cmd/go: accept more spaces in -gcflags arguments

Earlier versions of Go were not very picky about leading spaces
in the -gcflags values. Make the new pattern-enhanced parser
equally lax.

Fixes #22943.

Change-Id: I5cf4d3e81412e895a4b52af325853ed48d0b73f4
Reviewed-on: https://go-review.googlesource.com/81498
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -5177,7 +5177,7 @@ func TestGcflagsPatterns(t *testing.T) {
 	tg.setenv("GOPATH", "")
 	tg.setenv("GOCACHE", "off")
 
-	tg.run("build", "-v", "-gcflags=-e", "fmt")
+	tg.run("build", "-v", "-gcflags= \t\r\n -e", "fmt")
 	tg.grepStderr("fmt", "did not rebuild fmt")
 	tg.grepStderrNot("reflect", "incorrectly rebuilt reflect")
 
@@ -5186,7 +5186,7 @@ func TestGcflagsPatterns(t *testing.T) {
 	tg.grepStderr("reflect", "did not rebuild reflect")
 	tg.grepStderrNot("runtime", "incorrectly rebuilt runtime")
 
-	tg.run("build", "-x", "-v", "-gcflags=reflect=-N", "fmt")
+	tg.run("build", "-x", "-v", "-gcflags= \t\r\n reflect \t\r\n = \t\r\n -N", "fmt")
 	tg.grepStderr("fmt", "did not rebuild fmt")
 	tg.grepStderr("reflect", "did not rebuild reflect")
 	tg.grepStderr("compile.* -N .*-p reflect", "did not build reflect with -N flag")

src/cmd/go/internal/load/flag.go

@@ -41,6 +41,8 @@ func (f *PerPackageFlag) Set(v string) error {
 func (f *PerPackageFlag) set(v, cwd string) error {
 	f.present = true
 	match := func(p *Package) bool { return p.Internal.CmdlinePkg || p.Internal.CmdlineFiles } // default predicate with no pattern
+	// For backwards compatibility with earlier flag splitting, ignore spaces around flags.
+	v = strings.TrimSpace(v)
 	if v == "" {
 		// Special case: -gcflags="" means no flags for command-line arguments
 		// (overrides previous -gcflags="-whatever").
@@ -55,7 +57,7 @@ func (f *PerPackageFlag) set(v, cwd string) error {
 		if i == 0 {
 			return fmt.Errorf("missing <pattern> in <pattern>=<value>")
 		}
-		pattern := v[:i]
+		pattern := strings.TrimSpace(v[:i])
 		match = MatchPackage(pattern, cwd)
 		v = v[i+1:]
 	}