cmd/go: set expected filename when building a local package with -o is pointing to a folder

In the local package build process, when -o is pointing to an existing folder, the object
the filename is generated from files listed on the command line like when the -o is
not pointing to a folder instead of using the `importPath` that is going to be `command-line-arguments`

Fixes #34535

Change-Id: I09a7609c17a2ccdd83da32f01247c0ef473dea1e
GitHub-Last-Rev: b3224226a3914aa2573e47a6daff9fd5a48ca225
GitHub-Pull-Request: golang/go#34562
Reviewed-on: https://go-review.googlesource.com/c/go/+/197544
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 35 insertions, 13 deletions

diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index 205ecc596d..115bc29694 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -1391,26 +1391,51 @@ var cgoSyscallExclude = map[string]bool{
 
 var foldPath = make(map[string]string)
 
-// DefaultExecName returns the default executable name
-// for a package with the import path importPath.
+// exeFromImportPath returns an executable name
+// for a package using the import path.
 //
-// The default executable name is the last element of the import path.
+// The executable name is the last element of the import path.
 // In module-aware mode, an additional rule is used on import paths
 // consisting of two or more path elements. If the last element is
 // a vN path element specifying the major version, then the
 // second last element of the import path is used instead.
-func DefaultExecName(importPath string) string {
-	_, elem := pathpkg.Split(importPath)
+func (p *Package) exeFromImportPath() string {
+	_, elem := pathpkg.Split(p.ImportPath)
 	if cfg.ModulesEnabled {
 		// If this is example.com/mycmd/v2, it's more useful to
 		// install it as mycmd than as v2. See golang.org/issue/24667.
-		if elem != importPath && isVersionElement(elem) {
-			_, elem = pathpkg.Split(pathpkg.Dir(importPath))
+		if elem != p.ImportPath && isVersionElement(elem) {
+			_, elem = pathpkg.Split(pathpkg.Dir(p.ImportPath))
 		}
 	}
 	return elem
 }
 
+// exeFromFiles returns an executable name for a package
+// using the first element in GoFiles or CgoFiles collections without the prefix.
+//
+// Returns empty string in case of empty collection.
+func (p *Package) exeFromFiles() string {
+	var src string
+	if len(p.GoFiles) > 0 {
+		src = p.GoFiles[0]
+	} else if len(p.CgoFiles) > 0 {
+		src = p.CgoFiles[0]
+	} else {
+		return ""
+	}
+	_, elem := filepath.Split(src)
+	return elem[:len(elem)-len(".go")]
+}
+
+// DefaultExecName returns the default executable name for a package
+func (p *Package) DefaultExecName() string {
+	if p.Internal.CmdlineFiles {
+		return p.exeFromFiles()
+	}
+	return p.exeFromImportPath()
+}
+
 // load populates p using information from bp, err, which should
 // be the result of calling build.Context.Import.
 func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
@@ -1451,7 +1476,7 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 			p.Error = &PackageError{Err: e}
 			return
 		}
-		elem := DefaultExecName(p.ImportPath)
+		elem := p.DefaultExecName()
 		full := cfg.BuildContext.GOOS + "_" + cfg.BuildContext.GOARCH + "/" + elem
 		if cfg.BuildContext.GOOS != base.ToolGOOS || cfg.BuildContext.GOARCH != base.ToolGOARCH {
 			// Install cross-compiled binaries to subdirectories of bin.
@@ -2140,11 +2165,8 @@ func GoFilesPackage(gofiles []string) *Package {
 	pkg.Match = gofiles
 
 	if pkg.Name == "main" {
-		_, elem := filepath.Split(gofiles[0])
-		exe := elem[:len(elem)-len(".go")] + cfg.ExeSuffix
-		if cfg.BuildO == "" {
-			cfg.BuildO = exe
-		}
+		exe := pkg.DefaultExecName() + cfg.ExeSuffix
+
 		if cfg.GOBIN != "" {
 			pkg.Target = filepath.Join(cfg.GOBIN, exe)
 		} else if cfg.ModulesEnabled {