cmd/go: fix linux-amd64-clang builder

Fixes #10660

Fix the clang only builder by passing -extld down to the linker when needed.
The build passed on most hosts because gcc is almost always present. The bug
was verified by symlinking bin/false in place of gcc in my $PATH and running
the build.

Also, resolve a TODO and move the support logic into its own function.

Tested manually

    env CC=clang-3.5 ./all.bash	# linux/amd64
    env CC=gcc-4.8 ./all.bash	# linux/amd64
    ./all.bash			# linux/amd64
    ./all.bash			# darwin/amd64

Change-Id: I4e27a1119356e295500a0d19ad7a4ec14207bf10
Reviewed-on: https://go-review.googlesource.com/9526
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -1310,15 +1310,61 @@ func (b *builder) installShlibname(a *action) error {
 	return nil
 }
 
+// setextld sets the appropriate linker flags for the specified compiler.
+func setextld(ldflags []string, compiler []string) []string {
+	for _, f := range ldflags {
+		if f == "-extld" || strings.HasPrefix(f, "-extld=") {
+			// don't override -extld if supplied
+			return ldflags
+		}
+	}
+	ldflags = append(ldflags, "-extld="+compiler[0])
+	if len(compiler) > 1 {
+		extldflags := false
+		add := strings.Join(compiler[1:], " ")
+		for i, f := range ldflags {
+			if f == "-extldflags" && i+1 < len(ldflags) {
+				ldflags[i+1] = add + " " + ldflags[i+1]
+				extldflags = true
+				break
+			} else if strings.HasPrefix(f, "-extldflags=") {
+				ldflags[i] = "-extldflags=" + add + " " + ldflags[i][len("-extldflags="):]
+				extldflags = true
+				break
+			}
+		}
+		if !extldflags {
+			ldflags = append(ldflags, "-extldflags="+add)
+		}
+	}
+	return ldflags
+}
+
 func (b *builder) linkShared(a *action) (err error) {
 	// TODO(mwhudson): obvious copy pasting from gcToolchain.ld, should make a few
 	// changes to that function and then call it. And support gccgo.
 	allactions := actionList(a)
 	importArgs := b.includeArgs("-L", allactions[:len(allactions)-1])
-	// TODO(mwhudson): this does not check for cxx-ness, extldflags etc
 	ldflags := []string{"-installsuffix", buildContext.InstallSuffix}
 	ldflags = append(ldflags, "-buildmode="+ldBuildmode)
 	ldflags = append(ldflags, buildLdflags...)
+	cxx := a.p != nil && (len(a.p.CXXFiles) > 0 || len(a.p.SwigCXXFiles) > 0)
+	for _, a := range allactions {
+		if a.p != nil && (len(a.p.CXXFiles) > 0 || len(a.p.SwigCXXFiles) > 0) {
+			cxx = true
+		}
+	}
+	// If the user has not specified the -extld option, then specify the
+	// appropriate linker. In case of C++ code, use the compiler named
+	// by the CXX environment variable or defaultCXX if CXX is not set.
+	// Else, use the CC environment variable and defaultCC as fallback.
+	var compiler []string
+	if cxx {
+		compiler = envList("CXX", defaultCXX)
+	} else {
+		compiler = envList("CC", defaultCC)
+	}
+	ldflags = setextld(ldflags, compiler)
 	for _, d := range a.deps {
 		if d.target == "" { // omit unsafe etc
 			continue
@@ -2093,40 +2139,13 @@ func (gcToolchain) ld(b *builder, p *Package, out string, allactions []*action,
 	// appropriate linker. In case of C++ code, use the compiler named
 	// by the CXX environment variable or defaultCXX if CXX is not set.
 	// Else, use the CC environment variable and defaultCC as fallback.
-	extld := false
-	for _, f := range ldflags {
-		if f == "-extld" || strings.HasPrefix(f, "-extld=") {
-			extld = true
-			break
-		}
-	}
-	if !extld {
-		var compiler []string
-		if cxx {
-			compiler = envList("CXX", defaultCXX)
-		} else {
-			compiler = envList("CC", defaultCC)
-		}
-		ldflags = append(ldflags, "-extld="+compiler[0])
-		if len(compiler) > 1 {
-			extldflags := false
-			add := strings.Join(compiler[1:], " ")
-			for i, f := range ldflags {
-				if f == "-extldflags" && i+1 < len(ldflags) {
-					ldflags[i+1] = add + " " + ldflags[i+1]
-					extldflags = true
-					break
-				} else if strings.HasPrefix(f, "-extldflags=") {
-					ldflags[i] = "-extldflags=" + add + " " + ldflags[i][len("-extldflags="):]
-					extldflags = true
-					break
-				}
-			}
-			if !extldflags {
-				ldflags = append(ldflags, "-extldflags="+add)
-			}
-		}
+	var compiler []string
+	if cxx {
+		compiler = envList("CXX", defaultCXX)
+	} else {
+		compiler = envList("CC", defaultCC)
 	}
+	ldflags = setextld(ldflags, compiler)
 	ldflags = append(ldflags, "-buildmode="+ldBuildmode)
 	ldflags = append(ldflags, buildLdflags...)
 	return b.run(".", p.ImportPath, nil, buildToolExec, tool(archChar()+"l"), "-o", out, importArgs, ldflags, mainpkg)