cmd/go: allow -o to point to a folder that writes multiple execs

If -o points to a directory that exists then allow multiple
executables to be written to that directory.

Fixes #14295

Change-Id: Ic951e637c70a2ada5e7534bae9a43901a39fe2c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/167679
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/work/build.go

@@ -42,10 +42,10 @@ serving only as a check that the packages can be built.
 
 When compiling packages, build ignores files that end in '_test.go'.
 
-The -o flag, only allowed when compiling a single package,
-forces build to write the resulting executable or object
-to the named output file, instead of the default behavior described
-in the last two paragraphs.
+The -o flag forces build to write the resulting executable or object
+to the named output file or directory, instead of the default behavior described
+in the last two paragraphs. If the named output is a directory that exists,
+then any resulting executables will be written to that directory.
 
 The -i flag installs the packages that are dependencies of the target.
 
@@ -153,7 +153,7 @@ func init() {
 	CmdInstall.Run = runInstall
 
 	CmdBuild.Flag.BoolVar(&cfg.BuildI, "i", false, "")
-	CmdBuild.Flag.StringVar(&cfg.BuildO, "o", "", "output file")
+	CmdBuild.Flag.StringVar(&cfg.BuildO, "o", "", "output file or directory")
 
 	CmdInstall.Flag.BoolVar(&cfg.BuildI, "i", false, "")
 
@@ -316,8 +316,29 @@ func runBuild(cmd *base.Command, args []string) {
 	}
 
 	if cfg.BuildO != "" {
+		// If the -o name exists and is a directory, then
+		// write all main packages to that directory.
+		// Otherwise require only a single package be built.
+		if bs, err := os.Stat(cfg.BuildO); err == nil && bs.IsDir() {
+			a := &Action{Mode: "go build"}
+			for _, p := range pkgs {
+				if p.Name != "main" {
+					continue
+				}
+				p.Target = filepath.Join(cfg.BuildO, load.DefaultExecName(p))
+				p.Target += cfg.ExeSuffix
+				p.Stale = true
+				p.StaleReason = "build -o flag in use"
+				a.Deps = append(a.Deps, b.AutoAction(ModeInstall, depMode, p))
+			}
+			if len(a.Deps) == 0 {
+				base.Fatalf("go build: no main packages to build")
+			}
+			b.Do(a)
+			return
+		}
 		if len(pkgs) > 1 {
-			base.Fatalf("go build: cannot use -o with multiple packages")
+			base.Fatalf("go build: cannot write multiple packages to non-directory %s", cfg.BuildO)
 		} else if len(pkgs) == 0 {
 			base.Fatalf("no packages to build")
 		}

src/cmd/go/testdata/script/build_multi_main.txt

@@ -0,0 +1,27 @@
+# Verify build -o can output multiple executables to a directory.
+
+mkdir $WORK/bin
+go build -o $WORK/bin ./cmd/c1 ./cmd/c2
+! stderr 'multiple packages'
+
+! go build -o $WORK/bin ./pkg1 ./pkg1
+stderr 'no main packages'
+
+-- go.mod --
+module exmod
+
+-- cmd/c1/main.go --
+package main
+
+func main() {}
+
+-- cmd/c2/main.go --
+package main
+
+func main() {}
+
+-- pkg1/pkg1.go --
+package pkg1
+
+-- pkg2/pkg2.go --
+package pkg2