Combine extldflags lists when linking

_emit_go_link_action now scans gc_linkopts for -extldflags
arguments. The Go linker will only use the last list passed to it
(earlier lists are silently ignored), so we combine these into one
external flags list.

Fixes bazel-contrib#305

go/def.bzl

@@ -396,7 +396,7 @@ def _gc_linkopts(ctx):
   return gc_linkopts
 
 def _emit_go_link_action(ctx, importmap, transitive_libs, cgo_deps, lib,
-                        executable, gc_linkopts):
+                         executable, gc_linkopts):
   """Sets up a symlink tree to libraries to link together."""
   out_dir = executable.path + ".dir"
   out_depth = out_dir.count('/') + 1
@@ -431,6 +431,17 @@ def _emit_go_link_action(ctx, importmap, transitive_libs, cgo_deps, lib,
     if d.basename.endswith('.so'):
       dirname = _short_path(d)[:-len(d.basename)]
       ldflags += ["-Wl,-rpath,$ORIGIN/" + ("../" * pkg_depth) + dirname]
+  filtered_gc_linkopts = []
+  is_extldflags = False
+  for opt in gc_linkopts:
+    if is_extldflags:
+      is_extldflags = False
+      ldflags += [opt]
+    elif opt == "-extldflags":
+      is_extldflags = True
+    else:
+      filtered_gc_linkopts += [opt]
+  gc_linkopts = filtered_gc_linkopts
 
   link_cmd = [
       ('../' * out_depth) + ctx.file.go_tool.path,

tests/extldflags_static_link/BUILD

@@ -0,0 +1,37 @@
+load("//go:def.bzl", "go_binary", "cgo_library")
+
+cgo_library(
+    name = "cgo_default_library",
+    srcs = ["main.go"],
+)
+
+# Go binaries with Cgo code are dynamically linked by default.
+go_binary(
+    name = "dynamic_binary",
+    library = ":cgo_default_library",
+)
+
+go_binary(
+    name = "static_binary",
+    gc_linkopts = [
+        "-extldflags",
+        "-static",
+    ],
+    library = ":cgo_default_library",
+)
+
+sh_test(
+    name = "extldflags_link_test",
+    size = "small",
+    srcs = ["extldflags_link_test.sh"],
+    args = [
+        "$(location :dynamic_binary)",
+        "'dynamically linked'",
+        "$(location :static_binary)",
+        "'statically linked'",
+    ],
+    data = [
+        ":dynamic_binary",
+        ":static_binary",
+    ],
+)

tests/extldflags_static_link/README.md

@@ -0,0 +1,16 @@
+This test checks that flags passed to the external linker with -extldflags
+through gc_linkopts are correctly combined with other flags passed through
+-extldflags.
+
+`go tool link` only uses the last set of flags passed with -extldflags. If more
+than one set of flags is passed, the last set is passed to the external linker,
+and the rest are silently ignored.
+
+`go_binary` and `go_test` should look for -extldflags in gc_linkopts and
+combine the flags into a single list passed to `go tool link`. This includes
+flags generated by the rules themselves.
+
+This test checks this behavior by passing "-extldflags -static", which has an
+observable effect on a `go_binary` with cgo code (which is linked dynamically by
+default). This set of flags is combined with the linker flags generated by the
+Go rules themselves.

tests/extldflags_static_link/extldflags_link_test.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+result=0
+while [ $# -gt 0 ]; do
+  path=$1
+  pattern=$2
+  shift 2
+  if ! file --dereference "$path" | grep --quiet "$pattern"; then
+    echo "file '$path' did not match pattern '$pattern'" >&1
+    result=1
+  fi
+done
+
+exit $result

tests/extldflags_static_link/main.go

@@ -0,0 +1,13 @@
+package main
+
+/*
+int foo() {
+  return 42;
+}
+*/
+import "C"
+import "fmt"
+
+func main() {
+	fmt.Printf("%d\n", int(C.foo()))
+}