Pass large ldflags to cgo via response file instead of env variable. (#4386)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

Pass ldflags to cgo tool via a response file instead of `CGO_LDFLAGS`
env variable for go 1.23+. This avoids the "argument list too long"
error, which can happen if `CGO_LDFLAGS` is so large that it exceeds the
system limits.

Support to accept ldflags via file was added to `go` more than a year
ago. References:
- https://go-review.googlesource.com/c/go/+/584655
- Discussion: golang/go#66456

**Which issues(s) does this PR fix?**

Fixes #2654

---------

Co-authored-by: Dhiraj Goel <[email protected]>
Co-authored-by: Jay Conrod <[email protected]>

Author: 3 people

go/tools/builders/cgo2.go

@@ -28,6 +28,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"strconv"
 	"strings"
 )
 
@@ -95,7 +96,39 @@ func cgo2(goenv *env, goSrcs, cgoSrcs, cSrcs, cxxSrcs, objcSrcs, objcxxSrcs, sSr
 		}
 	}
 	combinedLdFlags = append(combinedLdFlags, defaultLdFlags()...)
-	os.Setenv("CGO_LDFLAGS", strings.Join(combinedLdFlags, " "))
+
+	// go 1.23+ supports ldflags file.
+	// https://go-review.googlesource.com/c/go/+/584655
+	canUseLdflagsFile, err := onVersionOrHigher(23)
+	if err != nil {
+		return "", nil, nil, err
+	}
+
+	var ldflagsFile *os.File
+	combinedLdFlagsStr := strings.Join(combinedLdFlags, " ")
+	if canUseLdflagsFile {
+		// Write linker flags to a temporary file instead of pasing via env variable.
+		// This avoids "argument list too long" error with extremely large CGO_LDFLAGS
+		// that can exceed system limits.
+		// Future versions of `go` may remove support for CGO_LDFLAGS entirely, so
+		// use file even for small ldflags.
+		// https://go-review.googlesource.com/c/go/+/596615
+		ldflagsFile, err = os.CreateTemp("", "cgo-ldflags-*.txt")
+		if err != nil {
+			return "", nil, nil, fmt.Errorf("failed to create temporary file for ldflags: %w", err)
+		}
+		defer os.Remove(ldflagsFile.Name())
+		if _, err := ldflagsFile.WriteString(combinedLdFlagsStr); err != nil {
+			ldflagsFile.Close()
+			return "", nil, nil, fmt.Errorf("failed to write ldflags to temporary file: %w", err)
+		}
+		if err := ldflagsFile.Close(); err != nil {
+			return "", nil, nil, fmt.Errorf("failed to close temporary ldflags file: %w", err)
+		}
+	} else {
+		// Fallback to env variable for backwards compatibility with older `go` versions.
+		os.Setenv("CGO_LDFLAGS", combinedLdFlagsStr)
+	}
 
 	// If cgo sources are in different directories, gather them into a temporary
 	// directory so we can use -srcdir.
@@ -142,6 +175,10 @@ func cgo2(goenv *env, goSrcs, cgoSrcs, cSrcs, cxxSrcs, objcSrcs, objcxxSrcs, sSr
 	}
 	// Trim the execroot from the //line comments emitted by cgo.
 	args := goenv.goTool("cgo", "-srcdir", srcDir, "-objdir", workDir, "-trimpath", execRoot)
+	if ldflagsFile != nil {
+		// The "@" prefix tells cgo to read arguments from the file.
+		args = append(args, "-ldflags", "@"+ldflagsFile.Name())
+	}
 	if packagePath != "" {
 		args = append(args, "-importpath", packagePath)
 	}
@@ -438,3 +475,18 @@ func copyOrLinkFile(inPath, outPath string) error {
 		return linkFile(inPath, outPath)
 	}
 }
+
+func onVersionOrHigher(version int) (bool, error) {
+	v := runtime.Version()
+	m := versionExp.FindStringSubmatch(v)
+	if len(m) != 2 {
+		return false, fmt.Errorf("failed to match against Go version %q", v)
+	}
+	mvStr := m[1]
+	mv, err := strconv.Atoi(mvStr)
+	if err != nil {
+		return false, fmt.Errorf("convert minor version %q to int: %w", mvStr, err)
+	}
+
+	return mv >= version, nil
+}

tests/core/cgo/BUILD.bazel

@@ -331,6 +331,30 @@ go_binary(
     cgo = True,
 )
 
+LARGE_EXT_LDFLAGS = [
+    flag
+    for i in range(10000)
+    for flag in [
+        "-extldflags",
+        "-Wl,-rpath,pathtolib{}".format(i),
+    ]
+]
+
+go_binary(
+    name = "binary_with_rpath",
+    srcs = [
+        "bar.cc",
+        "bar.go",
+    ],
+    out = "binary_with_rpath",
+    cgo = True,
+    gc_linkopts = LARGE_EXT_LDFLAGS,
+    target_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//os:macos",
+    ],
+)
+
 go_binary(
     name = "cc_deps",
     srcs = ["bar.go"],