Use go:build directives in generated files.

Build constraints are now inferred from go:build directives rather than +build
directives. +build directives are still emitted in generated files as required
in Go 1.16 and earlier.

Note that go/build/constraint was added in Go 1.16, so gVisor now requires Go
1.16.

PiperOrigin-RevId: 387240779

11 files changed, 361 insertions, 127 deletions

diff --git a/tools/constraintutil/BUILD b/tools/constraintutil/BUILD
new file mode 100644
index 000000000..004b708c4
--- /dev/null
+++ b/tools/constraintutil/BUILD
@@ -0,0 +1,18 @@
+load("//tools:defs.bzl", "go_library", "go_test")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "constraintutil",
+    srcs = ["constraintutil.go"],
+    marshal = False,
+    stateify = False,
+    visibility = ["//tools:__subpackages__"],
+)
+
+go_test(
+    name = "constraintutil_test",
+    size = "small",
+    srcs = ["constraintutil_test.go"],
+    library = ":constraintutil",
+)
diff --git a/tools/constraintutil/constraintutil.go b/tools/constraintutil/constraintutil.go
new file mode 100644
index 000000000..fb3fbe5c2
--- /dev/null
+++ b/tools/constraintutil/constraintutil.go
@@ -0,0 +1,169 @@
+// Copyright 2021 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package constraintutil provides utilities for working with Go build
+// constraints.
+package constraintutil
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"go/build/constraint"
+	"io"
+	"os"
+	"strings"
+)
+
+// FromReader extracts the build constraint from the Go source or assembly file
+// whose contents are read by r.
+func FromReader(r io.Reader) (constraint.Expr, error) {
+	// See go/build.parseFileHeader() for the "official" logic that this is
+	// derived from.
+	const (
+		slashStar     = "/*"
+		starSlash     = "*/"
+		gobuildPrefix = "//go:build"
+	)
+	s := bufio.NewScanner(r)
+	var (
+		inSlashStar = false // between /* and */
+		haveGobuild = false
+		e           constraint.Expr
+	)
+Lines:
+	for s.Scan() {
+		line := bytes.TrimSpace(s.Bytes())
+		if !inSlashStar && constraint.IsGoBuild(string(line)) {
+			if haveGobuild {
+				return nil, fmt.Errorf("multiple go:build directives")
+			}
+			haveGobuild = true
+			var err error
+			e, err = constraint.Parse(string(line))
+			if err != nil {
+				return nil, err
+			}
+		}
+	ThisLine:
+		for len(line) > 0 {
+			if inSlashStar {
+				if i := bytes.Index(line, []byte(starSlash)); i >= 0 {
+					inSlashStar = false
+					line = bytes.TrimSpace(line[i+len(starSlash):])
+					continue ThisLine
+				}
+				continue Lines
+			}
+			if bytes.HasPrefix(line, []byte("//")) {
+				continue Lines
+			}
+			// Note that if /* appears in the line, but not at the beginning,
+			// then the line is still non-empty, so skipping this and
+			// terminating below is correct.
+			if bytes.HasPrefix(line, []byte(slashStar)) {
+				inSlashStar = true
+				line = bytes.TrimSpace(line[len(slashStar):])
+				continue ThisLine
+			}
+			// A non-empty non-comment line terminates scanning for go:build.
+			break Lines
+		}
+	}
+	return e, s.Err()
+}
+
+// FromString extracts the build constraint from the Go source or assembly file
+// containing the given data. If no build constraint applies to the file, it
+// returns nil.
+func FromString(str string) (constraint.Expr, error) {
+	return FromReader(strings.NewReader(str))
+}
+
+// FromFile extracts the build constraint from the Go source or assembly file
+// at the given path. If no build constraint applies to the file, it returns
+// nil.
+func FromFile(path string) (constraint.Expr, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return FromReader(f)
+}
+
+// Combine returns a constraint.Expr that evaluates to true iff all expressions
+// in es evaluate to true. If es is empty, Combine returns nil.
+//
+// Preconditions: All constraint.Exprs in es are non-nil.
+func Combine(es []constraint.Expr) constraint.Expr {
+	switch len(es) {
+	case 0:
+		return nil
+	case 1:
+		return es[0]
+	default:
+		a := &constraint.AndExpr{es[0], es[1]}
+		for i := 2; i < len(es); i++ {
+			a = &constraint.AndExpr{a, es[i]}
+		}
+		return a
+	}
+}
+
+// CombineFromFiles returns a build constraint expression that evaluates to
+// true iff the build constraints from all of the given Go source or assembly
+// files evaluate to true. If no build constraints apply to any of the given
+// files, it returns nil.
+func CombineFromFiles(paths []string) (constraint.Expr, error) {
+	var es []constraint.Expr
+	for _, path := range paths {
+		e, err := FromFile(path)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read build constraints from %q: %v", path, err)
+		}
+		if e != nil {
+			es = append(es, e)
+		}
+	}
+	return Combine(es), nil
+}
+
+// Lines returns a string containing build constraint directives for the given
+// constraint.Expr, including two trailing newlines, as appropriate for a Go
+// source or assembly file. At least a go:build directive will be emitted; if
+// the constraint is expressible using +build directives as well, then +build
+// directives will also be emitted.
+//
+// If e is nil, Lines returns the empty string.
+func Lines(e constraint.Expr) string {
+	if e == nil {
+		return ""
+	}
+
+	var b strings.Builder
+	b.WriteString("//go:build ")
+	b.WriteString(e.String())
+	b.WriteByte('\n')
+
+	if pblines, err := constraint.PlusBuildLines(e); err == nil {
+		for _, line := range pblines {
+			b.WriteString(line)
+			b.WriteByte('\n')
+		}
+	}
+
+	b.WriteByte('\n')
+	return b.String()
+}
diff --git a/tools/constraintutil/constraintutil_test.go b/tools/constraintutil/constraintutil_test.go
new file mode 100644
index 000000000..eeabd8dcf
--- /dev/null
+++ b/tools/constraintutil/constraintutil_test.go
@@ -0,0 +1,138 @@
+// Copyright 2021 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package constraintutil
+
+import (
+	"go/build/constraint"
+	"testing"
+)
+
+func TestFileParsing(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		data string
+		expr string
+	}{
+		{
+			name: "Empty",
+		},
+		{
+			name: "NoConstraint",
+			data: "// copyright header\n\npackage main",
+		},
+		{
+			name: "ConstraintOnFirstLine",
+			data: "//go:build amd64\n#include \"textflag.h\"",
+			expr: "amd64",
+		},
+		{
+			name: "ConstraintAfterSlashSlashComment",
+			data: "// copyright header\n\n//go:build linux\n\npackage newlib",
+			expr: "linux",
+		},
+		{
+			name: "ConstraintAfterSlashStarComment",
+			data: "/*\ncopyright header\n*/\n\n//go:build !race\n\npackage oldlib",
+			expr: "!race",
+		},
+		{
+			name: "ConstraintInSlashSlashComment",
+			data: "// blah blah //go:build windows",
+		},
+		{
+			name: "ConstraintInSlashStarComment",
+			data: "/*\n//go:build windows\n*/",
+		},
+		{
+			name: "ConstraintAfterPackageClause",
+			data: "package oops\n//go:build race",
+		},
+		{
+			name: "ConstraintAfterCppInclude",
+			data: "#include \"textflag.h\"\n//go:build arm64",
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			e, err := FromString(test.data)
+			if err != nil {
+				t.Fatalf("FromString(%q) failed: %v", test.data, err)
+			}
+			if e == nil {
+				if len(test.expr) != 0 {
+					t.Errorf("FromString(%q): got no constraint, wanted %q", test.data, test.expr)
+				}
+			} else {
+				got := e.String()
+				if len(test.expr) == 0 {
+					t.Errorf("FromString(%q): got %q, wanted no constraint", test.data, got)
+				} else if got != test.expr {
+					t.Errorf("FromString(%q): got %q, wanted %q", test.data, got, test.expr)
+				}
+			}
+		})
+	}
+}
+
+func TestCombine(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		in   []string
+		out  string
+	}{
+		{
+			name: "0",
+		},
+		{
+			name: "1",
+			in:   []string{"amd64 || arm64"},
+			out:  "amd64 || arm64",
+		},
+		{
+			name: "2",
+			in:   []string{"amd64", "amd64 && linux"},
+			out:  "amd64 && amd64 && linux",
+		},
+		{
+			name: "3",
+			in:   []string{"amd64", "amd64 || arm64", "amd64 || riscv64"},
+			out:  "amd64 && (amd64 || arm64) && (amd64 || riscv64)",
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			inexprs := make([]constraint.Expr, 0, len(test.in))
+			for _, estr := range test.in {
+				line := "//go:build " + estr
+				e, err := constraint.Parse(line)
+				if err != nil {
+					t.Fatalf("constraint.Parse(%q) failed: %v", line, err)
+				}
+				inexprs = append(inexprs, e)
+			}
+			outexpr := Combine(inexprs)
+			if outexpr == nil {
+				if len(test.out) != 0 {
+					t.Errorf("Combine(%v): got no constraint, wanted %q", test.in, test.out)
+				}
+			} else {
+				got := outexpr.String()
+				if len(test.out) == 0 {
+					t.Errorf("Combine(%v): got %q, wanted no constraint", test.in, got)
+				} else if got != test.out {
+					t.Errorf("Combine(%v): got %q, wanted %q", test.in, got, test.out)
+				}
+			}
+		})
+	}
+}
diff --git a/tools/go_generics/go_merge/BUILD b/tools/go_generics/go_merge/BUILD
index 5e0487e93..211e6b3ed 100644
--- a/tools/go_generics/go_merge/BUILD
+++ b/tools/go_generics/go_merge/BUILD
@@ -7,6 +7,6 @@ go_binary(
     srcs = ["main.go"],
     visibility = ["//:sandbox"],
     deps = [
-        "//tools/tags",
+        "//tools/constraintutil",
     ],
 )
diff --git a/tools/go_generics/go_merge/main.go b/tools/go_generics/go_merge/main.go
index 801f2354f..81394ddce 100644
--- a/tools/go_generics/go_merge/main.go
+++ b/tools/go_generics/go_merge/main.go
@@ -25,9 +25,8 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
-	"strings"
 
-	"gvisor.dev/gvisor/tools/tags"
+	"gvisor.dev/gvisor/tools/constraintutil"
 )
 
 var (
@@ -131,6 +130,12 @@ func main() {
 	}
 	f.Decls = newDecls
 
+	// Infer build constraints for the output file.
+	bcexpr, err := constraintutil.CombineFromFiles(flag.Args())
+	if err != nil {
+		fatalf("Failed to read build constraints: %v\n", err)
+	}
+
 	// Write the output file.
 	var buf bytes.Buffer
 	if err := format.Node(&buf, fset, f); err != nil {
@@ -141,9 +146,7 @@ func main() {
 		fatalf("opening output: %v\n", err)
 	}
 	defer outf.Close()
-	if t := tags.Aggregate(flag.Args()); len(t) > 0 {
-		fmt.Fprintf(outf, "%s\n\n", strings.Join(t.Lines(), "\n"))
-	}
+	outf.WriteString(constraintutil.Lines(bcexpr))
 	if _, err := outf.Write(buf.Bytes()); err != nil {
 		fatalf("write: %v\n", err)
 	}
diff --git a/tools/go_marshal/gomarshal/BUILD b/tools/go_marshal/gomarshal/BUILD
index c2747d94c..aaa203115 100644
--- a/tools/go_marshal/gomarshal/BUILD
+++ b/tools/go_marshal/gomarshal/BUILD
@@ -18,5 +18,5 @@ go_library(
     visibility = [
         "//:sandbox",
     ],
-    deps = ["//tools/tags"],
+    deps = ["//tools/constraintutil"],
 )
diff --git a/tools/go_marshal/gomarshal/generator.go b/tools/go_marshal/gomarshal/generator.go
index 00961c90d..4c23637c0 100644
--- a/tools/go_marshal/gomarshal/generator.go
+++ b/tools/go_marshal/gomarshal/generator.go
@@ -25,7 +25,7 @@ import (
 	"sort"
 	"strings"
 
-	"gvisor.dev/gvisor/tools/tags"
+	"gvisor.dev/gvisor/tools/constraintutil"
 )
 
 // List of identifiers we use in generated code that may conflict with a
@@ -123,16 +123,18 @@ func (g *Generator) writeHeader() error {
 	var b sourceBuffer
 	b.emit("// Automatically generated marshal implementation. See tools/go_marshal.\n\n")
 
-	// Emit build tags.
-	b.emit("// If there are issues with build tag aggregation, see\n")
-	b.emit("// tools/go_marshal/gomarshal/generator.go:writeHeader(). The build tags here\n")
-	b.emit("// come from the input set of files used to generate this file. This input set\n")
-	b.emit("// is filtered based on pre-defined file suffixes related to build tags, see \n")
-	b.emit("// tools/defs.bzl:calculate_sets().\n\n")
-
-	if t := tags.Aggregate(g.inputs); len(t) > 0 {
-		b.emit(strings.Join(t.Lines(), "\n"))
-		b.emit("\n\n")
+	bcexpr, err := constraintutil.CombineFromFiles(g.inputs)
+	if err != nil {
+		return err
+	}
+	if bcexpr != nil {
+		// Emit build constraints.
+		b.emit("// If there are issues with build constraint aggregation, see\n")
+		b.emit("// tools/go_marshal/gomarshal/generator.go:writeHeader(). The constraints here\n")
+		b.emit("// come from the input set of files used to generate this file. This input set\n")
+		b.emit("// is filtered based on pre-defined file suffixes related to build constraints,\n")
+		b.emit("// see tools/defs.bzl:calculate_sets().\n\n")
+		b.emit(constraintutil.Lines(bcexpr))
 	}
 
 	// Package header.
@@ -553,11 +555,12 @@ func (g *Generator) writeTests(ts []*testGenerator) error {
 	b.reset()
 	b.emit("// Automatically generated marshal tests. See tools/go_marshal.\n\n")
 
-	// Emit build tags.
-	if t := tags.Aggregate(g.inputs); len(t) > 0 {
-		b.emit(strings.Join(t.Lines(), "\n"))
-		b.emit("\n\n")
+	// Emit build constraints.
+	bcexpr, err := constraintutil.CombineFromFiles(g.inputs)
+	if err != nil {
+		return err
 	}
+	b.emit(constraintutil.Lines(bcexpr))
 
 	b.emit("package %s\n\n", g.pkg)
 	if err := b.write(g.outputTest); err != nil {
diff --git a/tools/go_stateify/BUILD b/tools/go_stateify/BUILD
index 913558b4e..ad66981c7 100644
--- a/tools/go_stateify/BUILD
+++ b/tools/go_stateify/BUILD
@@ -6,7 +6,7 @@ go_binary(
     name = "stateify",
     srcs = ["main.go"],
     visibility = ["//:sandbox"],
-    deps = ["//tools/tags"],
+    deps = ["//tools/constraintutil"],
 )
 
 bzl_library(
diff --git a/tools/go_stateify/main.go b/tools/go_stateify/main.go
index 93022f504..7216388a0 100644
--- a/tools/go_stateify/main.go
+++ b/tools/go_stateify/main.go
@@ -28,7 +28,7 @@ import (
 	"strings"
 	"sync"
 
-	"gvisor.dev/gvisor/tools/tags"
+	"gvisor.dev/gvisor/tools/constraintutil"
 )
 
 var (
@@ -214,10 +214,13 @@ func main() {
 	// Automated warning.
 	fmt.Fprint(outputFile, "// automatically generated by stateify.\n\n")
 
-	// Emit build tags.
-	if t := tags.Aggregate(flag.Args()); len(t) > 0 {
-		fmt.Fprintf(outputFile, "%s\n\n", strings.Join(t.Lines(), "\n"))
+	// Emit build constraints.
+	bcexpr, err := constraintutil.CombineFromFiles(flag.Args())
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to infer build constraints: %v", err)
+		os.Exit(1)
 	}
+	outputFile.WriteString(constraintutil.Lines(bcexpr))
 
 	// Emit the package name.
 	_, pkg := filepath.Split(*fullPkg)
diff --git a/tools/tags/BUILD b/tools/tags/BUILD
deleted file mode 100644
index 1c02e2c89..000000000
--- a/tools/tags/BUILD
+++ /dev/null
@@ -1,11 +0,0 @@
-load("//tools:defs.bzl", "go_library")
-
-package(licenses = ["notice"])
-
-go_library(
-    name = "tags",
-    srcs = ["tags.go"],
-    marshal = False,
-    stateify = False,
-    visibility = ["//tools:__subpackages__"],
-)
diff --git a/tools/tags/tags.go b/tools/tags/tags.go
deleted file mode 100644
index f35904e0a..000000000
--- a/tools/tags/tags.go
+++ /dev/null
@@ -1,89 +0,0 @@
-// Copyright 2020 The gVisor Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// Package tags is a utility for parsing build tags.
-package tags
-
-import (
-	"fmt"
-	"io/ioutil"
-	"strings"
-)
-
-// OrSet is a set of tags on a single line.
-//
-// Note that tags may include ",", and we don't distinguish this case in the
-// logic below. Ideally, this constraints can be split into separate top-level
-// build tags in order to resolve any issues.
-type OrSet []string
-
-// Line returns the line for this or.
-func (or OrSet) Line() string {
-	return fmt.Sprintf("// +build %s", strings.Join([]string(or), " "))
-}
-
-// AndSet is the set of all OrSets.
-type AndSet []OrSet
-
-// Lines returns the lines to be printed.
-func (and AndSet) Lines() (ls []string) {
-	for _, or := range and {
-		ls = append(ls, or.Line())
-	}
-	return
-}
-
-// Join joins this AndSet with another.
-func (and AndSet) Join(other AndSet) AndSet {
-	return append(and, other...)
-}
-
-// Tags returns the unique set of +build tags.
-//
-// Derived form the runtime's canBuild.
-func Tags(file string) (tags AndSet) {
-	data, err := ioutil.ReadFile(file)
-	if err != nil {
-		return nil
-	}
-	// Check file contents for // +build lines.
-	for _, p := range strings.Split(string(data), "\n") {
-		p = strings.TrimSpace(p)
-		if p == "" {
-			continue
-		}
-		if !strings.HasPrefix(p, "//") {
-			break
-		}
-		if !strings.Contains(p, "+build") {
-			continue
-		}
-		fields := strings.Fields(p[2:])
-		if len(fields) < 1 || fields[0] != "+build" {
-			continue
-		}
-		tags = append(tags, OrSet(fields[1:]))
-	}
-	return tags
-}
-
-// Aggregate aggregates all tags from a set of files.
-//
-// Note that these may be in conflict, in which case the build will fail.
-func Aggregate(files []string) (tags AndSet) {
-	for _, file := range files {
-		tags = tags.Join(Tags(file))
-	}
-	return tags
-}