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 <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

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/alldocs.go

@@ -1844,6 +1844,7 @@
 // 	  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/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,