cmd/go/internal/imports: recognize "unix" build tag

For #20322
For #51572
Fixes #54712

Change-Id: I22fcfa820e83323bfdf1a40deee7286240f02b3e
GitHub-Last-Rev: cd2c653
GitHub-Pull-Request: #54716
Reviewed-on: https://go-review.googlesource.com/c/go/+/426296
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/dist/build.go

@@ -939,7 +939,8 @@ func packagefile(pkg string) string {
 }
 
 // unixOS is the set of GOOS values matched by the "unix" build tag.
-// This is the same list as in go/build/syslist.go.
+// This is the same list as in go/build/syslist.go and
+// cmd/go/internal/imports/build.go.
 var unixOS = map[string]bool{
 	"aix":       true,
 	"android":   true,

src/cmd/go/internal/imports/build.go

@@ -20,6 +20,7 @@ package imports
 
 import (
 	"bytes"
+	"cmd/go/internal/cfg"
 	"errors"
 	"fmt"
 	"go/build/constraint"
@@ -201,17 +202,22 @@ func matchTag(name string, tags map[string]bool, prefer bool) bool {
 		return prefer
 	}
 
-	have := tags[name]
-	if name == "linux" {
-		have = have || tags["android"]
-	}
-	if name == "solaris" {
-		have = have || tags["illumos"]
+	if tags[name] {
+		return true
 	}
-	if name == "darwin" {
-		have = have || tags["ios"]
+
+	switch name {
+	case "linux":
+		return tags["android"]
+	case "solaris":
+		return tags["illumos"]
+	case "darwin":
+		return tags["ios"]
+	case "unix":
+		return unixOS[cfg.BuildContext.GOOS]
+	default:
+		return false
 	}
-	return have
 }
 
 // eval is like
@@ -322,6 +328,24 @@ var KnownOS = map[string]bool{
 	"zos":       true,
 }
 
+// unixOS is the set of GOOS values matched by the "unix" build tag.
+// This is not used for filename matching.
+// This is the same list as in go/build/syslist.go and 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,
+}
+
 var KnownArch = map[string]bool{
 	"386":         true,
 	"amd64":       true,

src/cmd/go/testdata/script/import_unix_tag.txt

@@ -0,0 +1,32 @@
+# Regression test for https://go.dev/issue/54712: the "unix" build constraint
+# was not applied consistently during package loading.
+
+go list -x -f '{{if .Module}}{{.ImportPath}}{{end}}' -deps .
+stdout 'example.com/version'
+
+-- go.mod --
+module example
+
+go 1.19
+
+require example.com/version v1.1.0
+-- go.sum --
+example.com/version v1.1.0 h1:VdPnGmIF1NJrntStkxGrF3L/OfhaL567VzCjncGUgtM=
+example.com/version v1.1.0/go.mod h1:S7K9BnT4o5wT4PCczXPfWVzpjD4ud4e7AJMQJEgiu2Q=
+-- main_notunix.go --
+//go:build !(aix || darwin || dragonfly || freebsd || hurd || linux || netbsd || openbsd || solaris)
+
+package main
+
+import _ "example.com/version"
+
+func main() {}
+
+-- main_unix.go --
+//go:build unix
+
+package main
+
+import _ "example.com/version"
+
+func main() {}

src/go/build/syslist.go

@@ -33,7 +33,8 @@ var knownOS = map[string]bool{
 
 // 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.
+// This list also appears in cmd/dist/build.go and
+// cmd/go/internal/imports/build.go.
 var unixOS = map[string]bool{
 	"aix":       true,
 	"android":   true,