cmd/go: 'go get' should not delete binaries when run from $GOBIN

When 'go install' is run without arguments in a directory containing a
main package, it deletes an executable file with the same name as the
package (presumably created by 'go build' previously).

'go get' in module mode executes the same code after updating and
downloading modules. However, the special case was misfiring because
we passed an empty list of patterns to InstallPackages.

Fixes #32766

Change-Id: I19aca64ee1fb5a216777dd7d559e8e6a45b3e90c
Reviewed-on: https://go-review.googlesource.com/c/go/+/183846
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modget/get.go

@@ -472,7 +472,6 @@ func runGet(cmd *base.Command, args []string) {
 	// and the load and upgrade operations may only add and upgrade modules
 	// in the build list.
 	var matches []*search.Match
-	var install []string
 	for {
 		var seenPkgs map[string]bool
 		seenQuery := make(map[querySpec]bool)
@@ -664,7 +663,7 @@ func runGet(cmd *base.Command, args []string) {
 	}
 	work.BuildInit()
 	pkgs := load.PackagesForBuild(pkgPatterns)
-	work.InstallPackages(install, pkgs)
+	work.InstallPackages(pkgPatterns, pkgs)
 }
 
 // runQueries looks up modules at target versions in parallel. Results will be

src/cmd/go/testdata/mod/example.com_tools_v1.0.0.txt

@@ -0,0 +1,12 @@
+-- .info --
+{"Version": "v1.0.0"}
+-- .mod --
+module example.com/tools
+-- cmd/hello/hello.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("hello")
+}

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

@@ -0,0 +1,20 @@
+env GO111MODULE=on
+[short] skip
+
+# Test that when 'go get' is run from $GOBIN, it does not delete binaries
+# after it installs them. Verifies golang.org/issue/32766.
+
+go get example.com/tools/cmd/hello
+
+# 'go get' should not delete the command when run from $GOPATH/bin
+cd $GOPATH/bin
+exists hello$GOEXE
+go get example.com/tools/cmd/hello
+exists hello$GOEXE
+
+# 'go get' should not delete the command when run from a different $GOBIN
+mkdir $WORK/bin
+cd $WORK/bin
+env GOBIN=$WORK/bin
+go get example.com/tools/cmd/hello
+exists hello$GOEXE