Fix linker error if go_library with cgo references unresolved symbol

go/tools/builders/cgo2.go

@@ -197,7 +197,30 @@ func cgo2(goenv *env, goSrcs, cgoSrcs, cSrcs, cxxSrcs, objcSrcs, objcxxSrcs, sSr
 	args = append([]string{cc, "-o", mainBin, mainObj}, cObjs...)
 	args = append(args, combinedLdFlags...)
 	if err := goenv.runCommand(args); err != nil {
-		return "", nil, nil, err
+		// If linking the binary for cgo fails, this is usually because the
+		// object files reference external symbols that can't be resolved yet.
+		// Since the binary is only produced to have its symbols read by the cgo
+		// command, there is no harm in trying to build it allowing unresolved
+		// symbols - the real link that happens at the end will fail if they
+		// rightfully can't be resolved.
+		var allowUnresolvedSymbolsLdFlag string
+		switch os.Getenv("GOOS") {
+		case "windows":
+			// MinGW's linker doesn't seem to support --unresolved-symbols
+			// and MSVC isn't supported at all.
+			return "", nil, nil, err
+		case "darwin", "ios":
+			allowUnresolvedSymbolsLdFlag = "-Wl,-undefined,dynamic_lookup"
+		default:
+			allowUnresolvedSymbolsLdFlag = "-Wl,--unresolved-symbols=ignore-all"
+		}
+		if err2 := goenv.runCommand(append(args, allowUnresolvedSymbolsLdFlag)); err2 != nil {
+			// Return the original error if we can't link the binary with the
+			// additional linker flags as they may simply be incorrect for the
+			// particular compiler/linker pair and would obscure the true reason
+			// for the failure of the original command.
+			return "", nil, nil, err
+		}
 	}
 
 	cgoImportsGo := filepath.Join(workDir, "_cgo_imports.go")

tests/core/cgo/BUILD.bazel

@@ -414,3 +414,23 @@ go_bazel_test(
     name = "external_includes_test",
     srcs = ["external_includes_test.go"],
 )
+
+go_library(
+    name = "use_external_symbol",
+    srcs = ["use_external_symbol.go"],
+    cgo = True,
+    importpath = "github.com/bazelbuild/rules_go/tests/core/cgo/use_external_symbol",
+    tags = ["manual"],
+)
+
+go_binary(
+    name = "provide_external_symbol",
+    srcs = ["provide_external_symbol.go"],
+    cgo = True,
+    target_compatible_with = select({
+        "@platforms//os:osx": [],
+        "@platforms//os:linux": [],
+        "//conditions:default": ["@platforms//:incompatible"],
+    }),
+    deps = [":use_external_symbol"],
+)

tests/core/cgo/provide_external_symbol.go

@@ -0,0 +1,12 @@
+package main
+
+import "C"
+
+import "github.com/bazelbuild/rules_go/tests/core/cgo/use_external_symbol"
+
+//export external_symbol
+func external_symbol() {}
+
+func main() {
+	use_external_symbol.UseExternalSymbol()
+}

tests/core/cgo/use_external_symbol.go

@@ -0,0 +1,10 @@
+package use_external_symbol
+
+/*
+void external_symbol();
+*/
+import "C"
+
+func UseExternalSymbol() {
+	C.external_symbol()
+}