cmd/go: put user ldflags at the end of the linker invocation

If the user provided a key but no value via -ldflag -X,
another linker flag was used as the value.

Placing the user's flags at the end avoids this problem.
It also provides the user the opportunity to
override existing linker flags.

Fixes #8810.

Change-Id: I96f4190713dc9a9c29142e56658446fba7fb6bc8
Reviewed-on: https://go-review.googlesource.com/2242Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/go/build.go

@@ -1776,9 +1776,7 @@ func (gcToolchain) ld(b *builder, p *Package, out string, allactions []*action,
 			cxx = true
 		}
 	}
-	ldflags := buildLdflags
-	// Limit slice capacity so that concurrent appends do not race on the shared array.
-	ldflags = ldflags[:len(ldflags):len(ldflags)]
+	var ldflags []string
 	if buildContext.InstallSuffix != "" {
 		ldflags = append(ldflags, "-installsuffix", buildContext.InstallSuffix)
 	}
@@ -1824,6 +1822,7 @@ func (gcToolchain) ld(b *builder, p *Package, out string, allactions []*action,
 			}
 		}
 	}
+	ldflags = append(ldflags, buildLdflags...)
 	return b.run(".", p.ImportPath, nil, tool(archChar+"l"), "-o", out, importArgs, ldflags, mainpkg)
 }
 

test/linkx_run.go

@@ -16,6 +16,7 @@ import (
 )
 
 func main() {
+	// Successful run
 	cmd := exec.Command("go", "run", "-ldflags=-X main.tbd hello -X main.overwrite trumped -X main.nosuchsymbol neverseen", "linkx.go")
 	out, err := cmd.CombinedOutput()
 	if err != nil {
@@ -30,4 +31,12 @@ func main() {
 		fmt.Printf("got %q want %q\n", got, want)
 		os.Exit(1)
 	}
+
+	// Issue 8810
+	cmd = exec.Command("go", "run", "-ldflags=-X main.tbd", "linkx.go")
+	_, err = cmd.CombinedOutput()
+	if err == nil {
+		fmt.Println("-X linker flag should not accept keys without values")
+		os.Exit(1)
+	}
 }