go/build: recognize "unix" build tag

The new "unix" build tag matches any Unix or Unix-like system.
This is only recognized on go:build lines, not in file names.

For #20322
Fixes #51572

Change-Id: I3a991f9e69353b25e259bc6462709cdcd83640fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/389934
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Rob Pike <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/cmd/dist/build.go

@@ -976,12 +976,41 @@ func packagefile(pkg string) string {
  return pathf("%s/pkg/%s_%s/%s.a", goroot, goos, goarch, pkg)
 }
 
+// unixOS is the set of GOOS values matched by the "unix" build tag.
+// This is the same list as in go/build/syslist.go.
+var unixOS = map[string]bool{
+ "aix": true,
+ "android": true,
+ "darwin": true,
+ "dragonfly": true,
+ "freebsd": true,
+ "hurd": true,
+ "illumos": true,
+ "ios": true,
+ "linux": true,
+ "netbsd": true,
+ "openbsd": true,
+ "solaris": true,
+}
+
 // matchtag reports whether the tag matches this build.
 func matchtag(tag string) bool {
- return tag == "gc" || tag == goos || tag == goarch || tag == "cmd_go_bootstrap" || tag == "go1.1" ||
- (goos == "android" && tag == "linux") ||
- (goos == "illumos" && tag == "solaris") ||
- (goos == "ios" && tag == "darwin")
+ switch tag {
+ case "gc", "cmd_go_bootstrap", "go1.1":
+ return true
+ case "linux":
+ return goos == "linux" || goos == "android"
+ case "solaris":
+ return goos == "solaris" || goos == "illumos"
+ case "darwin":
+ return goos == "darwin" || goos == "ios"
+ case goos, goarch:
+ return true
+ case "unix":
+ return unixOS[goos]
+ default:
+ return false
+ }
 }
 
 // shouldbuild reports whether we should build this file.

src/cmd/go/internal/help/helpdoc.go

@@ -843,6 +843,7 @@ During a particular build, the following words are satisfied:
  GOOS environment variable.
  - the target architecture, as spelled by runtime.GOARCH, set with the
  GOARCH environment variable.
+ - "unix", if GOOS is a Unix or Unix-like system.
  - the compiler being used, either "gc" or "gccgo"
  - "cgo", if the cgo command is supported (see CGO_ENABLED in
  'go help environment').

src/go/build/build.go

@@ -1913,6 +1913,9 @@ func (ctxt *Context) matchTag(name string, allTags map[string]bool) bool {
  if ctxt.GOOS == "ios" && name == "darwin" {
  return true
  }
+ if name == "unix" && unixOS[ctxt.GOOS] {
+ return true
+ }
 
  // other tags
  for _, tag := range ctxt.BuildTags {

src/go/build/syslist.go

@@ -4,9 +4,9 @@
 
 package build
 
-// Past, present, and future known GOOS and GOARCH values.
-// Do not remove from this list, as these are used for go/build filename matching.
-
+// knownOS is the list of past, present, and future known GOOS values.
+// Do not remove from this list, as it is used for filename matching.
+// If you add an entry to this list, look at unixOS, below.
 var knownOS = map[string]bool{
  "aix": true,
  "android": true,
@@ -26,6 +26,27 @@ var knownOS = map[string]bool{
  "windows": true,
  "zos": true,
 }
+
+// unixOS is the set of GOOS values matched by the "unix" build tag.
+// This is not used for filename matching.
+// This list also appears in cmd/dist/build.go.
+var unixOS = map[string]bool{
+ "aix": true,
+ "android": true,
+ "darwin": true,
+ "dragonfly": true,
+ "freebsd": true,
+ "hurd": true,
+ "illumos": true,
+ "ios": true,
+ "linux": true,
+ "netbsd": true,
+ "openbsd": true,
+ "solaris": true,
+}
+
+// knownArch is the list of past, present, and future known GOARCH values.
+// Do not remove from this list, as it is used for filename matching.
 var knownArch = map[string]bool{
  "386": true,
  "amd64": true,