go/build: make -I/-L options in cgo flags absolute

Fixes #20266.

Change-Id: I51383820880e3d3566ef3d70650a0863756003ba
Reviewed-on: https://go-review.googlesource.com/44291
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

2 files changed, 41 insertions, 3 deletions

diff --git a/src/cmd/go/go_test.go b/src/cmd/go/go_test.go
index 90a95fd23d..205a1b14e2 100644
--- a/src/cmd/go/go_test.go
+++ b/src/cmd/go/go_test.go
@@ -4087,6 +4087,7 @@ func TestCgoFlagContainsSpace(t *testing.T) {
 		import (
 			"os"
 			"os/exec"
+			"strings"
 		)
 
 		func main() {
@@ -4105,13 +4106,13 @@ func TestCgoFlagContainsSpace(t *testing.T) {
 
 			var success bool
 			for _, arg := range os.Args {
-				switch arg {
-				case "-Ic flags":
+				switch {
+				case strings.Contains(arg, "c flags"):
 					if success {
 						panic("duplicate CFLAGS")
 					}
 					success = true
-				case "-Lld flags":
+				case strings.Contains(arg, "ld flags"):
 					if success {
 						panic("duplicate LDFLAGS")
 					}
diff --git a/src/go/build/build.go b/src/go/build/build.go
index 17446ee4ce..fd89871d42 100644
--- a/src/go/build/build.go
+++ b/src/go/build/build.go
@@ -1282,6 +1282,12 @@ func (ctxt *Context) saveCgo(filename string, di *Package, cg *ast.CommentGroup)
 		}
 
 		switch verb {
+		case "CFLAGS", "CPPFLAGS", "CXXFLAGS", "FFLAGS", "LDFLAGS":
+			// Change relative paths to absolute.
+			ctxt.makePathsAbsolute(args, di.Dir)
+		}
+
+		switch verb {
 		case "CFLAGS":
 			di.CgoCFLAGS = append(di.CgoCFLAGS, args...)
 		case "CPPFLAGS":
@@ -1322,6 +1328,37 @@ func expandSrcDir(str string, srcdir string) (string, bool) {
 	return res, ok && res != ""
 }
 
+// makePathsAbsolute looks for compiler options that take paths and
+// makes them absolute. We do this because through the 1.8 release we
+// ran the compiler in the package directory, so any relative -I or -L
+// options would be relative to that directory. In 1.9 we changed to
+// running the compiler in the build directory, to get consistent
+// build results (issue #19964). To keep builds working, we change any
+// relative -I or -L options to be absolute.
+//
+// Using filepath.IsAbs and filepath.Join here means the results will be
+// different on different systems, but that's OK: -I and -L options are
+// inherently system-dependent.
+func (ctxt *Context) makePathsAbsolute(args []string, srcDir string) {
+	nextPath := false
+	for i, arg := range args {
+		if nextPath {
+			if !filepath.IsAbs(arg) {
+				args[i] = filepath.Join(srcDir, arg)
+			}
+			nextPath = false
+		} else if strings.HasPrefix(arg, "-I") || strings.HasPrefix(arg, "-L") {
+			if len(arg) == 2 {
+				nextPath = true
+			} else {
+				if !filepath.IsAbs(arg[2:]) {
+					args[i] = arg[:2] + filepath.Join(srcDir, arg[2:])
+				}
+			}
+		}
+	}
+}
+
 // NOTE: $ is not safe for the shell, but it is allowed here because of linker options like -Wl,$ORIGIN.
 // We never pass these arguments to a shell (just to programs we construct argv for), so this should be okay.
 // See golang.org/issue/6038.