwargs
     )
 

go/private/rules/binary.bzl

@@ -71,7 +71,7 @@ def _go_binary_impl(ctx):
     """go_binary_impl emits actions for compiling and linking a go executable."""
     go = go_context(ctx)
 
-    library = go.new_library(go, importable = False)
+    library = go.new_library(go, importable = False, is_main = True)
     source = go.library_to_source(go, ctx.attr, library, ctx.coverage_instrumented())
     name = ctx.attr.basename
     if not name:

go/private/rules/nogo.bzl

@@ -66,6 +66,7 @@ def _nogo_impl(ctx):
         importmap = "nogomain",
         importpath_aliases = (),
         pathtype = EXPORT_PATH,
+        is_main = True,
         resolve = None,
     )
 

go/private/rules/test.bzl

@@ -133,6 +133,7 @@ def _go_test_impl(ctx):
         importmap = "testmain",
         importpath_aliases = (),
         pathtype = INFERRED_PATH,
+        is_main = True,
         resolve = None,
     )
     test_deps = external_archive.direct + [external_archive]

go/providers.rst

@@ -100,6 +100,12 @@ an input to the `library_to_source`_ helper method, which produces GoSource_.
 | A function called by `library_to_source`_ that can be used to resolve this                       |
 | library to a mode-specific GoSource_.                                                            |
 +--------------------------------+-----------------------------------------------------------------+
+| :param:`is_main`               | :type:`bool`                                                    |
++--------------------------------+-----------------------------------------------------------------+
+| Indicates whether the library should be compiled as a `main` package.                            |
+| `main` packages may have arbitrary `importpath` and `importmap` values,                          |
+| but the compiler and linker must see them as `main`.                                             |
++--------------------------------+-----------------------------------------------------------------+
 
 GoSource
 ~~~~~~~~