pkg/ifuzz: fix generate/build

Currently the commands we have in go:generate first create an empty file
and then write final contents. This breaks any parallel builds of the source.
Even running go generate ./... does not work.
Write output files atomically.

6 files changed, 36 insertions, 11 deletions

diff --git a/pkg/ifuzz/arm64/arm64.go b/pkg/ifuzz/arm64/arm64.go
index 3b835566f..a2d326339 100644
--- a/pkg/ifuzz/arm64/arm64.go
+++ b/pkg/ifuzz/arm64/arm64.go
@@ -1,7 +1,7 @@
 // Copyright 2024 syzkaller project authors. All rights reserved.
 // Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
 
-//go:generate bash -c "go run gen/gen.go gen/json/arm64.json | gofmt > generated/insns.go"
+//go:generate go run gen/gen.go gen/json/arm64.json generated/insns.go
 
 // Package arm64 allows to generate and mutate arm64 machine code.
 package arm64
diff --git a/pkg/ifuzz/arm64/gen/gen.go b/pkg/ifuzz/arm64/gen/gen.go
index 659fea202..ef5afbd46 100644
--- a/pkg/ifuzz/arm64/gen/gen.go
+++ b/pkg/ifuzz/arm64/gen/gen.go
@@ -5,6 +5,7 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -12,13 +13,14 @@ import (
 	"strings"
 
 	"github.com/google/syzkaller/pkg/ifuzz/arm64"
+	"github.com/google/syzkaller/pkg/osutil"
 	"github.com/google/syzkaller/pkg/serializer"
 	"github.com/google/syzkaller/pkg/tool"
 )
 
 func main() {
-	if len(os.Args) != 2 {
-		tool.Failf("usage: gen arm64.json")
+	if len(os.Args) != 3 {
+		tool.Failf("usage: gen arm64.json output.file")
 	}
 	jsonStr, err := os.ReadFile(os.Args[1])
 	if err != nil {
@@ -26,7 +28,8 @@ func main() {
 	}
 	insns := JSONToInsns(jsonStr)
 
-	fmt.Printf(`// Code generated by pkg/ifuzz/gen. DO NOT EDIT.
+	out := new(bytes.Buffer)
+	fmt.Fprintf(out, `// Code generated by pkg/ifuzz/gen. DO NOT EDIT.
 
 // go:build !codeanalysis
 
@@ -42,7 +45,10 @@ func init() {
 
 var insns_arm64 = 
 `)
-	serializer.Write(os.Stdout, insns)
+	serializer.Write(out, insns)
+	if err := osutil.WriteFileAtomically(os.Args[2], out.Bytes()); err != nil {
+		tool.Fail(err)
+	}
 
 	fmt.Fprintf(os.Stderr, "handled %v\n", len(insns))
 }
diff --git a/pkg/ifuzz/x86/gen/gen.go b/pkg/ifuzz/x86/gen/gen.go
index dc77ea497..c1af7a44e 100644
--- a/pkg/ifuzz/x86/gen/gen.go
+++ b/pkg/ifuzz/x86/gen/gen.go
@@ -7,6 +7,7 @@ package main
 
 import (
 	"bufio"
+	"bytes"
 	"errors"
 	"fmt"
 	"os"
@@ -16,14 +17,15 @@ import (
 
 	"github.com/google/syzkaller/pkg/ifuzz/iset"
 	"github.com/google/syzkaller/pkg/ifuzz/x86"
+	"github.com/google/syzkaller/pkg/osutil"
 	"github.com/google/syzkaller/pkg/serializer"
 	"github.com/google/syzkaller/pkg/tool"
 )
 
 // nolint: gocyclo, gocognit, funlen, dupl
 func main() {
-	if len(os.Args) != 2 {
-		tool.Failf("usage: gen instructions.txt")
+	if len(os.Args) != 3 {
+		tool.Failf("usage: gen instructions.txt output.file")
 	}
 	f, err := os.Open(os.Args[1])
 	if err != nil {
@@ -168,7 +170,8 @@ nextInsn:
 	fmt.Fprintf(os.Stderr, "deduped %v instructions\n", len(insns)-len(deduped))
 	insns = deduped
 
-	fmt.Printf(`
+	out := new(bytes.Buffer)
+	fmt.Fprintf(out, `
 // Code generated by pkg/ifuzz/x86/gen. DO NOT EDIT.
 
 //go:build !codeanalysis
@@ -183,7 +186,10 @@ func init() {
 
 var insns =
 `)
-	serializer.Write(os.Stdout, insns)
+	serializer.Write(out, insns)
+	if err := osutil.WriteFileAtomically(os.Args[2], out.Bytes()); err != nil {
+		tool.Fail(err)
+	}
 
 	fmt.Fprintf(os.Stderr, "handled %v, skipped %v\n", len(insns), skipped)
 }
diff --git a/pkg/ifuzz/x86/x86.go b/pkg/ifuzz/x86/x86.go
index a2825b9e5..053497176 100644
--- a/pkg/ifuzz/x86/x86.go
+++ b/pkg/ifuzz/x86/x86.go
@@ -1,7 +1,7 @@
 // Copyright 2017 syzkaller project authors. All rights reserved.
 // Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
 
-//go:generate bash -c "go run gen/gen.go gen/all-enc-instructions.txt > generated/insns.go"
+//go:generate go run gen/gen.go gen/all-enc-instructions.txt generated/insns.go
 
 // Package x86 allows to generate and mutate x86 machine code.
 package x86
diff --git a/pkg/osutil/osutil.go b/pkg/osutil/osutil.go
index 8e0ed7ea3..d16a76cb7 100644
--- a/pkg/osutil/osutil.go
+++ b/pkg/osutil/osutil.go
@@ -257,6 +257,19 @@ func WriteFile(filename string, data []byte) error {
 	return os.WriteFile(filename, data, DefaultFilePerm)
 }
 
+// WriteFileAtomically writes data to file filename without exposing and empty/partially-written file.
+// This is useful for writing generated source files. Exposing an empty file may break tools
+// that run on  source files in parallel.
+func WriteFileAtomically(filename string, data []byte) error {
+	// We can't use os.CreateTemp b/c it may be on a different mount,
+	// and Rename can't move across mounts.
+	tmpFile := filename + ".tmp"
+	if err := WriteFile(tmpFile, data); err != nil {
+		return err
+	}
+	return os.Rename(tmpFile, filename)
+}
+
 func WriteJSON[T any](filename string, obj T) error {
 	jsonData, err := json.MarshalIndent(obj, "", "\t")
 	if err != nil {
diff --git a/sys/fuchsia/fidlgen/main.go b/sys/fuchsia/fidlgen/main.go
index ab2bcf0cd..315d6a466 100644
--- a/sys/fuchsia/fidlgen/main.go
+++ b/sys/fuchsia/fidlgen/main.go
@@ -91,7 +91,7 @@ func main() {
 			return pos.File == file
 		}))
 
-		if err := osutil.WriteFile(file, desc); err != nil {
+		if err := osutil.WriteFileAtomically(file, desc); err != nil {
 			tool.Fail(err)
 		}
 	}