Add test coverage instrumentataion.

go/def.bzl

@@ -60,7 +60,7 @@ def _go_prefix(ctx):
   return prefix
 
 # TODO(bazel-team): it would be nice if Bazel had this built-in.
-def symlink_tree_commands(dest_dir, artifact_dict):
+def symlink_tree_commands(go_tool_cover, dest_dir, artifact_dict):
   """Symlink_tree_commands returns a list of commands to create the
   dest_dir, and populate it according to the given dict.
 
@@ -76,6 +76,7 @@ def symlink_tree_commands(dest_dir, artifact_dict):
     "mkdir -p " + dest_dir,
   ]
 
+  count = 0
   for old_path, new_path in artifact_dict.items():
     pos = new_path.rfind('/')
     if pos >= 0:
@@ -94,12 +95,25 @@ def symlink_tree_commands(dest_dir, artifact_dict):
       up resolving to 0 path elements, so 2 segments should be removed from the
       count."""
       up -= 2
-    cmds += [
-      "mkdir -p %s/%s" % (dest_dir, new_dir),
-      "ln -s %s%s %s/%s" % ('../' * up, old_path, dest_dir, new_path),
-    ]
+
+    cmds += ["mkdir -p %s/%s" % (dest_dir, new_dir)]
+
+    if go_tool_cover and old_path.endswith(".go"):
+      cmds += ["ln -s %s%s %s/%s.orig" % ('../' * up, old_path, dest_dir, new_path)]
+      cmds += ["%s -var=GoCover_%d -o %s/%s %s/%s.orig" % (go_tool_cover, count,
+          dest_dir, new_path, dest_dir, new_path)]
+      cmds += ["rm %s/%s.orig" % (dest_dir, new_path)]
+      cmds += ["cat %s/%s" % (dest_dir, new_path)]
+      count += 1
+    else:
+      cmds += ["ln -s %s%s %s/%s" % ('../' * up, old_path, dest_dir, new_path)]
   return cmds
 
+def get_go_tool_cover(ctx):
+  if ctx.coverage_instrumented():
+    return ' '.join([ctx.file.go_tool.path, "tool", "cover", "--mode=set"])
+  return None
+
 def go_environment_vars(ctx):
   """Return a map of environment variables for use with actions, based on
   the arguments. Uses the ctx.fragments.cpp.cpu attribute, if present,
@@ -243,13 +257,12 @@ def _emit_go_compile_action(ctx, sources, deps, out_lib,
   out_depth = out_dir.count('/') + 1
   if _is_external(out_dir):
     out_depth -= 2
-  cmds = symlink_tree_commands(out_dir, tree_layout)
+
+  cmds = ['export GOROOT=$(pwd)/%s/..' % ctx.file.go_tool.dirname]
+  cmds += symlink_tree_commands(get_go_tool_cover(ctx), out_dir, tree_layout)
 
   # cd into the out_dir.
-  cmds += [
-      'export GOROOT=$(pwd)/%s/..' % ctx.file.go_tool.dirname,
-      'cd ' + out_dir,
-  ]
+  cmds += ['cd ' + out_dir]
 
   # Filter source files using build tags.
   cleaned_go_source_paths = [
@@ -267,7 +280,7 @@ def _emit_go_compile_action(ctx, sources, deps, out_lib,
       '${UNFILTERED_GO_FILES[@]}',
   ])
   cmds += [
-      'UNFILTERED_GO_FILES=(%s)' % 
+      'UNFILTERED_GO_FILES=(%s)' %
           ' '.join(["'%s'" % f for f in cleaned_go_source_paths]),
       'FILTERED_GO_FILES=(%s)' %
           ' '.join(["'%s'" % f for f in cleaned_cgo_source_paths]),
@@ -514,7 +527,7 @@ def _emit_go_link_action(ctx, importmap, transitive_libs, cgo_deps, lib,
       main_archive,
   ]
 
-  cmds = symlink_tree_commands(out_dir, tree_layout)
+  cmds = []
   # Avoided -s on OSX but but it requires dsymutil to be on $PATH.
   # TODO(yugui) Remove this workaround once rules_go stops supporting XCode 7.2
   # or earlier.
@@ -525,6 +538,8 @@ def _emit_go_link_action(ctx, importmap, transitive_libs, cgo_deps, lib,
       "STAMP_XDEFS=()",
   ]
 
+  cmds += symlink_tree_commands(get_go_tool_cover(ctx), out_dir, tree_layout)
+
   stamp_inputs = []
   if ctx.attr.linkstamp:
     # read workspace status files, converting "KEY value" lines
@@ -804,8 +819,8 @@ def _cgo_codegen_impl(ctx):
   out_dir = (ctx.configuration.genfiles_dir.path + '/' +
              p + ctx.attr.outdir)
   cc = ctx.fragments.cpp.compiler_executable
-  cmds = symlink_tree_commands(out_dir + "/src", tree_layout) + [
-      "export GOROOT=$(pwd)/" + ctx.file.go_tool.dirname + "/..",
+  cmds = ["export GOROOT=$(pwd)/" + ctx.file.go_tool.dirname + "/.."]
+  cmds += symlink_tree_commands(get_go_tool_cover(ctx), out_dir + "/src", tree_layout) + [
       # We cannot use env for CC because $(CC) on OSX is relative
       # and '../' does not work fine due to symlinks.
       "export CC=$(cd $(dirname {cc}); pwd)/$(basename {cc})".format(cc=cc),